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Home , Christiaan Huygens, Ripple tank, Theory of relativity

HOW DO WE KNOW?

THE NATURE OF BY ANDREW ROBINSON What is light? It’s puzzled some of humanity’s greatest thinkers for 2,000 , especially since it appears to behave like as well as particles

HAT IS LIGHT? The question in the West as Alhazen, with the Da Vinci suggested that the eye is a has fascinated scientists – and nicknames ‘Ptolemy the ’ camera obscura – literally, ‘a darkened painters, poets, writers and and ‘The ’. He noted that chamber’ – into which light rays anyone who’s ever played you cannot look at the for long penetrate via a small aperture (the with a prism – since classical without great pain; and if you stare at pupil) and create an inverted image antiquity. Pythagoras, Euclid a bright object for half a and of an exterior scene on the back of and Ptolemy, for example, then close your eyes, a coloured image the eye (the retina). First adopted as accepted that light moved of the object floats into view. In each a by Kepler in 1604, the camera in straight lines. But rather case, something emitted by the object obscura became the dominant model W than assuming that light rays must have entered the eye. of human vision in the 17th . travelled from an object to the eye, The later of Alhazen’s they believed the eye emitted visual Book Of into led to its rays, like feelers, which touched being read by, among others, artist COMPETING IDEAS the object and thereby created the Leonardo da Vinci, physicist Galileo Two of light, apparently sensation of sight in the mind. Galilei, , incompatible, were in competition by Drop a needle on the ground, Euclid philosopher René Descartes and 1700. The first, proposed by noted in the 3rd Century BC, search , in 1678 and published in 1690, was an for it, and wonder why the needle is all of whom contributed to our undulatory : light transmitted not immediately visible. If light rays understanding of light and vision. as waves. Light waves spread in all are emanating instantaneously from directions from a light source, and an object to the eye, he hypothesized were detected by their creation of that you should see it immediately. in the retina. They were He decided the for the delay regarded as analogous to sound waves must be that the visual ray, searching spreading from a tuning fork, with the the ground, is yet to touch the needle. retina rather than the tympanum as Then, in a flash, you spot it. This detector. With sound, the undulating ancient idea of light and sight was medium was the air; with light it was taught in the West in various forms supposed to be an invisible, mysterious until the 12th Century AD. medium known as ‘the ether’. It was rejected by Ibn al-Haytham, In contrast, , who a 10th Century scientist from what A page from one of Leonardo Da Vinci’s late 15th Century began his optical experiments is now Iraq. Al-Haytham was known notebooks, showing his musings on the nature of light in 1666, favoured a corpuscular PHOTO: GETTY, PHOTO LIBRARY SCIENCE PHOTO GETTY, PHOTO:

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> IN A NUTSHELL

Light is a somewhat confusing phenomenon, in that it can behave both as a and as a particle. This counterintuitive property of light kept some of our brightest This prism is both refracting minds (pun intended!) guessing light that passes through it for 2,000 years… (causing the spectrum) and reflecting it internally

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theory: light as particles. Light The simplest phenomenon was the What about ? When rays spread from a light source transmission of light in straight lines, light strikes a mirror, the angle of in a stream of minute particles or as in shafts of sunlight through a cloud incidence is equal to the angle of ‘corpuscles’, shooting through empty or solar eclipses. Such behaviour was reflection. In the corpuscular theory, (rather than the ether) like expected for a stream of corpuscles, the explanation was straightforward: bullets, and were detected by their but not for a wave. Water waves the corpuscles must behave like impact on the retina. That said, rippling from a stone dropped into billiard balls bouncing off the cushion Newton was far from certain. He a pond could be seen to spread in all of a billiard table at equal angles. The waited until 1704 before publishing his directions and bend around obstacles undulatory theory, too, had no real , and presented his ideas as a to some extent. Sound waves could be difficulty. Once Huygens assumed that series of ‘Queries’ with answers that, heard to bend. Light, however, did not a light ray could be mathematically far from being definitive, occasionally appear to bend. In Newton’s emphatic modelled as the path of a point on a favoured the undulatory theory. words: “Sounds are propagated as wave front, he could easily deduce the Of course, the real test of the readily through crooked pipes as law of reflection. two theories was experiment. How through straight ones. But light was a more decisive effective was each theory in explaining is never known to follow crooked test. When light strikes the surface reflection, refraction and ? passages nor to bend into the shadow.” of water and passes through it, the

THE KEY The interference of water waves on a pond’s surface is a familiar phenomenon. EXPERIMENT showed that the same thing happens with beams of light

IN 1802, IN a lecture at London’s Royal Institution, Thomas Young demonstrated a new device now known as a ripple tank. Its basic principle is that water is stirred up in a trough with a glass bottom illuminated from below, so that the water waves and their patterns cast shadows onto a white screen above the trough. When two sets of ripples are created, they interfere with each other, producing a pattern of agitated water known as constructive interference – where the wave peaks coincide. A pattern of smooth water is known as destructive interference – where a wave peak is cancelled by a wave trough. If light were a wave, Young thought that it, too, might demonstrate constructive and destructive interference. A or two after his lecture, working at home, Young split a beam of light using two narrow slits. As the two beams were di!racted by the slits, they interfered with each other and created a pattern of bright and dark stripes on a screen. Thus, light added to light could produce more light or, more surprisingly, darkness. To Einstein, Young’s experiment was “the next great theoretical advance” after Newton’s on optics, while physicist Richard Feynman said it was the “heart of quantum ”.

