$(i)$ Describe an activity to find the approximate value of the 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 mirror?
$(iii)$ 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 the incident ray. Why does it happen so? Explain with the help of a diagram.

(N/A) $(i)$ Activity: Hold a concave mirror in your hand and direct it towards a distant object (like a window or a tree). Direct the light reflected by the mirror onto 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 the focal length of the mirror.
$(ii)$ As an object is moved gradually away from a convex mirror,the size of the image becomes smaller and it shifts towards the focus.
$(iii)$ When a light ray passes through a rectangular glass slab,it undergoes refraction at two parallel surfaces,$PQ$ (air-glass interface) and $SR$ (glass-air interface). The extent of bending of the ray of light at the first surface is equal and opposite to the bending at the second surface. Thus,the emergent ray is parallel to the incident ray,but it is laterally displaced. This is shown in the diagram below.