Two thin lenses are of same focal lengths $$(f)$$, but one is convex and the other one is concave. When they are placed in contact with each other, the equivalent focal length of the combination will be :

During a cloudy day, a primary and a secondary rainbow may be created, then the :

An astronomical refracting telescope is being used by an observer to observe planets in normal adjustment. The focal lengths of the objective and eye piece used in the construction of the telescope are 20 m and 2 cm respectively. Consider the following statements about the telescope :

(a) The distance between the objective and eye piece is 20.02 m

(b) The magnification of the telescope is ($$-$$) 1000

(c) The image of the planet is erect and diminished

(d) The aperture of eye piece is smaller than that of objective

The correct statements are :

A biconvex lens has radii of curvature, 20 cm each. If the refractive index of the material of the lens is 1.5, the power of the lens is