# Frank Solutions for Chapter 2.3 Dispersion through a Prism and Electromagnetic Spectrum Class 10 Physics ICSE

## Exercises

1. What do you mean by dispersion?
The phenomenon of splitting of white light into its constituent colours is known as dispersion.

2. Name the radiations of wavelength
(i) Longer than 8× 10-7 m.
(ii) Shorter than 4× 10-7 m.

3. What is the velocity of electromagnetic wave?
3×108 m/s is the velocity of electromagnetic radiation.

4. How does the deviation produced by a triangular prism depend on the colours (or wavelengths) of light incident on it? Which colour deviates the most and which the least?
Light of different colours have different speeds in a medium.
Therefore, the refractive index of glass (the material of prism) is different for different colours of light and the deviation caused by a prism is different for different colours of light.
Violet is deviated the most because in glass, speed of violet is least.
Red is deviated the least because in glass, speed of red is most.

5. What do you understand by the term spectrum?
The colour band obtained on a screen on passing white light through a prism is called the spectrum.

6. Fig. 10 shows a thin beam of white light from a source S striking on one face of a prism.

(a) Complete the diagram to show the effect of prism on he beam and to show that is seen on the screen.
(b) A slit is placed in between the prism and the screen to pass only the light of green colour. What will you then observe on the screen?
(c) What conclusion do you draw from the observation in part (b) above ?

We will observe the light of only green colour on the screen.
(c) We draw the conclusion that a prism by itself produces no colours

7. (i) If a monochromatic beam of light undergoes minimum deviation through an equiangular prism, how does the beam pass through the prism, with respect to its base?
(ii) If white light is used in same way as in (i) above, what change is expected in the emergent beam?
(i) It passes parallel with respect to the base of the prism.
(ii) The white light shall split into its constituent colours and a spectrum shall be formed.

8. Express nanometer (nm) in terms of Angstrom
1 nm = 10 Ã…

9. What do you mean by spectrum?
The colour band obtained on a screen on passing white light through a prism is called the spectrum.

10. What is the cause of dispersion?
Dispersion occurs because light of different colour bend through different angles while passing through a glass prism.

11. Briefly, describe an experiment to show that a prism itself produces no colour.
A Prism itself produces no colour. This can be demonstrated by the following experiment.
Experiment : In figure below, white light from a slit S is made to pass through a prism P which forms the spectrum VR on a white screen AB. A narrrow slit H is made on the screen AB parallel to the slit S to allow the light of a particular colour to pass through it. This light of a particular colour is made to fall on a second prism Q placed with its base in opposite direction to that of the prism P. The light after passing trough the second prism Q is received on another white screen M.

It is observed that the colour of light obtained on the screen M is same as that of the light incident on the second prism Q through the slit H. If green light is incident on the prism Q, the screen M has only green colour. This proves that the prism itself produces no colours.

12. (i) Draw a diagram to show the splitting of white light into its constituent colours.
(ii) Draw another diagram to show how the colours of spectrum of white light can be combined to give the effect of white light.

13. Complete the ray diagram given below to show the nature of light produced on the screen.
The completed ray diagram is as shown below :
The completed ray diagram is as shown below :

14. Name the scientist who discovered
(i) X-rays
(ii) Visible light
(iii) Microwaves
(v) Infra-red waves
(vi) Ultraviolet rays
(i) Roentgen
(ii) Newton
(iii) Hertz
(iv) Marconi
(v) Hershell
(vi) Ritter

15. The wavelengths for the light of red and blue colours are roughly 7 X 10-7 m and 4 X 10-7 m respectively. Which colour has the greater speed in vacuum? Which colour has the greater speed in glass?