Question

calculate the focal length of a lens if it's power is -2.5D​

Answer 1

Formula : Power = 1 / focal length . So the focal length is 20 cm. The lens has to be a concave lens since the focal length is negative

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Answer 2

Explanation:

p=1/f

2.5=1/f

f=1/2.5

f=0.4

-ve indicates lens is concave