Selina Textbook Solutions: Spectrum | Physics Class 10 ICSE PDF Download

 Page 1


 
 
Class X Chapter 6 – Spectrum of Light Physics  
 
 
EXERCISE- 6 (A) 
 
Question 1:  
Name three factors on which the deviation produces by a prism depends and state how does it 
depend on the factors stated by you. 
Solution 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. 
 
 
Question 2: 
How does the deviation produced by a triangular prism depend on the colours (or wavelengths) 
of light incident on it? 
Solution 2: 
The deviation caused by a prism increases with the decrease in the wavelength of light incident 
on it.  
 
 
Question 3: 
Which colour of white light is deviated by a glass prism the most and which the least? 
Solution 3: 
A glass prism deviates the violet light most and the red light least. 
 
 
 
 
 
 
Page 2


 
 
Class X Chapter 6 – Spectrum of Light Physics  
 
 
EXERCISE- 6 (A) 
 
Question 1:  
Name three factors on which the deviation produces by a prism depends and state how does it 
depend on the factors stated by you. 
Solution 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. 
 
 
Question 2: 
How does the deviation produced by a triangular prism depend on the colours (or wavelengths) 
of light incident on it? 
Solution 2: 
The deviation caused by a prism increases with the decrease in the wavelength of light incident 
on it.  
 
 
Question 3: 
Which colour of white light is deviated by a glass prism the most and which the least? 
Solution 3: 
A glass prism deviates the violet light most and the red light least. 
 
 
 
 
 
 
 
 
Class X Chapter 6 – Spectrum of Light Physics  
 
 
 
 
Question 4: 
Define the term dispersion of light. 
Solution 4: 
The phenomenon of splitting of white light by a prism into its constituent colours is known as 
dispersion of light. 
 
 
Question 5: 
Explain the cause of dispersion of white light through a prism. 
Solution 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. 
 
 
Question 6: 
Explain briefly, with the help of a neat labelled diagram, how white light gets dispersed by a 
prism. 
Solution 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). 
 
 
Page 3


 
 
Class X Chapter 6 – Spectrum of Light Physics  
 
 
EXERCISE- 6 (A) 
 
Question 1:  
Name three factors on which the deviation produces by a prism depends and state how does it 
depend on the factors stated by you. 
Solution 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. 
 
 
Question 2: 
How does the deviation produced by a triangular prism depend on the colours (or wavelengths) 
of light incident on it? 
Solution 2: 
The deviation caused by a prism increases with the decrease in the wavelength of light incident 
on it.  
 
 
Question 3: 
Which colour of white light is deviated by a glass prism the most and which the least? 
Solution 3: 
A glass prism deviates the violet light most and the red light least. 
 
 
 
 
 
 
 
 
Class X Chapter 6 – Spectrum of Light Physics  
 
 
 
 
Question 4: 
Define the term dispersion of light. 
Solution 4: 
The phenomenon of splitting of white light by a prism into its constituent colours is known as 
dispersion of light. 
 
 
Question 5: 
Explain the cause of dispersion of white light through a prism. 
Solution 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. 
 
 
Question 6: 
Explain briefly, with the help of a neat labelled diagram, how white light gets dispersed by a 
prism. 
Solution 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). 
 
 
 
 
Class X Chapter 6 – Spectrum of Light Physics  
 
 
 
 
Question 7: 
The diagram shown below in Fig. 6.11 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. 
 
Solution 7: 
 
 
 
Question 8: 
How does the speed of light in glass change on increasing the wavelength of light? 
Solution 8: 
Speed of light increases with increase in the wavelength. 
 
 
Question 9: 
Which colour of white light travels (a) fastest (b) slowest, in glass? 
Solution 9: 
Red colour travels fastest and Blue colour travels slowest in glass. 
 
 
Page 4


 
 
Class X Chapter 6 – Spectrum of Light Physics  
 
 
EXERCISE- 6 (A) 
 
Question 1:  
Name three factors on which the deviation produces by a prism depends and state how does it 
depend on the factors stated by you. 
Solution 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. 
 
 
Question 2: 
How does the deviation produced by a triangular prism depend on the colours (or wavelengths) 
of light incident on it? 
Solution 2: 
The deviation caused by a prism increases with the decrease in the wavelength of light incident 
on it.  
 
 
Question 3: 
Which colour of white light is deviated by a glass prism the most and which the least? 
Solution 3: 
A glass prism deviates the violet light most and the red light least. 
 
 
 
 
 
 
 
 
Class X Chapter 6 – Spectrum of Light Physics  
 
 
 
 
Question 4: 
Define the term dispersion of light. 
Solution 4: 
The phenomenon of splitting of white light by a prism into its constituent colours is known as 
dispersion of light. 
 
