Light: Reflection and Refraction
NCERT Exemplar Problems
Short Answer Part 1
Question 20: Identify the device used as a spherical mirror or lens in following cases, when the image formed is virtual and erect in each case.
(a) Object is placed between device and its focus, image formed is enlarged and behind it.
Answer: Concave mirror
(b) Object is placed between the focus and device, image formed is enlarged and on the same side as that of the object.
Answer: Convex lens
(c) Object is placed between infinity and device, image formed is diminished and between focus and optical centre on the same side as that of the object.
Answer: Concave lens
(d) Object is placed between infinity and device, image formed is diminished and between pole and focus, behind it.
Answer: Convex mirror
Question 21: Why does a light ray incident on a rectangular glass slab immersed in any medium emerges parallel to itself? Explain using a diagram.
Answer: Let us assume the medium in which glass slab is immersed is rarer than glass; because it is not possible to immerse a glass slab in a denser medium. When the ray of light travels from a rarer medium to a denser medium it bends towards the normal. When the ray of light emerges in the first medium (rarer) it beds away from the normal. In this case, the angle of incidence and angle of emergence are equal. This results in emergent ray being parallel to the incident ray.
Question 22: A pencil when dipped in water in a glass tumbler appears to be bent at the interface of air and water. Will the pencil appear to be bent to the same extent, if instead of water we use liquids like, kerosene or turpentine. Support your answer with reason.
Answer: The pencil appears to be bent because of refraction of light. We know, that refraction happens because of different refractive indices of the media involved. In case of liquids; like kerosene or turpentine also, refraction shall take place. The degree of bending would depend on comparative refractive indices of the two media through which the light has to travel.
Question 23: How is the refractive index of a medium related to the speed of light? Obtain an expression for refractive index of a medium with respect to another in terms of speed of light in these two media?
Answer: Lets us take two media 1 and 2 and let us assume that the speeds of light are respectively v1 and v2 in them. Then the refractive index of the second medium with respect to the first medium can be given by the following equation:
n21 = Speed of Light in 1st Medium ÷ Speed of Light in 2nd Medium
`=(v_1)/(v_2)`
Here; n21 is the refractive index of the second medium with respect to the first medium.
Question 24: Refractive index of diamond with respect to glass is 1.6 and absolute refractive index of glass is 1.5. Find out the absolute refractive index of diamond.
Answer: Absolute refractive index of glass = 1.5
Refractive index of diamond with respect to glass = 1.6
This means that glass is 1.5 times optically denser than vacuum and diamond is 1.6 times optically denser than glass.
So, absolute refractive index of diamond `=1.6×1.5=2.4`
Note: All figures on this page are taken from NCERT Exemplar Problems Class 10 Science Book