Two convex lenses have focal lengths of 50 cm and 25 cm, respectively. If these two lenses are placed in contact, then the net power of this combination will be eal to

The power of a lens (P) is defined as the reciprocal of its focal length (f) when expressed in meters, given by the formula P = 1/f. For a convex lens, the focal length and power are positive. The first lens has a focal length of 50 cm (0.5 m), resulting in a power of P1 = 1/0.5 = +2 dioptres [1]. The second lens has a focal length of 25 cm (0.25 m), resulting in a power of P2 = 1/0.25 = +4 dioptres. When two thin lenses are placed in contact, the net power of the combination is the algebraic sum of their individual powers: P = P1 + P2. Therefore, the net power is +2 D + 4 D = +6 dioptres. This additive property allows for the calculation of effective power in multi-lens systems.

Sources

1. [1] Science , class X (NCERT 2025 ed.) > Chapter 9: Light – Reflection and Refraction > 9.3.8 Power of a Lens > p. 157