Jethalal S. Zaveri
Prof. Muni Mahendra Kumar
Much earlier, there were two theories about light: One, which Newton favoured, was that it was composed of particles, called corpuscles; the other was that it was made of waves. A proper theory of the propagation of light did not come until 1865, when Maxwell succeeded in unifying the forces of electricity and magnetism. Maxwell's equations predicted that electromagnetic waves travel at a fixed speed. Thus, light is a rapidly alternating electromagnetic field travelling through space in the form of waves at a fixed speed.
In due course, it was established that radio-waves, light, and X- rays are all waves forming the electromagnetic spectrum, a tiny fraction of which - the visible spectrum - is visible in the form of light. This remained the accepted and proven theory of light up to 1905.
Einstein's theory of light was that it is composed of tiny particles called photons. A beam of light is analogous to a stream of bullets. To prove his theory, Einstein referred to a phenomenon called the photoelectric effect, in which, when light impinges on a metal surface, it sends electrons flying off. If a photon hits an electron, it knocks it away just as one billiard ball hitting another one knocks it away.
It was also found that the velocity of the rebounding electrons did not depend upon the intensity of the impinging light, but on its colour. Thus each photon of a given colour, say, green, has a certain amount of energy. Reducing intensity only reduces the number of photons, but each green photon has the same amount of energy. This brings us to the problem of colour.
Newton had discovered that if sunlight passes through a triangular-shaped piece of glass, one of the most beautiful phenomena occurs. From the other side of the glass comes not white light but every colour in the rainbow from red to violet with orange, yellow, green, blue, and indigo in between. This display of colours is called visible spectrum. When white light, which is a mixture of different colours, passes through prism, different colours are bent by different amounts - red the least, violet the most. Each colour has its own specific wavelength and frequency. The wavelength of red is longest while that of violet is shortest.
All the colours of visible light (together with infra-red and ultra-violet radiations) represent only a small portion of a band of radiations from high-energy X-rays to low-energy radio waves. The higher the energy, the shorter is the wavelength. Thus, high frequency light such as violet has a short wavelength and a high energy; low frequency light such as red has a long wavelength and a low energy. Velocity is the product of wave- length and frequency. The velocity of all electromagnetic waves - lights of all colours radio-waves, X-rays, and all the other forms of radiation - is always 1,86,000 miles per second (i.e. 3.00.000 kilometres per second). This constant speed of light is usually represented by the letter "c".
In the beginning of the 20th century, thus, there were two successful theories:
- Newtonian mechanics
- Maxwell's electrodynamics
The Newtonian monopoly of being the basis of all physics was destroyed.
