a) What is meant by the 'power of a lens'?
b) State and define the S.I. 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) The ability of a lens to converge or diverge a beam of the light ray is known as the Power (P) of the lens.
Power is equal to the reciprocal of the focal length.
That is, P = 
b) SI unit of the power of a lens is Dioptre.
1 diopter =  = 1 m^-1
1 m is the focal length of the lens.
c) Given,
Focal length, f = +25 cm
                   = 
 
Focal length, f₂ = -10 cm = 
Power, P₂ = 
Power of the combination of lens, P = P₁ + P₂
P = 4 D - 10 D = - 6 D