A narrow beam of white light is passing through a glass prism as shown in the diagram. Study the diagram and answer any four questions from (a) to (e).
(a) The phenomenon observed in above set-up is
(i) scattering of light
(ii) dispersion of light
(iii)reflection of light
(iv) refraction of light
Answer
In a prism, white light disperses into 7 different colors.
So, the correct answer is (ii)
(b) In nature, this phenomenon is observed in
(i) Formation of rainbow
(ii) Twinkling of stars
(iii) Blue colour of sky
(iv) Advance sunrise
Answer
Let’s look at each of the options
-
(i) Formation of rainbow
- Rainbow is
caused
by
dispersion of sunlight
by
tiny water droplets
present in the atmosphere
.
Since it is caused by dispersion, this option is correct
-
(ii) Twinkling of stars
- Stars
twinkle due
to atmospheric refraction.
Since it is not caused by dispersion, this option is incorrect
-
(iii) Blue colour of sky
- Color of sky is blue due to
scattering of light
Since it is not caused by dispersion, this option is incorrect
- (iv) Advance sunrise - This is due to atmospheric refraction
So, the correct answer is (i)
(c) Which of the following statements is correct about constituents of white light based on the above observations?
(i) White light consists of seven colours.
(ii) Violet colour suffers minimum deviation.
(iii) Red light suffers maximum deviation.
(iv) All the colours of the white light move with different speed in vacuum.
Answer
Let’s look at all the options one by one
-
(i) White light consists of seven colours
- This is
true
. White light
disperses
into 7 colors when passing throw a prism.
-
(ii) Violet colour suffers minimum deviation
- This is
false
. Red light suffers least deviation, violet light suffers max deviation.
-
(iii) Red light suffers maximum deviation
- This is
false
. Red light suffers least deviation, violet light suffers max deviation.
- (iv) All the colours of the white light move with different speed in vacuum - All colors of light moves at the same speed in vaccum. So, this is also false.
So, the correct answer is (i)
(d) The cause of dispersion of light is
(i) All the colors of light travel with the speed more than the speed of light.
(ii) All the colors have different angles of deviation.
(iii) All the colors do not travel with the same speed of light.
(iv) All the colors have the same wavelength.
Ans.
Dispersion is caused because
- White light is composed of 7 colours . In vacuum, these 7 colours travel with the same speed (c=3 x 10 8 ).
- In any medium, due to refractive index, speed of light changes (Since n=c/c’).
- For white light, all 7 colors have different wavelengths , thus different refractive index, thus different speed of light .
Hence, we can say that all colors do not travel with the same speed of light.
So, the correct answer is (iii)
(e) Read the following statements carefully
(I) The prism behaves the same as that of a rectangular glass slab.
(II) All the colours have different angles of deviation in case of dispersion through prism.
(III) All the colours travel with the same speed of light in glass.
(IV) Dispersion of light is observed in the case of rectangular glass slabs.
Which of the above statement/s is true?
(i) Only II
(ii) I and II only
(iii)I, II and IV only
(iv) All of the above
Ans.
Let’s look at all the options one by one
(I) The prism behaves the same as that of a rectangular glass slab -
- A prism is made up of glass in which the faces are at a certain angle but in the case of rectangular glass slabs , the opposite faces are parallel to each other.
- As the light enters in the prism, all the colours travel with the different speed because glass is a denser medium as compared to air
- Dispersion of light is not observed in case of a rectangular glass slab .
So, both the prism and rectangular glass slab behave differently. Thus, this statement is
incorrect.
(II) All the colours have different angles of deviation in case of dispersion through prism -
- Angles of deviation depends on the frequency of light. Since all colors have different frequencies, thus, all colours have different angles of deviation .
Thus, this statement is correct
(III) All the colours travel with the same speed of light in glass
-
- Speed of light depends on the frequency of light. Since all colors have different frequencies, thus, all colours have different speeds.
Thus, this statement is
incorrect.
(IV) Dispersion of light is observed in the case of rectangular glass slabs -
- Dispersion is not observed in a rectangular glass slab, rather later displacement is observed.
Thus, this statement is incorrect.
Since only statement (II) is correct.
So, the correct answer is (i)
