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Light - Part XV |
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Dispersion
of light The
splitting of a ray into its component colours is known as dispersion of
light and the band of colours is known as a spectrum. A spectrum of
yellow light from a sodium lamp will remain yellow itself. This means
that light rays from a sodium lamp do not show dispersion. To understand how white light is made up of seven different colours, do the following. Take a disc and mark seven sectors on it. Colour each sector as red, orange, yellow, green, blue, indigo and violet. Now rotate the disc at a high speed. You will notice that the disc, instead of showing seven colours, will start looking white to your eyes. White
light consists of photons of various wavelengths (or colours). The
wavelength of red photon is the longest and the wavelength of the violet
photon is the shortest. Thus when a white light passes through a glass
medium like a prism, different photons cross the medium at different
speeds. The red colour appears at the top of the spectrum because it is
deviated the least or it is refracted the least (less bending). On the
other hand, the violet end of the spectrum is bent the most or refracted
most, as it takes longer to traverse the glass medium. When
two prisms are placed with their vertices and bases in the opposite
direction as shown below, a ray of white light will come out as a white
light only. The
first prism PQR disperses the white light into seven different colours.
The second prism P’Q’R’ recombines the dispersed light rays into
one white light again. The emerging white light appears to be the same.
This is the reason why we do not see any spectrum with a glass block. The rainbow : A rainbow is seen when the sun appears in the sky after the rains. The moisture in the atmosphere behaves like tiny prisms, dispersing the sun’s rays into seven colours (Vibgyor). The red colour appears on the top of the rainbow and the violet colour appears at the bottom. Colours of objects : Visually we see colours because of the reflection of that particular colour only. For example a leaf will look green in the sun, because it absorbs all the other six colours from the sun’s rays and will not absorb the green photons of the spectrum. These green photons reach our eyes and give us a sensation that the leaf is green in colour. If you observe a leaf under a sodium lamp, which gives off yellow light, the leaf will appear black in colour. Similaraly, if an object looks black, the entire spectrum of light is absorbed by the black object, hence it looks black.[1] When an object seems white in colour, it means that it is reflecting all the light back and is not absorbing any photons of the seven colours. Any colour that we see other than the seven colours are actually a combinations of two or more of the seven colours of the spectrum. [1] It is because of this reason that we wear dark colours in winter. The dark colours absorb all the light falling on it and hence keep the wearer warm. In summer season, when we do not want to wear clothes that absorb sun light, we tend to wear white or light coloured clothes.
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