SPLENDOURS OF LIGHT
P.R.Vishwanath

(a) Early History

Among things purportedly created by the Maker, Light occupies the foremost place (God said ‘ Let there be light’!). While it was believed (wrongly) by the Greeks that light originates in the eye, they also made two correct guesses : ( 1) The speed of light has to be very fast since one can see thousands of stars when one closes the eye and opens it immediately (2) Light travels in straight lines. However the proof had to wait till 11th century AD when the Arab scientist Al Hazen did experiments by sending light through a small hole. He also hypothesized that light does not originate in the eye but in the thing perceived.
Glass, which is very important to the science of Light ( called Optics) is made up of a mixture of ash and limestone. Since this mixture has to be heated to ~500 degrees, natural glass was first made probably in volcanoes or lightning hits. The use of glass can be seen in ancient civilizations in the form of beads etc. The buildings in Roman empire used glass for windows. Chinese had mastery over the making of gas and the eastern methods became known to the west at the time of crusades. An easternmost European city exploited this advantage of being at cross roads. This is how Venice became famous for glass ! It is also no wonder that appreciation for Galileo’s telescope really started from Venice.

(b) Classical theories of light

One of the first fundamental experiments on optics was done by the young Newton. When the thirty year old scientist sent light through a hole and made it fall on a glass prism., he found that it had got separated into various colours. The results published in 1672 showed that the order of the colours coming out of the prism was the same as that in the rainbow: the red had bent least and violet the most. When all these colours were again made to impinge on a prism, it was white (ordinary) light that came out of the prism. This made Newton hypothesize that white light is a mixture of lights of various colours. While this did explain colours, there was no explanation of light itself. Later Newton came out with the theory that light is made up of small particles which he called corpuscles.
However, there was another theory of light which was gaining prominence. This theory championed by Hookes, Huyghens and others proposed that light travels in the form of waves (like water waves, sound waves etc) . Many experiments done in the next hundred years gave proof for the wave theory of light.
Towards the end of his book on Optics, Newton made several prescient remarks which agree with 20th century developments in spirit if not in details. He said that light could be both waves and particles. He also wondered if it is not possible to interchange particles and light. While the modern photon is certainly not the particle which Newton was contemplating, it is well known that photon can create particles (like in the phenomena of pair production) .
A Danish astronomer called Romer tried to determine the velocity of light in 1676 by looking at the moons of Jupiter. While the value he got was much less than the present value ( ~ 300000 km/sec) , it was important to show that light has a finite velocity! Newton used these numbers to get a value of 8 minutes for the time taken for light to travel from sun to earth. This is not far from the present value of 8.3 minutes !
The next important step in understanding light was when theory and experiment came together in 19th century in the form of two British scientists. Michael Faraday, one of the greatest experimental physicists of all times, was at first an apprentice of Sir Humphry Davy. While Faraday like his mentor did many important experiments in chemistry, his greatest work was in the field of electricity and magnetism. This man, whose photograph was one of the few to adorn Einstein’s study, was well known for his humility . Apart from rejecting the presidentship of the prestigious Royal society, he had refused to accept many honors! 1n 1831 Faraday showed that electricity and magnetism were nothing but two faces of the same phenomena. 33 years later James Clerk Maxwell gave a theoretical understanding of Faraday’s experiments. He unified the fields of electricity, magnetism and optics by showing that an electron could give raise to all these effects under different circumstances. This is the famous electromagnetic theory which along with Newton’s gravitation theory was able to explain most classical phenomena. Light was just another electromagnetic wave !

© The electromagnetic spectrum:

