Question

A spherical mirror and a thin lens have each a focal length of –20 cm.

• A. Both are concave
• B. Both are convex
• C. The mirror is concave and the lens is convex
• D. The mirror is convex and the lens is concave

Answer 1

The correct option is A

Both are concave

1. The focal length is the distance from the pole to the focus.
2. Focus is the point at which the incident parallel rays meet after reflection or refraction.
3. The rays may meet actually or virtually.
4. Depending on this, the focal length may be positive or negative.

Optical instruments	focal length sign
concave mirror	negative
concave lens	negative
convex mirror	positive
convex lens	positive

As we can see in the table, only concave mirror and concave lens have negative focal length

Therefore, we can say that both lens and mirror whose focal length is -20 cm are concave.

Hence, the correct option is (A).