EPICURUS (341 – 270 BCE)

Third Century BCE

“Epicurus’s philosophy combines a physics based on an atomistic materialism with a rational hedonistic ethics that emphasizes moderation of desires and cultivation of friendships.”

Summarized by the Roman author Lucretius, who wrote ‘On the Nature of the Universe’ in 55 BCE – “The light and heat of the Sun; these are composed of minute atoms which, when they are shoved off, lose no time in shooting right across the interspace of air in the direction imparted by the shove”. This may be considered as accurate for the time, when most people thought that sight was associated with something reaching out from the eye (EMPEDOCLES) .

Plato wrote of a marriage between the inner light and the outer light.

Euclid worried about the speed with which sight worked. He pointed out that if you close your eyes, then open them again, even the distant stars reappear immediately in your sight, although the influence of sight has had to travel all the way from your eyes to the stars and back again before you could see them.

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AL-BIRUNI (973-1050)

The Persian scholar al-Biruni lived around the same time as ibn-Sina. He pioneered the idea that light travels faster than sound, promoted the idea that the Earth rotates on its axis and measured the density of 18 precious stones and metals.

portrait of al-biruni

He classified gems according to the properties: colour; powder colour; dispersion (whether white light splits up into the colours of the rainbow when it goes through the gem); hardness; crystal shape; density.
He used crystal shape to help him decide whether a gemstone was quartz or diamond.

He noted that flowers have 3,4,5 or 8 petals, but never 7 or 9.

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IBN SINA (AVICENNA) (980-1037)

‘al Qann fi al-Tibb’ (The Canon of Medicine), also ‘ The Book of the Remedy

Avicenna lived under the Sammarid caliphs in Bukhara. He identified different forms of energy – heat, light and mechanical – and the idea of a force.

drawing of Ibn Sina ©


Before GALEN, scientists describing nature followed the old Greek traditions of giving the definitions and following them up with the body of logical development. The investigator was then obliged merely to define the various types of ‘nature’ to be found. With Galen this procedure was changed.

Instead of hunting for these natures and defining more and more of them, reproducing ARISTOTLE’s ideas, AVICENNA, a Persian physician, planned inductive and deductive experimental approaches to determine the conditions producing observable results.

His tome surveyed the entire field of medical knowledge from ancient times up to the most up to date Muslim techniques. Avicenna was the first to note that tuberculosis is contagious; that diseases can spread through soil and water and that a person’s emotions can affect their state of physical health. He was the first to describe meningitis and realize that nerves transmit pain. The book also contained a description of 760 drugs. Its comprehensive and systematic approach meant that once it was translated into Latin in the twelfth century it became the standard medical textbook in Europe for the next 600 years.

Arabic Canon of Medicine by Avicenna 1632. Many physicians in the Islamic world were outstanding medical teachers and practitioners. Avicenna (980-1037 CE) was born near Bokhara in Central Asia. Known as the 'Prince of Physicians', his Canon of Medicine (medical encyclopedia) remained the standard text in both the East and West until the 16th century and still forms the basis of Unani theory and practice today. Divided into five books, this opening shows the start of the third book depicting diseases of the brain.

Arabic Canon of Medicine by Avicenna 1632

Avicenna thought of light as being made up of a stream of particles, produced in the Sun and in flames on Earth, which travel in straight lines and bounce off objects that they strike.

A pinhole in a curtain in a darkened room causes an inverted image to be projected, upside-down, onto a wall opposite the curtained window. The key point is that light travels in straight lines. A straight line from the top of a tree some distance away, in a garden that the window of the camera obscura faces onto – passing through the hole in the curtain – will carry on down to a point near the ground on the wall opposite. A straight line from the base of the tree will go upwards through the hole to strike the wall opposite near the ceiling. Straight lines from every other point on the tree will go through the hole to strike the wall in correspondingly determined spots, the result is an upside-down image of the tree (and of everything else in the garden).

He realized that refraction is a result of light traveling at different speeds in water and in air.

He used several logical arguments to support his contention that sight is not a result of some inner light reaching outward from the eye to probe the world around it, but is solely a result of light entering the eye from the world outside – realizing that ‘after-images’ caused by a bright light will persist when the eyes are closed and reasoning that this can only be the result of something from outside affecting the eyes. By effectively reversing the extro-missive theory of Euclid, he formulated the idea of a cone emanating from outside the eye entering and thus forming an image inside the eye – he thus introduced the modern idea of the ray of light.

The idea which was to have the most profound effect on the scientific development of an understanding of the behaviour of light was the thought of the way images are formed on a sunny day by the ‘camera obscura’.

AL HAZEN (c.965-1039)

Born in Basra and working in Egypt under al-Hakim, Abu Ali al-Hassan ibn al-Haytham was one of the three greatest scientists of Islam (along with al-Biruni and ibn-Sina). He explained how vision works in terms of geometric optics and had a huge influence on Western science. He is regarded as one of the earliest advocates of the scientific method.

The mathematical technique of ‘casting out of nines’, used to verify squares and cubes, is attributed to al-Hazen.

