Power of a lens is the reciprocal of ______.

The correct answer is P = 1/f.

Explanation:

Power of a lens

• The power of a lens is defined as the reciprocal of its focal length. 
• It is represented by the letter P. 
• The power (P) of a lens of focal length (f) is given by \(P=\frac{1}{f}\)P=1fP=1f
• The SI unit of power of a lens is ‘dioptre’. It is denoted by the letter D.
• f is expressed in metres, and power is expressed in dioptres.
• Thus, 1 dioptre is the power of a lens whose focal length is 1 metre i.e., 1 D = 1 m–1.

Additional InformationConcave Lens

• A concave lens is known as a diverging lens. 
• The rays of light passing through the concave lens appears to be coming from a point known as focus. 

Convex lens

• The convex lens is a converging lens that converges the ray of light to point. 
• The ray of light converges at a point after refraction which is called the focus. 

Sign convention

• Sign convention for a convex lens is as shown. It is also applicable for a concave lens in terms of sides.