Question

Focal lengths of both a spherical mirror and a lens are -25 cm. Most probably the mirror and lens are:

• A. mirror convex and lens concave
• B. mirror concave and lens convex
• C. both convex
• D. both concave

Hint:

Concave mirrors and concave lenses have negative focal lengths, while convex mirrors and convex lenses have positive focal lengths.

Answer 1

The Correct Option is D

In optics, the focal length (\(f\)) of a mirror or lens determines how it focuses light. The sign of the focal length indicates the type of mirror or lens: - For concave mirrors and concave lenses, the focal length is negative. - For convex mirrors and convex lenses, the focal length is positive.
Step 1: Focal length of the mirror.
Given that the focal length of the spherical mirror is -25 cm, this indicates that it is a concave mirror (since concave mirrors have negative focal lengths).
Step 2: Focal length of the lens.
Similarly, the focal length of the lens is also -25 cm, which indicates that the lens is concave (since concave lenses have negative focal lengths).
Step 3: Conclusion.
Therefore, both the mirror and the lens are concave. The correct answer is (D) both concave.