Al-Hazen dissented with the J’bir Ayam hypothesis of transmutation, thus providing two different strands for Alchemy in Europe from the Islāmic world.

diagram explaining the working of the eye

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bust said to depict a likeness of Socrates

The speculative Greek philosophers, considering the great overarching principles that controlled the Cosmos, were handicapped by a reluctance to test their speculations by experimentation.
At the other end of the spectrum were the craftsmen who fired and glazed pottery, who forged weapons out of bronze and iron. They in turn were hindered by their reluctance to speculate about the principles that governed their craft.

WESTERN SCIENCE is often credited with discoveries and inventions that have been observed in other cultures in earlier centuries.
This can be due to a lack of reliable records, difficulty in discerning fact from legend, problems in pinning down a finding to an individual or group or simple ignorance.

The Romans were technologists and made little contribution to pure science and then from the fall of Rome to the Renaissance science regressed. Through this time, science and technology evolved independently and to a large extent one could have science without technology and technology without science.

Later, there developed a movement to ‘Christianise Platonism’ (Thierry of Chartres).

Platonism at its simplest is the study and debate of the various arguments put forward by the Greek philosopher PLATO (428/7-348/7 BCE).
The philosopher Plotinus is attributed with having founded neo-Platonism, linking Christian and Gnostic beliefs to debate various arguments within their doctrines. One strand of thought linked together three intellectual states of being: the Good (or the One), the Intelligence and the Soul. The neo-Platonic Academy in Greece was closed by the Emperor Justinian (CE 483-565) in CE 529.
During the early years of the Renaissance, texts on neo-platonism began to be reconsidered, translated and discoursed.

Aristotle’s four causes from the ‘Timaeus’ were attributed to the Christian God, who works through secondary causes (such as angels).

Efficient Cause – Creator – God the Father

Formal Cause – Secondary agent – God the Son

Material Cause – The four elements: earth, air, fire & water.
Because these four are only fundamental forms of the single type of matter, they cannot be related to any idea of ‘elements’ as understood by modern science – they could be transmuted into each other. Different substances, although composed of matter would have different properties due to the differing amounts of the qualities of form and spirit. Thus a lump of lead is made of the same type of matter (fundamental form) as a lump of gold, but has a different aggregation of constituents. Neither lead nor gold would contain much spirit – not as much as air, say, and certainly not as much as God, who is purely spiritual. ( ALCHEMY )

Final Cause – Holy Spirit

All other is ‘natural’ – underwritten by God in maintaining the laws of nature without recourse to the supernatural.
Science was the method for investigating the world. It involved carrying out careful experiments, with nature as the ultimate arbiter of which theories were right and which were wrong.

Robert Grosseteste (1168-1253) Bishop of Lincoln (Robert ‘Bighead’) – neo-platonic reading of Genesis – emanation of God’s goodness, like light, begins creation. Light is thus a vehicle of creation and likewise knowledge (hence ‘illumination’), a dimensionless point of matter with a dimensionless point of light superimposed upon it (dimensions are created by God). Spherical radiation of light carries matter with it until it is dissipated. Led to studies of optical phenomena (rainbow, refraction, reflection).

stained glass window depicting Robert Grosseteste (created 1896)

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ROGER BACON (1214- 94)

(Doctor Mirabilis) ‘The Marvelous Doctor’

(Franciscan friar) Oxford – 1257

‘Mathematics (The first of the sciences, the alphabet of philosophy, door & key to the sciences), not Logic, should be the basis of all study’

Converted from Aristotelian to a neo-Platonist.

Etching of ROGER BACON Franciscan friar (1214- 94)


The Multiplication of Species; the means of causation (change) radiate from one object to another like the propagation of light.

‘An agent directs its effect to making the recipient similar to itself because the recipient is always potentially what the agent is in actuality.’

Thus heat radiating from a fire causes water placed near the fire,
but not in it, to become like the fire (hot). The quality of fire is multiplied in the water (multiplication of species).

All change may be analysed mathematically. Every multiplication is according to line, angles or figures. This thinking comes from the ninth century al-Kinde and his thoughts on rays and leads to a mathematical investigation into light.

Fear of the Mongols, Muslims and the Anti-Christ motivated the Franciscans. Franciscan neo-Platonism was based on Augustinian thought with a mathematical, Pythagorean, approach to nature. Bacon subscribed to this apocalyptical view, suffered trial and was imprisoned.
The Dominicans chose Aristotle – with a qualitative, non-mathematical approach to the world.

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1502 – Florence, Italy

‘In the Renaissance science was reinvented’

Image of the VITRUVIAN MAN


Leonardo is celebrated as the Renaissance artist who created the masterpieces ‘The Last Supper’ (1495-97) and ‘The Mona Lisa’ (1503-06). Much of his time was spent in scientific enquiry, although most of his work remained unpublished and largely forgotten centuries after his death. The genius of his designs so far outstripped contemporary technology that they were rendered literally inconceivable.

The range of his studies included astronomy, geography, palaeontology, geology, botany, zoölogy, hydrodynamics, optics, aerodynamics and anatomy. In the latter field he undertook a number of human dissections, largely on stolen corpses, to make detailed sketches of the body. He also dissected bears, cows, frogs, monkeys and birds to compare their anatomy with that of humans.

It is perhaps in his study of muscles where Leonardo’s blend of artistry and scientific analysis is best seen. In order to display the layers of the body, he developed the drawing technique of cross-sections and illustrated three-dimensional arrays of muscles and organs from different perspectives.

