. D ISPERSION BY A P RISM It has been known for a long time that when a narrow beam of sunlight, usually called white light, is incident on a glass prism, the emergent light is seen to be consisting of several colours. There is actually a continuous variation of colour, but broadly, the different component colours that appear in sequence are: v iolet, i ndigo, b lue, g reen, y ellow, o range and r ed (given by the acronym VIBGYOR). The red light bends the least, while the violet light bends the most (Fig.
. ). The phenomenon of splitting of light into its component colours 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 Chapter the electro- magnetic spectrum over the large range of wavelengths, from " -rays to radio waves, of which the spectrum of light (visible spectrum) is only a small part. Though the reason for appearance of spectrum is now common knowledge, it was a matter of much debate in the history of physics. Does the prism itself create colour in some way or does it only separate the colours already present in white light? In a classic experiment known for its simplicity but great significance, Isaac 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 (Fig. . ). The resulting emergent beam was found to be white light.
The explanation was clear— the first prism splits the white light into its 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, as 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 that colour is associated with wavelength of light. In the visible spectrum, red light is at the long wavelength end (~ nm) while the violet light is at the short wavelength end (~ nm).
Dispersion takes place because the refractive index of medium for different wavelengths (colours) is different. For example, the bending FIGURE . Dispersion of sunlight or white light on passing through a glass prism. The relative deviation of different colours shown is highly exaggerated.
FIGURE . Schematic diagram of Newton’s classic experiment on dispersion of white light.