It has been known for a long time that when a narrow beam of sunlight , usally called white light, is incident on a glass prism, the emergent light is seen to be consisting of several colour. There is actually a continuous variation of colour, but broadly, the different component colour that appear in sequence are: violet, indigo, blue, green, yellow, orange and red (given by the acronym VIBGYOR). The red light bends the least while the violet light bends the most picture.
The phenomenon of splitting of light into component colour is known as dispersion. The pattern of colour components of light is called the spectrum of light. The word spectrum is now used in a much more general sense: we discussed in article the electromagnetic spectrum over the large range of wavelengths, from ४-ray to radio waves of which the spectrum of light (visible spectrum) is only a small part.
Through the reason for appearance of spectrum is now common- knowledge, it was a matter of much debate in does it only separate the colours already present in white light?
In classic experiment known for its simplicity but great significance, Issac Newton settled the issue once for all. He put another similar prism, but in an inverted position, and let the emergent beam from the first prism fall on the second prism. The resulting emergent beam was found to be white light. The explanation was clear- the first prism splits the white light into it's component colours, while the inverted prism recombines them to give white light.Thus, white light itself consists of light of different colours which are separated by the prism.
It must be understood here that a ray of light, ad defined mathematically does not exist. An actual ray is really a beam of many rays of light. Each ray splits into component colours when it enters the glass prism. When those coloured rays come out on the other side, they again produce a white beam.
We now, know colour is associated with wavelength of light. In the visible spectrum, red light is at the long wavelength end (~700 nm) while the violet light is at the short wavelength ey (~400 nm). Dispersion takes place became the refractive index of medium for different wavelengths (colour) is different. For example, the bending of red components of white is least while it most for the violet.
Equivalently, red light travels faster than violet light in a glass prism. The variation of refractive index with wavelength may be more pronounced in some media than the other.In vacuum of cours, the speed of light in independent of wavelengths. Thus, vaccum (or air approximately) is a non-dispersive medium in which all colours travel with the same speed. This also follow from the fact that sunlight reaches us in the from of white light and not ad it's components. On the other hand, glass is a despersive medium.