Leonardo’s superlative skill in illustration and his obsession with accuracy made his anatomical drawings the finest the world had ever seen. One of Leonardo’s special interests was the eye and he was fascinated by how the eye and brain worked together. He was probably the first anatomist to see how the optic nerve leaves the back of the eye and connects to the brain. He was probably the first, too, to realise how nerves link the brain to muscles. There had been no such idea in GALEN’s anatomy.

Possibly the most important contribution Leonardo made to science was the method of his enquiry, introducing a rational, systematic approach to the study of nature after a thousand years of superstition. He would begin by setting himself straightforward scientific queries such as ‘how does a bird fly?’ He would observe his subject in its natural environment, make notes on its behaviour, then repeat the observation over and over to ensure accuracy, before making sketches and ultimately drawing conclusions. In many instances he would directly apply the results of his enquiries into nature to designs for inventions for human use.

Self portrait of LEONARDO DA VINCI


He wrote ‘Things of the mind left untested by the senses are useless’. This methodical approach to science marks a significant stepping-stone from the DARK AGES to the modern era.

1469 Leonardo apprenticed to the studio of Andrea Verrocchio in Florence

1482 -1499 Leonardo’s work for Ludovico Siorza, the Duke of Milan, included designs for weaponry such as catapults and missiles.
Pictor et iggeniarius ducalis ( painter and engineer of the Duke )’.
Work on architecture, military and hydraulic engineering, flying machines and anatomy.

1502 Returns to Florence to work for Pope Alexander VI’s son, Cesare Borgia, as his military engineer and architect.

1503 Begins to paint the ‘Mona Lisa’.

1505-07 Wrote about the flight of birds and filled his notebooks with ideas for flying machines, including a helicopter and a parachute. In drawing machines he was keen to show how individual components worked.

1508 Studies anatomy in Milan.

1509 Draws maps and geological surveys of Lombardy and Lake Isea.

1516 Journeys to France on invitation of Francis I.

1519 April 23 – Dies in Clos-Luce, near Amboise, France.

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1621 – Holland

woodblock print portrait of WILLEBRORD SNELL ©


‘During refraction of light, the ratio of the sines of the angles of incidence ( i ) and refraction ( r ) is a constant equal to the refractive index of the medium’

In equation form: n1sini = n2 sinr 
where n1 and n2 are the respective refractive indices of the two media.

The refractive index of a substance is a measure of its ability to bend light. The higher the number the better light is refracted. The refractive index of diamond, 2.42, is the highest of all gems.

Refraction is the change in direction of a ray of light when it crosses the boundary between two media. It happens because light has different speeds in different media. A ray of light entering a medium where the speed of light is less (from air to water, for example) bends towards the perpendicular to the boundary of the two media. It bends away from the perpendicular when it crosses from water to air. Refraction was known to ancient Greeks, but Snell, a Dutch mathematician, was the first to study it.

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ROBERT HOOKE (1635-1703)

1670 – England

‘Within the limits of elasticity, the extension ( Strain ) of an elastic material is proportional to the applied stretching force ( Stress )’

Hooke’s law applies to all kinds of materials, from rubber balls to steel springs. The law helps define the limits of elasticity of a material.

In equation form; the law is expressed as F = kx, where F is force, x change in length and k is a constant. The constant is known as Young’s Modulus, after THOMAS YOUNG who in 1802 gave physical meaning to k.

Boyle and Hooke formed the nucleus of scientists at Gresham College in Oxford who were to create the Royal Society in 1662 and Hooke served as its secretary until his death. Newton disliked Hooke’s combative style (Hooke accused Newton of plagiarism, sparking a lifelong feud between the two) and refused to attend Royal Society meetings while Hooke was a secretary.

Hooke mistrusted his contemporaries so much that when he discovered his law he published it as a Latin anagram, ceiiinosssttvu, in his book on elasticity.

Two years later, when he was sure that the law could be proved by experiments on springs, he revealed that the anagram meant Ut tensio sic vis. That is, the power of any spring is in the same proportion with the tension thereof.

At the same time, in 1665 Hooke published his work Micrographia presenting fifty-seven illustrations drawn by him of the wonders seen with the microscope.

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1674 – Netherlands

Portrait of Leeuwenhoek

Leeuwenhoek was probably inspired to take up microscopy after seeing a copy of HOOKE’s Micrographia, though as a draper he was likely to have already been using lenses to examine cloth.
Unlike Hooke, Leeuwenhoek did not use a two lens compound microscope, but a single high quality lens, which could be described simply as a magnifying glass rather than a microscope. Leeuwenhoek is known to have made over 500 of these single–lens microscopes. They are simple devices just a few inches long, with the lens mounted in a tiny hole in a brass plate. The specimen is mounted on a point that sticks up in front of the lens. Two screws move the specimen for focusing. All else that is needed is careful lighting and a very steady, sharp eye.

After an introduction to Henry Oldenburg of the Royal Society in London from Dutch physician and anatomist Regnier de Graaf (discoverer of the egg-making follicles in the human ovary which now bear his name), Leeuwenhoek was encouraged to write to the Society’s journal ‘Philosophical Transactions’.

Leeuwenhoek’s letters were translated into Latin and English from the Dutch and he reported seeing tiny creatures in lake-water.

