# Power of the Prism and the Nature of Light

August 30, 2014

One of the most fascinating things about modern physics is that this apparently highly complex and abstract branch of science is, for the most part, based upon very simple and elegant principles. A good example would be simple harmonic motion, which, in mathematics and physics, describes periodic motion. Even though this piece of elegant mathematics was invented to describe such simple phenomena as the motion of springs and pendula, it can be found all over modern physics, starting with simple dynamics and ending with string theory. A similar pattern can be noticed in experimental and observational physics as well. In this case my favourite example is the dispersion of light. To understand why this part of physics is so significant, we have to go back to the past.

Back in 1706, the great British scientist and mathematician Isaac Newton published one of his famous works — Optics. By then Newton had already forged himself a name as one of the leading figures in science for developing the law of gravity and differential calculus. Another little known passion of Newton was optics. In 1666 he observed the phenomenon of light dispersion — the splitting of white light into colours by a prism. This finding was correctly interpreted as refraction of different colours by different angles by the prism. Ok, so you might be thinking, what is the use of this discovery (besides inspiring an album cover by the Pink Floyd)? To answer that, we need to dig deeper in the history of physics.

Newton’s simple experiment with the prism illustrated that colour was an inherent property of light, which settled a long debate going on at the time among the leading physicists. Another important discovery regarding the nature of light was made in 1800 by Sir Frederick William Herschel. Herschel discovered what is now known as the infrared part of the electromagnetic spectrum. Now the way he did it was ingeniously simple — he used a prism, as Newton did before him, to form the spectrum of light and measured the temperature of each colour. What Herschel found was that each colour had a slightly different temperature, red having the highest temperature. What is even more fascinating, is that Herschel, after placing the thermometer beyond the red part of the spectrum, found an even hotter region. The conclusion drawn from this by the great astronomer was that there must be another type of light — the infrared part of the spectrum.

The discoveries of the dispersion of light, the invisible parts of the spectrum, the diffraction of light and the mathematical formulation of the wave behaviour by Maxwell formed the basis for the wave theory of light. This powerful set of ideas turned out to be invaluable in different parts of experimental physics. Not only the understanding of the behaviour of light could be used to study materials, but it could answer such mind-blowing questions like what is the mass and the chemical composition of the distant galaxies and stars.

To understand how astronomical spectroscopy works, we have to look back at another major discovery, which interestingly happened on the same year as that of Herschel. In 1800 Joseph von Fraunhofer constructed extremely pure prisms, which displayed dark lines in otherwise continuous spectrum of light. Soon after this discovery, that would eventually cause a revolution in the science of astronomy, Fraunhofer combined a telescope and a prism to observer the spectrum of stars and planets.

The black lines observed by Fraunhofer corresponded to the missing parts of the spectrum at certain wavelengths. When the sunlight is reflected from Venus, for example, part of it is absorbed by the elements found on the planet. By finding out the wavelength of these lines scientists can then determine the corresponding energy levels of the elements that absorb them. So to put it simply, by studying the absorption lines scientists can get a good idea of the chemical composition of stars and planets.

Similarly, the spectra of stars and other objects might contain extra intense lines, which correspond to physical processes that result in intense radiation in that wavelength. Probably the best known example would be Hα (656.28) line corresponding to the radiation emitted from the Solar layer called the Chromosphere.

So far so good, but how about the masses of stars and, say, the size of their orbits. Once again we can get a clue of these quantities by studying the spectrum of the stars. One of the most intuitive methods employs Doppler shift. The method works best for the so called spectroscopic binary stars (stars that are so close together that an astronomers only see one object). Even though such stars look as a single object, the light coming from them is Doppler shifted, i.e. their spectral lines are shifted and that shift changes over time. By studying this change, astronomers can determine the velocity of the stars, the size of their orbits and, through Newton’s laws of gravity and Kepler’s laws, the mass of the stars in the binary system.

It is needless to say that there are many other more sophisticated methods of determining the chemical compositions and masses of stars and planets. However, these few examples were chosen to illustrate how intuitive and elegant a lot of modern physics is — beneath all the complicated jargon and long mathematical equations we still find the basic principles discovered by such scientists as Newton, Herschel and Fraunhofer.