High School

The power of a lens is -4 D. Find the focal length and type of the lens.

A. 25 cm, convex lens
B. 25 cm, concave lens
C. 40 cm, concave lens
D. 40 cm, convex lens

Answer :

Final answer:

The lens with a power of -4 D has a focal length of -25 cm, indicating that it is a concave lens, hence the answer B. 25 cm, concave lens is correct.

Explanation:

The power of a lens (P) is given as the inverse of the focal length (f), which is measured in meters. The formula for power is P = 1/f. Given the power of a lens is -4 D (diopters), we can find the focal length by rearranging the formula: f = 1/P.

So the focal length, f = 1/(-4 D) = -0.25 m or -25 cm. The negative sign indicates that the lens is of concave type, which diverges light rays. Therefore, the correct answer is B. 25 cm, concave lens.