‘ I found floating therein divers earthly particles, and some green streaks, spirally wound serpentwise, and orderly arranged after the manner of copper or tin worms which distillers use to cool their liquors as they distil over. The whole circumference of each of these streaks was about the thickness of a hair of one’s head ’

Leeuwenhoek’s descriptions of ‘animalcules’ in water from different sources – rainwater, pond water, well water, sea water and so on – were verified by independent witnesses, including the vicar of Delft. Hooke too confirmed his findings with his own observations performed in front of expert witnesses, including Sir Christopher Wren.
Leeuwenhoek came close to understanding that bacteria were germs that cause disease but it took another century before LOUIS PASTEUR made that step.

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ISAAC NEWTON (1642-1727)

1687 – England

‘Any two bodies attract each other with a force proportional to the product of their masses and inversely proportional to the square of the distance between them’

portrait of NEWTON ©


The force is known as gravitation
Expressed as an equation:

F = GmM/r2

where F is Force, m and M the masses of two bodies, r the distance between them and G the gravitational constant
This follows from KEPLER’s laws, Newton’s laws of motion and the laws of conic sections. Gravitation is the same thing as gravity. The word gravity is particularly used for the attraction of the Earth for other objects.

Newton stated that the law of gravitation is universal; it applies to all bodies in the universe. All historical speculation of different mechanical principles for the earth from the rest of the cosmos were cast aside in favour of a single system. He demonstrated that the planets were attracted toward the Sun by a force varying as the inverse square of the distance and generalized that all heavenly bodies mutually attract one another. Simple mathematical laws could explain a huge range of seemingly disconnected physical facts, providing science with the straightforward explanations it had been seeking since the time of the ancients. That the constant of gravitation is in fact constant was proved by careful experiment, that the focus of a body’s centre of gravity appears to be a point at the centre of the object was proved by his calculus.

The angle of curve, by definition, is constantly changing, so it is difficult to calculate at any particular point. Similarly, it is difficult to calculate the area under a curve. Using ARCHIMEDES’ method of employing polygons and rectangles to work out the areas of circles and curves, and to show how the tangent or slope of any point of a curve can be analyzed, Newton developed his work on the revolutionary mathematical and scientific ideas of RENE DESCARTES, which were just beginning to filter into England, to create the mathematics of calculus. Calculus studies how fast things change.
The idea of fluxions has become known as differentiation, a means of determining the slope of a line, and integration, of finding the area beneath a curve.

Newton’s ideas on universal gravitation did not emerge until he began a controversial correspondence with ROBERT HOOKE in around 1680. Hooke claimed that he had solved the problem of planetary motion with an inverse square law that governed the way that planets moved. Hooke was right about the inverse square law, but he had no idea how it worked or how to prove it, he lacked the genius that permitted Newton to combine Kepler’s laws of planetary motion with the assumption that an object falling towards Earth was the same kind of motion as the Earth’s falling toward the Sun.
It was not until EDMUND HALLEY challenged Newton in 1684 to show how planets could have the elliptical orbits described by Johannes Kepler, supposing the force of attraction by the Sun to be the reciprocal of their distance from it – and Newton replied that he already knew – that he fully articulated his laws of gravitation.

It amounts to deriving Kepler’s first law by starting with the inverse square hypothesis of gravitation. Here the Sun attracts each of the planets with a force that is inversely proportional to the square of the distance of the planet from the Sun. From Kepler’s second law, the force acting on the planets is centripetal. Newton says this is the same as gravitation.

In the previous half century, Kepler had shown that planets have elliptical orbits and GALILEO had shown that things accelerate at an even pace as they fall towards the ground. Newton realized that his ideas about gravity and the laws of motion, which he had only applied to the Earth, might apply to all physical objects, and work for the heavens too. Any object that has mass will be pulled towards any other object. The larger the mass, the greater the pull. Things were not simply falling but being pulled by an invisible force. Just as this force (of gravity) pulls things towards the Earth, it also keeps the Moon in its orbit round the Earth and the planets moving around the Sun. With mathematical proofs he showed that this force is the same everywhere and that the pull between two things depends on their mass and the square of the distance between them.

Title page of Philosophiae Naturalis Principia Mathematica

Title page of Philosophiae Naturalis Principia Mathematica

Newton published his law of gravitation in his magnum opus Philosophiae Naturalis Principia Mathematica (Mathematical Principles of Natural Philosophy) in 1687. In it Newton analyzed the motion of orbiting bodies, projectiles, pendulums and free fall near the Earth.

The first book of Principia states the laws of motion and deals with the general principles of mechanics. The second book is concerned mainly with the motion of fluids. The third book is considered the most spectacular and explains gravitation.

Why do two objects attract each other?
‘I frame no hypotheses’, said Newton

It was Newton’s acceptance of the possibility that there are mysterious forces in the world, his passions for alchemy and the study of the influence of the Divine that led him to the idea of an invisible gravitational force – something that the more rationally minded Galileo had not been able to accept.
Newton’s use of mathematical expression of physical occurrences underlined the standard for modern physics and his laws underpin our basic understanding of how things work on an everyday scale. The universality of the law of gravitation was challenged in 1915 when EINSTEIN published the theory of general relativity.

1670-71 Newton composes ‘Methodis Fluxionum‘, his main work on calculus, which is not published until 1736. His secrecy meant that in the intervening period, the German mathematician LEIBNIZ could publish his own independently discovered version – he gave it the name calculus, which stuck.


