Exercise 6-A

Q.1. Name three factors on which the deviation produced by a prism depends and state how does it depend on the factors stated by you.
Ans.
The deviation produced by the prism depends on the following four factors: you can write any three of below.

(a)The angle of incidence – As the angle of incidence increases, first the angle of deviation decreases and reaches to a minimum value for a certain angle of incidence. By further increasing the angle of incidence, the angle of deviation is found to increase.

(b)The material of prism (i.e., on refractive index) – For a given angle of incidence, the prism with a higher refractive index produces a greater deviation than the prism which has a lower refractive index.

(c)Angle of prism- Angle of deviation increases with the increase in the angle of prism.

(d)The colour or wavelength of light used- Angle of deviation increases with the decrease in wavelength of light.

Q.2. How does the deviation produced by a triangular prism depend on the colours or wave lengths of light incident on it?
Ans.
The deviation caused by a prism increases with the decrease in the wavelength of light incident on it.

Q.3. How does the speed of light in glass change on increasing the wave length of light.
Ans.
Speed of light increases with increase in the wavelength.

Q.4. Which colour of white light travels (a) fastest (b) slowest in glass ?
Ans. Red colour travels fastest and Blue colour travels slowest in glass.

Q.5. Name the subjective property of light related to its wavelength.
Ans.Colour of light is related to its wavelength.

Q.6. What is the range of wavelength of the spectrum of white light in (i) Å, (ii) nm?
Ans.(i) 4000 Å to 8000 Å
(ii) 400 nm to 800 nm

Q.7. Write the approsimate wavelengths for (i) blue, and (ii) red light.
Ans. (i) 4800 Å (ii) 8000 Å

Q.8. Write the seven prominent colours present in white light in the order of increasing wavelength.
Ans. Seven prominent colours of the white light spectrum in order of their increasing wavelength:
violet, indign, blue, green, yellow, orange, and red.

Q.9. Name the seven prominent colours of the white light spectrum in order of their increasing frequencies.
Ans. Seven prominent colours of the white light spectrum in order of their increasing frequencies:
red, orange, yellow, green, blue, indigo and violet.

Q.10. Name four colors of the spectrum of white light which have wave length longer than blue light.
Ans. green, yellow, orange and red.

Q.11. Which colour of the white light is deviated by a glass prism (i) the most, and (ii) the least ?
Ans. A glass prism deviates the violet light most and the red light least.

Q.12. The wavelengths for the light of red and blue colours are nearly 7.8 X 10-7 m and 4.8 X 10-7 m respectively.
(a) which colour has the greater speed in vacuum?
(b) Which colour has the greater speed in glass?

Ans. (a)In vacuum, both have the same speeds.
(b)In glass, red light has a greater speed.

Q.13. Define the term dispersion of light.
Ans. The phenomenon of splitting of white light by a prism into its constituent colours is known as dispersion of light.

Q.14. Explain the cause of dispersion of white light through a prism.
Ans. When white light is incident on the first surface of a prism and enters in glass, light of different colours due to different speeds in glass, is refracted or deviated through different angles. Thus the dispersion of white light into its constituent colours takes place at the first surface of prism. Thus the cause of dispersion is the change in speed of light with wavelength or frequency.

Q.15. Explain briefly with the help of a neat labelled diagram, how does white light get dispersed by a prism.
Ans. When white light is incident on the first surface of a prism and enters in glass, light of different colours due to different speeds in glass, is refracted or deviated through different angles. Thus the dispersion of white light into its constituent colours takes place at the first surface of prism.

On the second surface, only refraction takes place and different colours are deviated through different angles. As a result, the colours get further separated on refraction at the second surface (violet being deviated the most and red the least).

Q.16. What do you understand by the term spectrum?
Ans. The colour band obtained on a screen on passing white light through a prism is called the spectrum.

Q.17.
Ans. (a)Violet, Indigo, Blue, Green, Yellow, Orange, Red.
(b)No, different colours have different widths in the spectrum.
(c)(i) Violet colour is deviated the most. (ii) Red colour is deviated the least.

Q.18.

Q.19.
Ans. (a)Constituent colours of white light are seen on the screen after dispersion through the prism.

(b)When a slit is introduced in between the prism and screen to pass only the light of green colour, only green light is observed on the screen.
(c)From the observation, we conclude that prism itself produces no colour.

Q.20.
Ans.
(a)If a monochromatic beam of light undergoes minimum deviation through an equi-angular prism, then the beam passes parallel to the base of prism.
(b)White light splits into its constituent colours i.e., spectrum is formed.
(c)We conclude that white light is polychromatic.

Numerals
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