 
Question 5: 
Explain the cause of dispersion of white light through a prism. 
Solution 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. 
 
 
Question 6: 
Explain briefly, with the help of a neat labelled diagram, how white light gets dispersed by a 
prism. 
Solution 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). 
 
 
 
 
Class X Chapter 6 – Spectrum of Light Physics  
 
 
 
 
Question 7: 
The diagram shown below in Fig. 6.11 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. 
 
Solution 7: 
 
 
 
Question 8: 
How does the speed of light in glass change on increasing the wavelength of light? 
Solution 8: 
Speed of light increases with increase in the wavelength. 
 
 
Question 9: 
Which colour of white light travels (a) fastest (b) slowest, in glass? 
Solution 9: 
Red colour travels fastest and Blue colour travels slowest in glass. 
 
 
 
 
Class X Chapter 6 – Spectrum of Light Physics  
 
 
 
 
Question 10: 
Fig 6.12 shows a thin beam of white light from a source S striking on one face of a prism. 
 
(a) Complete the diagram to show effect of prism on the beam and to show what 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? 
Solution 10: 
(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. 
 
 
Question 11: 
(a) 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? 
(b) If white light is used in same way as in part (a) above, what change is expected in the 
emergent beam? 
(c) what conclusion do you draw about the nature of white light in part (b)? 
Solution 11: 
(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. 
Page 5


 
 
Class X Chapter 6 – Spectrum of Light Physics  
 
 
EXERCISE- 6 (A) 
 
Question 1:  
Name three factors on which the deviation produces by a prism depends and state how does it 
depend on the factors stated by you. 
Solution 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. 
 
 
Question 2: 
How does the deviation produced by a triangular prism depend on the colours (or wavelengths) 
of light incident on it? 
Solution 2: 
The deviation caused by a prism increases with the decrease in the wavelength of light incident 
on it.  
 
 
Question 3: 
Which colour of white light is deviated by a glass prism the most and which the least? 
Solution 3: 
A glass prism deviates the violet light most and the red light least. 
 
 
 
 
 
 
 
 
Class X Chapter 6 – Spectrum of Light Physics  
 
 
 
 
Question 4: 
Define the term dispersion of light. 
Solution 4: 
The phenomenon of splitting of white light by a prism into its constituent colours is known as 
dispersion of light. 
 
 
Question 5: 
Explain the cause of dispersion of white light through a prism. 
Solution 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. 
 
 
Question 6: 
Explain briefly, with the help of a neat labelled diagram, how white light gets dispersed by a 
prism. 
Solution 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). 
 
 
 
 
Class X Chapter 6 – Spectrum of Light Physics  
 
 
 
 
Question 7: 
The diagram shown below in Fig. 6.11 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. 
 
Solution 7: 
 
 
 
Question 8: 
How does the speed of light in glass change on increasing the wavelength of light? 
Solution 8: 
Speed of light increases with increase in the wavelength. 
 
 
Question 9: 
Which colour of white light travels (a) fastest (b) slowest, in glass? 
Solution 9: 
Red colour travels fastest and Blue colour travels slowest in glass. 
 
 
 
 
Class X Chapter 6 – Spectrum of Light Physics  
 
 
 
 
Question 10: 
Fig 6.12 shows a thin beam of white light from a source S striking on one face of a prism. 
 
(a) Complete the diagram to show effect of prism on the beam and to show what 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? 
Solution 10: 
(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. 
 
 
Question 11: 
(a) 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? 
(b) If white light is used in same way as in part (a) above, what change is expected in the 
emergent beam? 
(c) what conclusion do you draw about the nature of white light in part (b)? 
Solution 11: 
(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. 
 
 
Class X Chapter 6 – Spectrum of Light Physics  
 
 
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(b) White light splits into its constituent colours i.e., spectrum is formed. 
(c) We conclude that white light is polychromatic. 
 
 
Question 12: 
What do you understand by the term spectrum? 
Solution 12: 
The colour band obtained on a screen on passing white light through a prism is called the 
spectrum. 
 
 
Question 13: 
A ray of white light is passed through a glass prism and spectrum is obtained on a screen. 
(a) Name the seven colours of the spectrum in order. 
(b) Dow the colours have the same width in the spectrum? 
(c) which of the colour of the spectrum of white light deviates (i) the most, (ii) the least? 
Solution 13: 
(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. 
 
 
Question 14: 
The wavelengths for the light of red and blue colours are roughly 7.8 × 10
-7
 m and 4.8 × 10
-7
 m 
respectively. 
(a) Which colour has the greater speed in vacuum? 
(b) Which colour has the greater speed in vacuum? 
Solution 14: 
(a) In vacuum, both have the same speeds. 
(b) In glass, red light has a greater speed.