ICSE-X-Physics
06: Spectrum of Light
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- #Exercise 6 A
- #Section : A
- Qstn #1Name three factors on which the deviation produces by a prism depends and state how does it depend on the factors stated by you.Ans : 1 The deviation produced by the prism depends on the following four factors:
(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.
- Qstn #2How does the deviation produced by a triangular prism depend on the colours (or wavelengths) of light incident on it?Ans : 2 The deviation caused by a prism increases with the decrease in the wavelength of light incident on it.
- Qstn #3Which colour of white light is deviated by a glass prism the most and which the least?Ans : 3 A glass prism deviates the violet light most and the red light least.
- Qstn #4Define the term dispersion of light.Ans : 4 The phenomenon of splitting of white light by a prism into its constituent colours is known as dispersion of light.
- Qstn #5Explain the cause of dispersion of white light through a prism.Ans : 5 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.
- Qstn #6Explain briefly, with the help of a neat labelled diagram, how white light gets dispersed by a prism.Ans : 6 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).
- Qstn #7The diagram shown below in Fig. shows the path taken by a narrow beam of yellow monochromatic light passing through an equiangular glass prism. Now the yellow light is replaced by a narrow beam of white light incident at the same angle. Draw another diagram to show the passage of white light through the prism and label it to show the effect of prism on the white light.
Ans : 7
- Qstn #8How does the speed of light in glass change on increasing the wavelength of light?Ans : 8 Speed of light increases with increase in the wavelength.
- Qstn #9Which colour of white light travels (a) fastest (b) slowest, in glass?Ans : 9 Red colour travels fastest and Blue colour travels slowest in glass.
- Qstn #10Fig. shows a thin beam of white light from a source S striking on one face of a prism.
Ans : 10
- #10-aComplete the diagram to show effect of prism on the beam and to show what is seen on the screen.Ans : Constituent colours of white light are seen on the screen after dispersion through the prism.
- #10-bA 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?Ans : 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.
- #10-cWhat conclusion do you draw from the observation in part
(b) above?Ans : From the observation, we conclude that prism itself produces no colour.