# Describe an activity to find the approximate value of focal length of a concave mirror

(i) Describe an activity to find the approximate value of focal length of a concave mirror.
(ii) What happens to the size of the image of an object when it is moved gradually away from a convex minor?
(iii) In an experiment to study refraction
through a glass slab, it is observed that a ray of light undergoing refraction emerges parallel to the direction of incident ray. Why does it happen so? Explain with the help of a diagram.

(i) Activity : Hold a concave mirror in your hand and direct it towards the screen. Direct the light reflected by the mirror on to a sheet of paper held close to the mirror. Move the sheet back and forth slowly until a bright, sharp spot of light is seen on the paper. The distance of this image from the position of the mirror gives the approximate value of focal length of the mirror.

(iii) Size of the image becomes smaller.
(iii) The extent of bending of the ray of light at the opposite parallel faces PQ (air - glass interface) and SR (glass -air interface) of the rectangular glass slab is equal and opposite.