Question

a lens has a power of -2.5D what is it's focal length
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Answer 1

-2.5. .........0.........

Answer 2

Here,

Power (P) = -2.5D

Focal length (f) = ?

$Now, \: P = \frac{1}{f} \\ \\ = > f = \frac{1}{P} \\ \\ = \frac{1}{ - 2.5} \\ \\ = - 0.4 \:m$