Young’s double-slit experiment at the turn of the 19th Century proved decisive in showing that light can behave like a wave PHOTO: SCIENCE PHOTO LIBRARY, SCIENCE & SOCIETY X4, THINKSTOCK SCIENCE & SOCIETY X4, LIBRARY, SCIENCE PHOTO PHOTO:

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angles of incidence and refraction differ. The angle of refraction is less than the angle of incidence, and so CAST OF Five great minds who contributed to a ray is bent towards the normal CHARACTERS our understanding of how light behaves (the perpendicular). Although their relationship had been formulated in 1621 as Snell’s Law, it still required a Christiaan Huygens physical explanation. (1629-95) This Newton’s attempt was not very mathematician, convincing. He proposed that light’s physicist and in water was faster than in air astronomer came from – a counter-intuitive idea, given that a rich and influential water is a denser medium. Once they Dutch family. He is entered the water, said Newton, the perhaps best known corpuscles were acted upon by a for his of the which increased their velocity and Isaac Newton -regulated altered the direction of their . (1642-1727) This , but also However, the nature of this force was farmer’s son became improved the inexplicable, and it had no supporting president of the Royal of his and evidence from other phenomena. Society and needs discovered the rings Huygens, by contrast, assumed no introduction as a of . His greatest that light travelled more slowly in mathematician and achievement was his water than in air. He then used the physicist. His laws of wave theory of light. undulatory theory in a direct and mechanics and theory simple way, without postulating any of gravitation are his new force, to calculate Snell’s Law. lasting monument, But this verdict was by no means but his research into conclusive. What precisely was the optics was also highly Thomas Young velocity of light in air and in water? influential, despite his (1773-1829) This Ole Rømer made the first modern dogged adherence child prodigy born in estimate of light’s velocity in the 1670s to the corpuscular modest circumstances using astronomical measurements, theory of light. to English Quakers but not until 1850 was it measured became probably the accurately enough to prove that light greatest polymath of moves more slowly in water than in his . ‘The last man air, as Huygens had assumed. who knew everything’ was a London physician by profession, but his FRINGE THEORIES reputation was made in Finally, there was diffraction, a , the physiology phenomenon first observed by of the eye and colour Francesco Grimaldi in the mid-17th James Clerk Maxwell vision, and Egyptology. Century. He allowed a beam of light (1831-79) Maxwell was into a darkened room through a small a Scottish physicist circular aperture, then passed it who founded the through a second aperture and onto Cavendish Laboratory a screen. He noticed that the spot of at Cambridge. He made light on the screen was slightly larger key contributions to than the second aperture, and had , the (1879-1955) Einstein is coloured fringes. When he placed a kinetic theory of gases generally regarded as thin obstacle in the beam, its shadow and colour vision, the greatest physicist was not sharp: there were bright but he’s principally since Newton, for bands, very narrow and coloured, remembered for his his General Theory following the outer edge of the field equations of of Relativity and for shadow. In other words, light could be , his contributions to bent – diffracted – by apertures and which defined light as an quantum theory. Born obstacles, even if only very slightly. electromagnetic wave. in Germany, he worked When Newton repeated Grimaldi’s in Switzerland as a experiments, he too observed patent clerk, returned coloured fringes. Perplexed, he to his homeland as a claimed that the edges of an aperture professor and finally or obstacle interfered with the paths emigrated to the United of the corpuscles. Although the States to avoid Nazi undulatory theory could not persecution of Jews.

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explain the colours, it explained diffraction better than its rival. It took to arrive at our current understanding Nevertheless, Newton’s prestige of light – even today, the picture isn’t entirely clear allowed his theory of light to dominate the Age of the Enlightenment. In 1771, the first edition of the Encyclopaedia Britannica boldly declared: “Light Ibn al-Haytham shows that vision depends consists of an inconceivably great on light rays reflected TH number of particles flowing from from objects, not visual a luminous body in all manner of rays emitted by eyes. 11 directions; and these particles are His work is translated CENTURY so small, as to surpass all human into Latin in the 13th comprehension.” Not until the Century, influencing later European thinkers. early 19th Century were corpuscles seriously challenged by undulations. Around 1804, Thomas Young Christiaan Huygens demonstrated the interference of light, publishes a wave in which two overlapping beams of theory of light. It explains refraction and light produced bright and dark stripes, 1690 di!raction, but depends comparable to the interference of two on the existence of an water waves (see ‘The Key Discovery’). ‘ether’, for which there is Corpuscles could not produce such no evidence. patterns. Subsequent experiments on polarised light by French physicist Augustin Fresnel suggested a transverse light wave. Its components After of experimentation, Isaac Newton publishes his Opticks. It explains oscillate at right angles to each other some optical phenomena (but not others) and to the direction of propagation, as with a particle theory of light, which does 1704 opposed to a such not require the existence of an ether. as sound, which oscillates only in the direction of propagation.

