1.

A thin lens has a focal length of `- 25 cm`. What is the power of the lens? It is convex or concave ?

Answer» Correct Answer - `- 4 D` ; concave
Here, `f = - 25 cm, P = ?`
From `P = 100/f = 100/(-25) = -4 D`. Negative sign shows that the lens is concave.


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