What do you understand by the focus and focal length of a spherical mirror ? Show them on the separate diagrams for each of a concave mirror and a convex mirror.
Explain the following :
(a) A coin placed at the bottom of a vessel appears to be raised when water is poured in the vessel.
(b) A straight stick partly dipped in water obliquely, appears to be bent at the surface of water.
(c) The sun is seen before the sunrise and after the sunset.