1687 – England

  • First Law: An object at rest will remain at rest and an object in motion will remain in motion at that velocity until an external force acts on the object

  • Second Law: The sum of all forces (F) that act on an object is equal to the mass (m) of the object multiplied by the acceleration (a), or F = ma

  • Third Law: To every action, there is an equal and opposite reaction

The first law

introduces the concept of inertia, the tendency of a body to resist change in its velocity. The law is completely general, applying to all objects and any force. The inertia of an object is related to its mass. Things keep moving in a straight line until they are acted on by a force. The Moon tries to move in a straight line, but gravity pulls it into an orbit.
Weight is not the same as mass.

The second law

explains the relationship between mass and acceleration, stating that a force can change the motion of an object according to the product of its mass and its acceleration. That is, the rate and direction of any change depends entirely on the strength of the force that causes it and how heavy the object is. If the Moon were closer to the Earth, the pull of gravity between them would be so strong that the Moon would be dragged down to crash into the Earth. If it were further away, gravity would be weaker and the Moon would fly off into space.

The third law

shows that forces always exist in pairs. Every action and reaction is equal and opposite, so that when two things crash together they bounce off one another with equal force.


1672 – New Theory about Light and Colours is his first published work and contains his proof that white light is made up of all colours of the spectrum. By using a prism to split daylight into the colours of the rainbow and then using another to recombine them into white light, he showed that white light is made up of all the colours of the spectrum, each of which is bent to a slightly different extent when it passes through a lens – each type of ray producing a different spectral colour.

Newton also had a practical side. In the 1660s his reflecting telescope bypassed the focusing problems caused by chromatic aberration in the refracting telescope of the type used by Galileo. Newton solved the problem by swapping the lenses for curved mirrors so that the light rays did not have to pass through glass but reflected off it.

At around the same time, the Dutch scientist CHRISTIAAN HUYGENS came up with the convincing but wholly contradictory theory that light travels in waves like ripples on a pond. Newton vigorously challenged anyone who tried to contradict his opinion on the theory of light, as Robert Hooke and Leibniz, who shared similar views to Huygens found out. Given Newton’s standing, science abandoned the wave theory for the best part of two hundred years.

1704 – ‘Optiks’ published. In it he articulates his influential (if partly inaccurate) particle or corpuscle theory of light. Newton suggested that a beam of light is a stream of tiny particles or corpuscles, traveling at huge speed. If so, this would explain why light could travel through a vacuüm, where there is nothing to carry it. It also explained, he argued, why light travels in straight lines and casts sharp shadows – and is reflected from mirrors. His particle theory leads to an inverse square law that says that the intensity of light varies as the square of its distance from the source, just as gravity does. Newton was not dogmatic in Optiks, and shows an awareness of problems with the corpuscular theory.

In the mid-eighteenth century an English optician John Dolland realized that the problem of coloured images could largely be overcome by making two element glass lenses, in which a converging lens made from one kind of glass was sandwiched together with a diverging lens made of another type of glass. In such an ‘achromatic’ lens the spreading of white light into component colours by one element was cancelled out by the other.

During Newton’s time as master of the mint, twenty-seven counterfeiters were executed.

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1690 – Holland



‘Every point on a wavefront can act as a new source of waves’

A line perpendicular to the wave fronts is called a ray and this ray shows the direction of the wave.

The Huygens construction, published in ‘Traite de la Lumiere‘ (’Treatise on Light’, 1690) gives an explanation for the way light is reflected and refracted.

Huygens said that light consists of a disturbance spreading from its source as spherical pressure waves having wave fronts perpendicular to the direction of their motion and correctly anticipated that in a denser medium light would travel more slowly. This hypothesis was largely ignored at the time as it conflicted with NEWTON‘s theory. Huygens’ view, when re-discovered and championed by THOMAS YOUNG (1773-1829) would eventually become the more commonly accepted version.

He invented a pendulum clock (1656) and also discovered Titan, the first observed moon of Saturn (1665).

Saturn's moon Titan. Notable Features - Relatively smooth surface with almost no craters; Color variation across the planet (previously thought to be seas of methane, but that has been disproved. True origin has not been discovered.) At least one lake of liquid ethane is on the surface at the present time

Huygens discovered that a simple pendulum does not keep perfect time but completes smaller swings faster than big swings. This is because the weight or ‘bob’ of the pendulum follows a circular path. Huygens’ realisation that a pendulum mimicking a circle’s curve does not maintain a perfectly equal swing and that in order to do this it actually needs to follow a ‘cycloidal’ arc, set him on the path to designing the first successful pendulum clock.

Published ‘Horologium‘ (1658), ‘Horologium Oscillatorium‘ (1673) in which he showed that if the bob’s path were a cycloid (the curved path traced out by a point on the rim of a wheel as it rolls along) instead of a circle, it would be isynchronous (keeping equal time) no matter what the length of the swing. He made the pendulum’s swing cycloidal by suspending a rigid pendulum rod on two chords whose swing either way was limited by two plates called cycloidal checks.

GALILEO had considered the timekeeping possibilities of a swinging pendulum and Huygens successfully tied it with an escapement mechanism.
He explored the mathematics associated with pendulums – which led him, together with HOOKE, to an early prediction of the link between the elliptical orbits of the planets and the inverse square law of gravity. His work was a milestone, playing a key part in the understanding of centrifugal force. It helped to confirm Newton’s laws of motion by showing how an object will travel in a straight line unless pulled into a curved path by some other force.

