(a) What is meant by 'power of a lens' ? (b) State and define the SI unit of power of a lens. (c) A convex lens of focal length 25 cm and a concave lens of focal length 10 cm are placed in close contact with each other. Calculate the lens power of this combination.

(a) What is meant by 'power of a lens' ? (b) State and define the SI

1.	(a) What is meant by 'power of a lens' ? (b) State and define the SI unit of power of a lens. (c) A convex lens of focal length 25 cm and a concave lens of focal length 10 cm are placed in close contact with each other. Calculate the lens power of this combination.
Answer» Solution :(a) and (b) - For power of lens and its SI UNIT, see Point Numbers 42 under the heading "Chapter At A GLANCE". (C) Focal length of convex lens `f_(1) = + 25 cm = + 0.25 cm` `therefore` Power of convex lens `P_(1) = (1)/(f_(1) "in m") = (1)/(0.25 m) = + 4.0 D` Again focal length of concave lens `f_(2) = - 10 cm = - 0.1 m` `therefore` Power of concave lens `P_(2) = (1)/(f_(2) "in m") = (1)/((-0.10)m) = -10.0 D` `therefore` Power of the lens combination `P = P_(1) + P_(2) = + 4.0 - 10.0 = - 6.0 D` The -ve sign signifies that the lens combination behaves as a concave (diverging) lens.