Q: COMPREHENSION
A block with a concave mirror of radius of curvature 1 m attached to one of its sides floats, with exactly half of its length immersed in water and the other half exposed to air.

Any ray originating from an object O1 and O2 (as shown in figure) floating on the surface of water first gets reflected by the mirror. This then gets refracted by the water surface if the image formed by reflection is real (I1). If the image is virtual (I2), then the reflected ray never encounters the air-water interface and hence there is no refraction. The image for the next three questions refers to the final image, formed after both the reflection and refraction (if it occurs at all) has taken place.
1: The final image formed is unique (i.e. only one image is formed) if the point object floats on the surface of
water at a distance x in front of the mirror where
(A) x is less than 50 cm
(B) x is less than 1m
(C) x is between 50 cm and 1m
(D) for any value of x
2: There is no refraction of light rays reflected from the mirror if the point object floats on the surface of water at
a distance x in front of the mirror where
(A) x is less than 50 cm
(B) x is less than 1 m
(C) x is between 50 cm and 1 m
(D) for any value of x
3: The final image is not unique (i.e. more than one image for the same object) if the point object is placed a distance ëyí above the surface of water and a distance x in front of the mirror (rays are not paraxial), where y = 20 cm and x is :
(A) less than 50 cm
(B) less than 1 m
(C) between 50 cm and 1 m
(D) for any value of
Click to See Solution :
Ans: 1: (D) ; 2:(A) ; 3: (D)
Sol: 1: There is onlyone point image corresponding to a point object, as long as the object lies on the water surface
(principal axis of the mirror).Any object lying at some distance from the princpal axis results in multiple
image points.
2. If light rays diverge outward (forming a virtual image behind the mirror) after reflection, there is no refraction
at water surface after reflection. This is the case when the object lies between the focus and the