A spherical mirror and a thin spherical lens have each a focal length of -15 cm. The mirror and the lens are likely to be (A) both concave (B) both convex (C) the mirror is concave and the lens is convex (D) the mirror is convex, but the lens is concave.

Answer: (A) both concave 

The distance from the lens to the focal point is called the focal length. For converging lenses, the focal length is always positive, while diverging lenses always have negative focal lengths. Here the answer is both concave. Both the concave mirror and concave lens have a negative focal length.

Concave mirror

If a hollow sphere is cut into parts and the outer surface of the cut part is painted, then it becomes a mirror with its inner surface as the reflecting surface. This type of mirror is known as a concave mirror.

Concave lens

A concave lens is a lens that diverges a straight light beam from the source to a diminished, upright, virtual image.

 

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