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Q: A spherical mirror and a thin spherical lens each have a focal length of -20 cm. Which of the following is likely to be true in such a scenario?
  • A. Both are convex.
  • B. The mirror is concave and the lens is convex.
  • C. The mirror is convex and the lens is concave.
  • D. Both are concave.
Correct Answer: Option D - The inverse of the lens's focal length is called the power of the lens if the focal length of a lens is f m., Then its power is P = 1 / f diopter. While a spherical mirror and a thin spherical lens, each having a focal length of −20 cm, both the mirror and the lens will be concave. The power of a convex lens is positive and the power of a concave lens is negative.
D. The inverse of the lens's focal length is called the power of the lens if the focal length of a lens is f m., Then its power is P = 1 / f diopter. While a spherical mirror and a thin spherical lens, each having a focal length of −20 cm, both the mirror and the lens will be concave. The power of a convex lens is positive and the power of a concave lens is negative.

Explanations:

The inverse of the lens's focal length is called the power of the lens if the focal length of a lens is f m., Then its power is P = 1 / f diopter. While a spherical mirror and a thin spherical lens, each having a focal length of −20 cm, both the mirror and the lens will be concave. The power of a convex lens is positive and the power of a concave lens is negative.