Question

if the power of a lens is -2.5 D, the focal length of the lens and type is
(A)
+0.40 m and convex lens
(B) - 0.40 m and convex lens
(C) +0.40 m and concave lens
(D) -0.40 m and concave lens

Answer 1

Answer:

If the power of a lens is -2.5 D, the focal length of the lens and type is

(A)+0.40 m and convex lens

(B) - 0.40 m and convex lens ✓

(C) +0.40 m and concave lens

(D) -0.40 m and concave lens

Focal length (f) = 1/P . Hence , the given lens is a concave lens . So , option (b) is correct.

Answer 2

Answer:

Therefore the focal length is  -0.40 m and it is a concave lens.

Explanation:

The negative sign indicates that the lens is concave lens.

Power (P) = -2.5D

Focal length (f) = ?? remember, focal length is in meters.

$\sf{\longrightarrow}\:P=\dfrac{1}{f} \\\\\sf{\longrightarrow}\:2.5=\dfrac{1}{-f}\\\\\sf{\longrightarrow}\:-f=\dfrac{1}{2.5} \\\\\sf{\longrightarrow}\:f=-0.40$

Therefore the focal length is  -0.40 m and it is a concave lens.