Question

Two lenses with focal lengths -25 cm and +20 cm. Determine the nature and total power of the lens combination.

A) Convex-concave, total power = -0.05 D
B) Concave-convex, total power = 0.05 D
C) Convex-convex, total power = 0.05 D
D) Concave-concave, total power = -0.05 D

Answer 1

The two lenses represent a concave-convex lens combination with a total power of 1 D.

Step-by-step explanation:

The two lenses, with focal lengths -25 cm and +20 cm, represent a combination of a concave lens and a convex lens. The negative focal length of the first lens indicates that it is a diverging lens or a concave lens, while the positive focal length of the second lens indicates that it is a converging lens or a convex lens. The lens combination can be classified as a concave-convex lens combination.

The total power of the lens combination can be calculated by simply adding the powers of the individual lenses. The formula to calculate the power of a lens is P = 1/f, where P is the power of the lens in diopters and f is the focal length of the lens in meters. Therefore, the total power of the concave-convex lens combination is (-1/0.25) + (1/0.2) = -4 + 5 = 1 diopter. So, the nature and total power of the lens combination is concave-convex and the total power is 1 D.