Huygens was one of the founders of the French Académie des sciences in Paris.

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THOMAS YOUNG (1773-1829)

1801 – England

‘Interference between waves can be constructive or destructive’

Huygens‘ wave theory was neglected for more than a hundred years until it was revived by Young in the opening years of the nineteenth century. Young rejected Newton‘s view that if light consisted of waves it would not travel in a straight line and therefore sharp shadows would not be possible. He said that if the wavelength of light was extremely small, light would not spread around corners and shadows would appear sharp. His principle of interference provided strong evidence in support of the wave theory.

Young’s principle advanced the wave theory of light of CHRISTIAAN HUYGENS. Further advances came from EINSTEIN and PLANCK.

In Young’s double slit experiment a beam of sunlight is allowed to enter a darkened room through a pinhole. The beam is then passed through two closely spaced small slits in a cardboard screen. You would expect to see two bright lights on a screen placed behind the slits. Instead a series of alternate light and dark stripes are observed, known as interference fringes, produced when one wave of light interferes with another wave of light.

Two identical waves traveling together either reinforce each other (constructive interference) or cancel each other out (destructive interference). This effect is similar to the pattern produced when two stones are thrown into a pool of water.

portrait of THOMAS YOUNG ©


The mathematical explanation of this effect was provided by AUGUSTIN FRESNEL (1788-1827). The wave theory was further expanded by EINSTEIN in 1905 when he showed that light is transmitted as photons.

Light, an electromagnetic radiation, is transported in photons that are guided along their path by waves. This is known as ‘wave-particle duality’.

The current view of the nature of light is based on quantum theory.

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1823 – Germany

‘Why is the sky dark at night?’

This question puzzled astronomers for centuries and no, the answer is not because the Sun is on the other side of the planet.

Olbers pointed out that if there were an infinite number of stars evenly distributed in space, the night sky should be uniformly bright. He believed that the darkness of the night sky was due to the adsorption of light by interstellar space.

This is wrong.

Heinrich-Wilhelm-Matthias-Olbers ©


Olbers’ question remained a paradox until 1929 when it was discovered that the galaxies are moving away from us and the universe is expanding. The distant galaxies are moving away so fast that the intensity of light we receive from them is diminished.

diagram explaining reduced light intensity as the observer travels further from the source

What is light intensity?

In addition, this light is shifted towards the red end of the spectrum. These two effects significantly reduce the light we receive from distant galaxies, leaving only the nearby stars, which we see as points of light in a darkened sky.

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1823 – Germany

‘The spectroscope’

A significant improvement on the apparatus used by Newton. Sunlight, instead of passing through a pinhole before striking a prism, is passed through a long thin slit in a metal plate. This creates a long ribbon-like spectrum, which may be scanned from end to end with a microscope.

image of the visible portion of the electromagnetic spectrum showing a series of dark fraunhofer lines

Cutting across the ribbon of rainbow colours are thin black lines. The lines are present even when a diffraction grating is used instead of a prism, proving that the lines are not produced by the material of a prism, but are inherent in sunlight.

An equivalent way of describing colours is as light waves of different sizes.
The wavelength of light is fantastically small, on average about a thousandth of a millimeter, with the wavelength of red light being about twice as long as that of blue light.

Fraunhofer’s black lines correspond to missing wavelengths of light.

By 1823 Fraunhofer had measured the positions of 574 spectral lines, labeling the most prominent ones with the letters of the alphabet. The lines labeled with the letters ‘H’ and ‘K’ correspond to light at a wavelength of 0.3968 thousandths of a millimeter and 0.3933 thousandths of a millimeter, respectively. The lines are present in the spectrum of light from stars, usually in different combinations.

Fraunhofer died early at the age of 39 and it was left to the German GUSTAV KIRCHOFF to make the breakthrough that explained their significance.

Astronomers today know the wavelengths of more than 25,000 ‘Fraunhofer lines’.

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1831 – England

‘A changing magnetic field around a conductor produces an electric current in the conductor. The size of the voltage is proportional to the rate of change of the magnetic field’

portrait drawing of MICHAEL FARADAY English chemist and physicist (British Library) (1791-1867)

This phenomenon is called ‘electromagnetic induction’ and the current produced ‘induced current’. Induction is the basis of the electric generator and motor.

Faraday developed HANS CHRISTIAN OERSTED’s 1820 discovery that electric current could deflect a compass needle. In his experiment Faraday wrapped two coils of insulated wire around opposite sides of an iron ring. One coil was connected to a battery, the other to a wire under which lay a magnetic compass needle. He anticipated that if he passed a current through the first wire it would establish a field in the ring that would induce a current in the second wire. He observed no effect when the current was steady but when he turned the current on and off he noticed the needle moving. He surmised that whenever the current in the first coil changed, current was induced in the second. To test this concept he slipped a magnet in and out of a coil of wire. While the magnet was moving the compass needle registered a current, as he pushed it in it moved one way, as he pulled it out the needle moved in the opposite direction. This was the first production of electricity by non-chemical means.

In 1831, by rotating a copper disc between the poles of a magnet, Faraday was able to produce a steady electric current. This was the world’s first dynamo.