According to the wave theory of light, what distinguishes one color from the next is the spacing between two waves, a distance called the wavelength . Blue light has a shorter wavelength than red light. Light is also differentiated using frequency, the reciprocal of the wavelength. William Hershel, in a classic experiment with a thermometer kept next to a glass prism , had shown that there are invisible rays of light called the infrared light beyond red. Several years later John Ritter found another set of invisible rays called ultra violet, again with a prism! He was looking for reactions of light of different colour on silver chloride. He found almost no reaction with red light but there was maximum reaction when he exposed the silver chloride to a region beyond violet ! Thus there were invisible rays beyond both ends of the visible spectrum ! James Maxwell hypothesized that there should also be waves with longer and longer wavelength which also wont be ‘ visible ‘ . These were called radio waves . A German scientist called Hertz discovered these rays in 1890. Jagadeesh Chandra Bose also made significant experiments regarding these waves!
While various theories of what constitutes light were the focal point of many discussions, it has to be asked that what is it that gives raise to light. A simple candle emits a glow (One set of Michael Faraday’s popular Christmas lectures to young adults was about the candle: The chemical history of a candle! . Faraday , in these lectures given in 1860, spent moe than 5 hours discussing how and what makes a candle glow!). Thus we know that a hot body gives out light ! The type of light depends on the temperature of the body. As the temperature of a body increases from zero, it starts giving out Infra red radiation. Most mammals give out much more infra red radiation than cold blooded creatures like reptiles. This is the radiation that one feels but does not see. When a chunk of coal is lit up, initially one feels the heat due to Infra red rays. The fire of a hot stove at ~600 degrees gives out red light. The familiar incandescent bulb at ~ 2000 degrees gives out yellow light. The sun, at a surface temperature of ~6000 degrees givers out a yellowish blue light. Thus the hotter object is the more energetic the light would be!
We have seen that red light has longer wavelength than blue light. Further, radio waves are of still longer wavelength . The range from radio to blue light represented the so called electromagnetic spectrum. Is blue photon the highest frequency photon that nature is capable of producing ? Answer to this came in the form of a ‘ new color’ when a scientist’s wife put her hand in the way of some new rays that her husband was experimenting with!
The scientific discovery which almost immediately affected the common man also took place when a 50 year old German scientist William Roentgen, a professor in Wurzberg university, decided to look deeper into the experiments conducted with Cathode Rays. These experiments done with an evacuated tube showed a stream of particles traveling from one electrode called the Cathode towards the other one called the Anode. The English physicist Thompson was able to show later that the particles were nothing but electrons which had been sought after for a long time. However, Roentgen was not interested in the stream of particles itself but its effect on nearby matter. What attracted Roentgen in 1895 October was that a barium screen kept nearby would start shining when the voltage was put on the two electrodes ! He found it so interesting that he stopped teaching and began to spend all the time in the laboratory. He found that the new rays would pass through the flesh in his wife’s hand but not the bones! He announced his discovery in December and published his results in January 1986.
The spooky picture of a living person’s hand showing only the bones and the ring was published in many newspapers of the world. The New York Times in its 13th January 1986 issue called it a new type of photography. It also prophesied that since metals inside the body could be recognized with these rays, these would revolutionize medical science ! In fact, surgeons started using these rays to locate bone fractures and bullets lodged inside the body! It is no wonder that there were lot of misuses also in the initial days of excitement. Only slowly ,adverse effects - skin burns, loss of hair etc - were also recognized. The discovery of these X-rays got Roentgen the first Nobel Prize ever. Out of the 13 Nobel prizes given out in the first two decades of the 20th century 4 were due to phenomena connected with x-rays ! Roentgen himself showed that these rays were similar to ordinary light in many ways.
It was found later that these rays have energies of about 1000 times that of optical light. The first few decades also saw the discovery of still higher energy light called gamma rays. As we will see later, ordinary light is emitted when there is a change in the energy level of outer electrons in an atom. A change in the energy level of inner electrons would be found to be responsible for the X-rays. Gamma rays, first seen in radioactive phenomena, are emitted when a nucleus changes its energy levels. These gamma rays have energies of about 1000 times more than X-rays and thus a million times more than ordinary light. Electrons traversing magnetic fields also emit radiation called synchrotron radiation. Depending on the energy of the electrons, the emitted radiation could belong to any part of the electromagnetic spectrum.
Astronomy is a study of celestial objects through the light they emit. Till the 20th century, astronomy was synonymous with optical astronomy. But a celestial object can be studied through all type of photons it emits, irrespective of the energy. Different energies reveal different aspects of the source ! Curiosity about certain type of light in the laboratory 100 years ago has helped us today not just in medical science, but understand mysteries of the far off objects in the universe through fields like x-ray and gamma ray astronomy!

