Dispersion of White Light By A Glass Prism
When white light is incident on a glass prism, it splits into a band of colours as the refracting surfaces of the glass prism are inclined at an angle.
Example 1.
Case Based:
Take a thick sheet of cardboard and make a smalt hole or narrow slit in its middle. Allow sunlight to fall on the narrow slit. This gives a narrow beam of white light. Now, take a glass prism and allow the light from the slit to fall on one of its faces as shown in figure.
Turn the prism slowly until the light that comes out of it appears on a nearby screen.
(A) When the prism is turned slowly in the above activity, it is observed that:
(I) White light comes out of the prism on a nearby screen
(II) A band of seven colours is observed on a nearby screen with red and Indigo colours at the two ends.
(III) A band of seven colours is observed on a nearby screen with Yellow and Violet colours at the two ends.
(IV) A band of seven colours is observed on a nearby screen with red and Violet colours at the two ends.
Select the incorrect observations from the above:
(a) Both (I) and (II)
(b) Both (II) and (III)
(c) (I), (II) and (III)
(d) (I), (III) and (IV)
Answer:
(c) (I), (II) and (III)
Explanation: When the prism is turned slowly, a band of seven colours is observed on a nearby screen, with red and violet colours at the two ends of the band. The various colours seen are Violet, Indigo, Blue, Green, Yellow, Orange and Red, as shown in figure. The band of the coloured components of a light beam is called its spectrum.
(B) The colour of light that has the minimum ve¬locity in the glass prism is:
(a) Red
(b) Green
(c) Blue
(d) Violet
Answer:
(d) Violet
Explanation: Violet colour has the minimum wavelength among the visible colours of the spectrum and deviates the maximum. This is because it has the minimum velocity in the glass prism since refractive index of glass is maximum for violet.
(C) Which colour of light deviates the most on passing white light through a prism?
Answer:
Violet colour deviates the most when white light is passed through a glass prism. This is be-cause violet has the least wavelength among the seven visible colours of spectrum and re-fractive index of glass for violet light is the maximum.
(D) When white light enters a glass prism from air, for which colour is the angle of deviation the least?
Answer:
The angle of deviation will be least for red colour as red colour has the longest wavelength among the visible colours of the spectrum and therefore refractive index of glass for red colour will be minimum. So, red will suffer the minimum deviation.
(E) Assertion (A): The rays of different colours emerge along different paths from a glass prism.
Reason (R): Light rays undergo refraction and bend away from the nor¬mal when they travel from glass prism to air.
(a) Both (A) and (R) are true and (R) is the correct explanation of the assertion.
(b) Both (A) and (R) are true, but (R) is not the correct explanation of the assertion.
(c) (A) is true, but (R) is false.
(d) (A) is false, but (R) is true.
Answer:
(b) Both (A) and (R) are true, but (R) is not the correct explanation of the (A).
Explanation: The rays of different colours emerge along different paths from a glass slab as the refractive index of glass is different for different colours due to which different colours bend by different amounts on passing through the glass prism. The reason given, though correct, does not explain the splitting of light into its constituent colours.
Dispersion of White Light
The splitting of light into its component colours is called dispersion of light.
Spectrum
The band of the coloured components of a light beam is known as its spectrum.The sequence of colours is Violet, Indigo, Blue, Green, Yellow, Orange and Red and is given by the famous acronym – VIBGYOR
Cause of Dispersion
Dispersion occurs because light rays of different colours travel with different speeds in a refractive medium. The refractive index of glass is largest for violet colour and least for red colour due to which violet colour is dispersed the maximum and red colour the least.
Recombination of Spectrum Colours
If a second identical prism is placed in an inverted position with respect to the first prism such that all colours of the spectrum pass through the second prism, then a beam of white light emerges from the other side of the second prism.
This observation gave Newton the idea that the sunlight is made up of seven colours.
Formation of Rainbow
A rainbow is formed in the sky after a rain shower due to the dispersion of sunlight by the tiny droplets of water present in the atmosphere. It is always formed in a direction opposite to that of the Sun. The tiny droplets of water act like small prisms which refract and disperse the incident sunlight, then reflect the light internally and finally refract it again when it comes out of the raindrop. Different colours reach the observers eyes due to the dispersion of light and internal reflection.