NEWTON, with his concept of gravity, had introduced the idea of an invisible force that exerted its effect through empty space, but the idea of ‘action-at-a-distance’ was rejected by an increasing number of scientists in the early nineteenth century. By 1830, THOMAS YOUNG and AUGUSTIN FRESNEL had shown that light did not travel as particles, as Newton had said, but as waves or vibrations. But if this was so, what was vibrating? To answer this, scientists came up with the idea of a weightless matter, or ‘aether’.

Faraday had rejected the concept of electricity as a ‘fluid’ and instead visualised its ‘fields’ with lines of force at their edges – the lines of force demonstrated by the pattern of iron fillings around a magnet. This meant that action at a distance simply did not happen, but things moved only when they encountered these lines of force. He believed that magnetism was also induced by fields of force and that it could interrelate with electricity because the respective fields cut across each other. Proving this to be true by producing an electric current via magnetism, Faraday had demonstrated electromagnetic induction.

When Faraday was discovering electromagnetic induction he did so in the guise of a natural philosopher. Physics, as a branch of science, was yet to be given a name.

The Russian physicist HEINRICH LENZ (1804- 65) extended Faraday’s work when in 1833 he suggested that ‘the changing magnetic field surrounding a conductor gives rise to an electric current whose own magnetic field tends to oppose it.’ This is now known as Lenz’s law. This law is in fact LE CHATELIER‘s principle when applied to the interactions of currents and magnetic fields.


Fluctuating Electromagnetic Fields and EM Waves

It took a Scottish mathematician by the name of JAMES CLERK MAXWELL to provide a mathematical interpretation of Faraday’s work on electromagnetism.

Describing the complex interplay of electric and magnetic fields, he was able to conclude mathematically that electromagnetic waves move at the speed of light and that light is just one form of electromagnetic wave.
This led to the understanding of light and radiant heat as moving variations in electromagnetic fields. These moving fields have become known collectively as radiation.

Faraday continued to investigate the idea that the natural forces of electricity, magnetism, light and even gravity are somehow ‘united’, and to develop the idea of fields of force. He focused on how light and gravity relate to electromagnetism.
After conducting experiments using transparent substances, he tried a piece of heavy lead glass, which led to the discovery of the ‘Faraday Effect’ in 1845 and proved that polarised light may be affected by a magnet. This opened the way for enquiries into the complete spectrum of electromagnetic radiation.

In 1888 the German physicist HEINRICH HERTZ confirmed the existence of electromagnetic waves – in this case radio waves – traveling at the speed of light.

The unit of capacitance, farad (F) is named in honour of Faraday.

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Faraday Lecture -‘The chemical history of a candle’
Faraday as a discoverer

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ARMAND FIZEAU (1819- 96)

1849 – France

‘The first successful experiment to determine the speed of light’



Prior to this experiment it was believed that light had an infinite speed – although in 1676 the Danish astronomer Ole Røemer (1644-1710) had used GALILEO’s 1610 discovery of the four largest moons of Jupiter to describe the way of measuring the speed of light by measuring the times at which the moons were eclipsed by Jupiter itself.
The timing of the eclipses is affected by whether the Earth is on the same side of the Sun as Jupiter, or on the opposite side. Rømer explained the differences in the eclipse timings as due to the extra time required for light from the moons to reach the Earth when it is on the opposite side of the Sun.

Diagram explaining how the speed of light may be determined from observation of the moons of Jupiter from the earth if the distance between the planets is known.

Using modern measurements, it is calculated that it takes light more than eight minutes, traveling at 300,000km per second, to reach us from the Sun, across half the diameter of the Earth’s orbit; so the maximum delay in observing an eclipse of one of the moons of Jupiter is twice that – more than a quarter of an hour.

photo portrait of ARMAND FIZEAU ©


Fizeau carried out his experiment in Paris between the belvedere of a house at Montmartre and a hill at Suresnes – a distance of 8.67 kilometres.
He placed a rotating toothed wheel with 720 gaps at Montmartre and a mirror at Suresnes. When the wheel was at rest, light passed through one gap and was reflected. When the wheel was rotated slowly the light was completely eclipsed from the observer. When the wheel was turned rapidly the reflected light passed through the next gap. Fizeau observed this at a maximum speed of 25 revolutions per second. Therefore the time required by light to travel a distance of 8.67 × 2 kilometres was 1/25 × 1/720 of a second. This gave a speed of 312,320 kilometres per second (the correct value is 299,792 kilometres per second).

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1860 – Germany

‘Each chemical element, when heated to incandescence, produces its own characteristic lines in the spectrum of light’

For example, sodium produces two bright yellow lines.
Bunsen developed the Bunsen burner in 1855.
In the flame test the Bunsen burner’s non-luminous flame does not interfere with the coloured flame given off by the sample.


Kirchhoff was a professor of physics at Heidelberg. Bunsen and Kirchhoff together developed the first spectroscope, a device used to produce and observe a spectrum. They used their spectroscope to discover two new elements, caesium (1860) and rubidium (1861).

In 1860 Kirchhoff made the discovery that when heated to incandescence, each element produces its own characteristic lines in the spectrum.

This means that each element emits light of a certain wavelength – sodium’s spectrum has two yellow lines (wavelengths about 588 and 589 nanometres). The Sun’s spectrum contains a number of dark lines, some of which correspond to these wavelengths.