(d) Einstein’s photon

We have seen that temperature of a body is reflected in the type of radiation that it puts out. It was Kirchoff who first put this statement as a law. However, the light given out by the idealized black body will be of several colours and it was important to know the proportion of different colours at any given temperature. Wien gave an equation which worked well for violet light but not for red. Similarly Rayleigh and Jeans gave an equation which would work only for the red. Thus there was not one equation which would explain both the high and low wavelength regions resulting in the so called black body problem !
The end of 19th century is a very interesting time in the history of physics. Complacency had set in since many scholars felt that they knew whatever was to be known and it was only a matter of just details . However, results of new experiments started coming in which would tear this complacency apart. It was like the proverbial lull before the storm. The great English physicists J.J.Thomson and Ernest Rutherford and Henri Becquerel and the Curie couple in France were in the midst of discovering new phenomena. But what unsettled the classical theories immediately were the black body problem and a series of experiments started by the German physicist Hertz. He found that light falling on metals would generate streams of electrons. Further experiments in 1902 by the German physicist Lenard showed anomalies in this so called Photoelectric effect. The observed properties of electrons were very different from what were expected. It was also seen that the color of light was of importance. Red light was not able to get electrons out, whereas blue light had no such problem. Thus there were at least two experimental results which awaited proper understanding. This is the time when the greatest ideas of modern physics were put forth .
The beginning of the 20th century coincided with the German physicist Max Planck’s theory of transport of energy. At first Planck found an equation which would agree with the experiments on the black body spectrum. But he thought it was just a mathematical jugglery but later realized the revolutionary concept in the theory. He said that energy flows in discrete quanta - one quanta or two quanta or three quanta but not in between. As the famous popularizer George Gamow put it, nature does not allow one to drink one and half glasses of beer - either one glass or two glasses! He also introduced a quantity called ‘ h ‘ (later to be known as Planck’s constant) which had dimensions of action. Many scholars ignored Planck’s ideas when they came out.
This is the time when a 26 year old clerk in a patent office in Switzerland made history ! In a series of epoch making publications in 1905, Albert Einstein, by sweeping aside tenets of old physics, laid the foundation for the new physics. In one of those papers he showed that the results of the photoelectric effect could be understood completely only if light is thought of as consisting of particles, later to be called photons. . It was not as though the wave theory was brushed aside completely. The energy of these particles , could be known only from a knowledge of the wavelength . These were related by the constant ‘h ‘ introduced by Max Plank just 5 years earlier. A longer wavelength implied lower energy and thus there was no wonder red light could not liberate electrons from the metal ! One can extrapolate further and see that radio photons would have very very low energy whereas gamma rays would have the highest energies. Thus light is a packet of pure energy! Einstein got the Nobel Prize for this discovery in 1912.
There were still more revolutionary ideas in the other papers of that monumental year of 1905. While it may be out of place to discuss all of them, familiarity with some of them is necessary: 1) The velocity of light is constant in all frames of reference. This is the maximum velocity possible for any particle in the universe 2) Energy and Mass are related by velocity of light. This gives rise to the most famous equation known to mankind >>. E =M c*c where M and E are the energy and the mass of a particle and c is the velocity of light. It can be seen from the equation that Photon ( whose mass is zero) which exists purely as energy can give rise to particles with mass.

