Question

A beam of white light falling on a glass prism gets split up into seven colours marked 1 to 7 as shown in the diagram. A student makes the following statements about the spectrum observed on the screen.
(a) The colours at positions marked 3 and 5 are similar to the colour of the sky and the colour of gold metal respectively. Is the above statement made by the student correct or incorrect? Justify.
(b) Which two positions correspond closely to the colour of (i) brinjal. (i) 'danger' or stop signal lights?

Answer 1

Hint:When light passes through a prism it splits into seven colours. Position of these colours depends on the frequency or the wavelength of the light in the medium.

Complete answer:
We have given here a glass prism which splits light into seven colours marked from 1 to 7 as shown in the diagram. Now, we know that a prism splits light into seven colours and the positions of these lights depend on the wavelength of the light or the frequency of the light.If the wavelength is higher the light bends less passing through the prism and if the wavelength is lower the light bends more.

Now, passing through a prism light comes in order of ‘V I B G Y O R’ starting from the end.Where V is the violet, I is the indigo, B is the blue , G is the green, Y is the yellow, O is the orange and R is the red. So, here the number 7 colour will be Violet and the number 1 will be Red.

(a) number 3 colour is the yellow which is not the colour of the sky and the number 5 colour is the blue which is not the colour of the gold. So, the student is stating the colour in the opposite order.

(b) (i) the colour of brinjal is in the domain of the colour violet so it corresponds to the number 7 position.
(ii) the danger or stop signal colour is red so the number 1 colour is the colour of danger or stop signal.

Note:The scattering of light is directly related to its wavelength. According to Cauchy’s law the intensity of scattered light is inversely proportional to the square of the wavelength of the light. Since, violet has the highest wavelength so it bends more after refracting from the prism and the red light bends less due to its higher wavelength.