State the condition at which total internal reflection occurs.

16 A beam of red light is allowed to fall in turn on 
(a) a black cloth 
(b) red cloth 
(c) white cloth 
(d) blue cloth. Describe and explain the appearance in each case. 17 An object is placed between the pole and focus of a concave mirror. What is the nature and size of image? 19 A stamp collector uses a lens (convex) of focal length 7 cm, to examine a stamp. What is the power of the lens? 20 What is critical angle? Draw a ray diagram showing light incident at the critical angle.