(e) Bohr’s atom

The concept of atom had taken root in early 19th century with Dalton’s atomic theory. Chemistry has a fascinating history starting with the alchemists. While they themselves went on a wild goose chase, their methods marked the beginnings of a modern laboratory. The transition from alchemy to chemistry was due to a galaxy of chemists starting with Newton’s contemporary Robert Boyle and including Henry Cavendish, Joseph Priestley, Antoine Lavoisser etc. While properties of various atoms were well known, what really constituted atom was still a mystery.
Research on Cathode rays was the most important experimental activity in the last decade of the 19th century. It has already been remarked how these experiments gave birth to X-rays. A new fundamental branch of physics called Particle Physics opened up in 1897 when J.J.Thomson found that the cathode rays were negatively charged particles. He identified them with electrons. Few years later his student Ernest Rutherford showed that an atom consists of a positively charged heavy central core. Rutherford also gave a model for the atom : A nucleus at the centre surrounded by orbiting electrons. But the problem with the Rutherford model of the atom was that orbiting electrons would keep on losing speed and eventually fall into the nucleus. This threatened the very existence of the atom! Apart from this theoretical objection, the model could also not explain some of the experimental features connected with atoms !
Spectroscopy had become a mature branch of physics. It was routine to see what spectral lines would be seen when atoms are radiated with light. Each atom gave out specific spectral lines .i.e. was capable of giving out only certain colored light. However, the electron would accelerate because of the Coulomb force attraction and it was well known that an accelerating electron would continuously give out radiation. This would mean that in Rutherford’s model the light coming out has to be a continuum and not just few colours. For eg, Hydrogen gave out lines called Balmer’s lines (three in flue and one in red) which meant the light has to be of only particular energies.
A 26 year old Danish theoretical physicist came to Cambridge to continue his research. He found the great Thompson too distant and started working with Rutherford who had shifted to Manchester. The young Dane struck up a good relationship with the great experimentalist in spite of Rutherford’s dislike for theoretical ideas. The fact that Niels Bohr could play football redeemed him in Rutherford’s eyes ! Bohr knew the problems with Rutherford’s model of the atom. How to make the atom stable ? The answer lay in making the electron stable. Bohr proposed a bold solution on the models of the solar system. The elctron would be orbiting at a certain distance and when the electron is in such orbits, it just keepa on rotating and does not give out any light. In the solar system a planet can be at any distance from the sun. But in an atom, the electron can be only at certain distances from the central nucleus ! But then how are Balmer’s lines produced ? Bohr had another ingenious answer. He said that electron jumping from one allowed orbit to the other allowed orbits would radiate photons ! Each orbit is associated with a particular energy and the differences in energies would account for the energy (and thus color ) of the photon. Thus Bohr could calculate the frequency of the lines in Balmer spectra ! It is important to see that questions of how light is produced are first answered here in a quantitative manner!
If a list has to be made of the great physicists of the first half of the 20th century, all would agree that it has to be headed by Einstein and Bohr. Both belonged to the first generation which ushered in the quantum revolution, but they parted company when the older theory gave way to quantum mechanics of Schrodinger, Heisenberg, Pauli, Dirac etc . Pauli known for his wit called the later developments as the Physics of the kids ! The younger set, which was not comfortable with the ideas of orbits, electron jumps etc in atoms introduced the concept of energy levels .
Einstein was not philosophically in tune with ideals like probability, uncertainty etc which were the basic tenets of quantum mechanics. His oft quoted statement ‘ God does not play dice ‘ was a result of his beliefs. However, Bohr imbibed the later ideas and made significant contributions. Importantly, he was also the mouthpiece for the philosophical aspects. Thus , Bohr and Einstein held entirely opposite views regarding what comprised physical reality. However, they remained great friends arguing the matter out till the end of their lives.
Since the two scientists had also great authority, they also played important roles in fields outside science. World peace was dear to both of them and they strived hard to see that scientific discoveries not become tools in the hands of unscrupulous world leaders. Nuclear fission had been discovered in Europe and Bohr presented these results in the United States in 1939. There was immediately the fear that Hitler would use it which resulted in Einstein writing a letter to the American President to start research on atomic energy . (He later regretted it as ‘ my greatest mistake’). Bohr helped many Jewish scientists to escape from Europe and also tried to convince Winston Churchill about the futility of war. Both Einstein and Bohr tried unsuccessfully to diffuse the situation by interacting with American authorities. Even after the war, Bohr spent his life in spreading awareness about the danger of atomic weapons. Einstein interacted with world leaders like Nehru to dissuade the big nations from making atomic weapons.
We end this article with a colorful account of how a great mind can still be playful ! .In 1913, two gentlemen were traveling in a train which was approaching Zurich railway station. One was the venerable Max Planck and the other Nernst, known for his research in thermodynamics. Nernst, was frequently putting his head outside the window to look at the platform. He announced : Yes, he is there to which Planck asked : What colour is it . The young man whom they were expecting to meet on the platform had told them that he would bring a white rose if he is not interested in the job they were offering him. The train reached the station and when the two got out the smiling young man presented them with a red rose! And Albert Einstein spent his next 20 years