ELECTROMAGNETISM C.1804 In the 1830s and ’40s, Michael Faraday demonstrated the inter-relationship of electric and magnetic fields of force. James Clerk Maxwell then took Thomas Young establishes beyond doubt that Faraday’s fields and merged them light can behave as a wave by passing a light mathematically into a single concept: an beam through two narrow slits and observing an interference pattern on a screen. electromagnetic wave with transverse electric and magnetic components, which Maxwell calculated would Building on the work propagate through the ether at a of Michael Faraday, similar to the measured speed of James Clerk light, around 300,000km/s. Different Maxwell predicts the existence of colours of light now corresponded to electromagnetic 1865 different and . waves of varying In the 1880s, German physicist and Heinrich Hertz proved such waves’ , existence experimentally. including light waves. Maxwell solved one problem brilliantly, but offered no physical explanation for the ether. If light was a wave, what was vibrating? Persistent Albert Einstein’s Special attempts to detect the ether failed postulates that light during the latter part of the 19th moves at a speed Century. After 1905, with the arrival independent of the of Albert Einstein’s Special Theory 1905 observer and does not of Relativity, Maxwell’s ether was require Maxwell’s ether. abandoned as an unnecessary concept. Separately, he uses the Relativity started with Einstein’s photoelectric e!ect to postulate light quanta. ‘thought experiment’ in 1895 about chasing a light ray. Ten years later PHOTO: SCIENCE PHOTO LIBRARY X3, SCIENCE & SOCIETY, GETTY, ALAMY GETTY, SCIENCE & SOCIETY, X3, LIBRARY SCIENCE PHOTO PHOTO:

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NEED TO KNOW Key terms relating to the behaviour and study of light

REFRACTION 1 Light passing from one medium to another is bent by refraction – hence the ability of lenses to focus light rays, and the fact that a pencil placed in a half-glass of water looks bent. The of a medium is the ratio of the velocity of light in a to its velocity in the medium.

DIFFRACTION 2 Di!raction is the bending of all waves by apertures and obstacles. The di!raction of light is not so obvious in ordinary life as its refraction, but it can be observed in the iridescent colours formed by a CD or DVD under visible light. The surface of these discs is ruled with very close lines, which form a di!raction grating.

PHOTOELECTRIC EFFECT 3 High-frequency electromagnetic radiation such as X-rays can displace A young Isaac Newton conducting light experiments using a prism in the mid-. Newton favoured a electrons from certain metals. However, corpuscular theory of light, and his great standing ensured that idea dominated scientific thought for many decades this occurs only above a certain threshold frequency, never below it, regardless of the postulated that the is its phenomena. But its underlying intensity of the radiation. The explanation for always the same, independent of how physical nature remains a puzzle. this requires the radiation to be quantised, the emitting source or the detector A few years before his 1955 death, with the of each quantum dependent move, and without the need for any Einstein remarked: “All the 50 years of on its frequency. universal such conscious brooding have brought me as the ether. If this were not radical no closer to the answer to the question, enough, in 1905 Einstein also proposed ‘What are light quanta?’ Of course a comeback for Newton’s corpuscular today every rascal thinks he knows the he wrote: “If I pursue a beam of light theory in a more modern form. Instead answer, but he is deluding himself.” with the velocity c (velocity of light of corpuscles, there were now light Since the 1980s, delicate in a vacuum), I should observe such a ‘quanta’ (later termed ). experiments have replayed a version beam of light as a spatially oscillatory In 1900, had postulated of Young’s key experiment and electromagnetic field at rest. However, that the electromagnetic spectra from shown that a single can there seems to be no such thing, red-hot bodies could be explained somehow interfere with itself. In whether on the basis of experience or only if radiation could be emitted his much-praised study Catching according to Maxwell’s equations.” or absorbed in discrete packets, The Light, quantum physicist Arthur Were we to travel faster than light, or ‘quanta’. Now Einstein, after Zajonc admits: “Light remains as Einstein imagined a situation in which considering the photoelectric effect – fundamentally mysterious as ever”. „ we might run away from a light signal the way in which certain metals emit and catch up with previously sent light electrons when exposed to radiation signals. The most recently sent light – proposed that not only were the signal would be detected first by our emission and absorption of light ANDREW ROBINSON’s books include eyes, and then we’d see progressively quantised (formed into quanta), light biographies of Einstein and Thomas Young older light signals. “We should catch itself was quantised. Between them, them in a reverse order to that in Planck and Einstein launched a theory Find out more which they were sent, and the train that would determine the course of Melvyn Bragg and guests of happenings on our Earth would physics. discuss the life and work of appear like a film shown backwards, Today, are compelled James Clerk Maxwell in this beginning with the happy ending.” to accept that light behaves as both episode of Radio 4’s In Our programme. The idea of catching or overtaking a particle and a wave, depending bbc.co.uk/programmes/p005491g light was clearly absurd. Einstein on how they choose to measure

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