Two identical glass ($$\mu $$_g = 3/2) equiconvex lenses of focal length $$f$$ each are kept in contact. The space between the two lenses is filled with water $$\left( {{\mu _w} = 4/3} \right)$$. The focal length of the combination is

A person can see clearly objects only when they lie between 50 cm and 400 cm from his eyes . In order to increase the maximum distance of distinct vision to infinity, the type and power of the correcting lens, the person has to use. will be

Match the corresponding entries of column 1 with column 2. [Where m is the magnification produced by the mirror]

Column 1			Column 2
(A)	m = - 2		(p)	Convex mirror
(B)	m = -$${1 \over 2}$$		(q)	Concave mirror
(C)	m = +2		(r)	Real image
(D)	m = +$${1 \over 2}$$		(s)	Virtual image

A astronomical telescope has objective and eyepiece of focal lengths 40 cm and 4 cm respectively. To view an object 200 cm away from the objective, the lenses must be separated by a distance