The Swedish scientist ANDERS ANGSTROM had, four years earlier, found that a gas always absorbs light at the same wavelength that it emits light. If the gas is hotter than the light source, then more light is emitted by the gas than absorbed, creating a bright line in the spectrum of the light source. If the gas is cooler than the light source the opposite happens; more light is absorbed by the gas than is emitted, creating a dark line.
The dark solar D lines told Kirchhoff that sodium is present in the relatively cool outer atmosphere of the Sun. This could be tested in the laboratory by burning a piece of chalk in a hot oxygen-hydrogen torch. The intensely bright limelight that is produced may be passed through a cooler sodium flame and the light emerging examined through a spectroscope. Crossing the spectrum of the artificial light occur black lines at the same wavelength that a sodium flame emits light. This solved the mystery of the FRAUNHOFER LINES.

Scientists now had a means to determine the presence of elements in stars. By comparing the dark lines in the spectra of light from the stars with the bright lines produced by substances in the laboratory, Kirchhoff had been able to identify the elements that made up a celestial body millions of miles away in space.



In England the astronomer William Huggins recorded the spectra of hundreds of stars and showed the unmistakable fingerprints of familiar elements that are found on the Earth’s surface. The stars are made of exactly the same kind of atoms as the Earth.

In 1868 Norman Lockyer described a spectral line in the yellow region very close to the wavelength of the two ‘D’ spectral lines of sodium. After repeated attempts to discover a substance that produced the same line on Earth, it appeared that the line did not correspond to any hitherto known element. Lockyer gave the element the name ‘helium’, the gas later to be found associated with radioactive decay in ores containing uranium.


Helium had not previously been found on Earth because it is both inert and lighter than air, ironic because after hydrogen, helium is the second most common element in the universe.

In 1904 RUTHERFORD would declare that the presence of helium in the Sun was evidence that sunlight was a product of radioactive processes. The absence of any FRAUNHOFER lines in sunlight that corresponded to radium dealt a blow to this hypothesis. Was there another way of releasing atomic energy than radioactivity?


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ALBERT MICHELSON (1852-1931) EDWARD MORLEY (1838-1923)

1887 – USA

‘The aim of the experiment was to measure the effect of the Earth’s motion on the speed of light’

This celebrated experiment found no evidence of there being an effect.

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1890 – Ireland
1904 – Holland

‘A moving object appears to contract’

The contraction is negligible unless the object’s speed is close to the speed of light.

In 1890 Fitzgerald suggested that an object moving through space would shrink slightly in its direction of travel by an amount dependent on its speed.

In 1904 Lorentz independently studied this problem from an atomic point of view and derived a set of equations to explain it. A year later, Einstein derived Lorentz’s equations independently from his special theory of relativity.

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1895 – Germany

picture of the diploma awarded to Roentgen by the NOBEL foundation. link to:link to: http://www.nobelprize.org/nobel_prizes/physics/laureates/1901/index.html
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‘X-Rays are high energy radiation given off when fast-moving electrons lose energy very rapidly’

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1905 – Switzerland

  1. ‘the relativity principle: All laws of science are the same in all frames of reference.
  2. constancy of the speed of light: The speed of light in a vacuüm is constant and is independent of the speed of the observer’
photo portrait of Albert Einstein &copy:


The laws of physics are identical to different spectators, regardless of their position, as long as they are moving at a constant speed in relation to each other. Above all the speed of light is constant. Classical laws of mechanics seem to be obeyed in our normal lives because the speeds involved are insignificant.

Newton’s recipe for measuring the speed of a body moving through space involved simply timing it as it passed between two fixed points. This is based on the assumptions that time is flowing at the same rate for everyone – that there is such a thing as ‘absolute’ time, and that two observers would always agree on the distance between any two points in space.
The implications of this principle if the observers are moving at different speeds are bizarre and normal indicators of velocity such as distance and time become warped. Absolute space and time do not exist. The faster an object is moving the slower time moves. Objects appear to become shorter in the direction of travel. Mass increases as the speed of an object increases. Ultimately nothing may move faster than or equal to the speed of light because at that point it would have infinite mass, no length and time would stand still.

‘The energy (E) of a body equals its mass (m) times the speed of light (c) squared’

This equation shows that mass and energy are mutually convertible under certain conditions.

The mass-energy equation is a consequence of Einstein’s theory of special relativity and declares that only a small amount of atomic mass could unleash huge amounts of energy.

Two of his early papers described Brownian motion and the ‘photoelectric’ effect (employing PLANCK’s quantum theory and helping to confirm Planck’s ideas in the process).

1915 – Germany

‘Objects do not attract each other by exerting pull, but the presence of matter in space causes space to curve in such a manner that a gravitational field is set up. Gravity is the property of space itself’

From 1907 to 1915 Einstein developed his special theory into a general theory that included equating accelerating forces and gravitational forces. This implies light rays would be bent by gravitational attraction and electromagnetic radiation wavelengths would be increased under gravity. Moreover, mass and the resultant gravity, warps space and time, which would otherwise be ‘flat’, into curved paths that other masses (e.g. the moons of planets) caught within the field of the distortion follow. The predictions from special and general relativity were gradually proven by experimental evidence.

Einstein spent much of the rest of his life trying to devise a unified theory of electromagnetic, gravitational and nuclear fields.

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