Find the focal length of a lens of power

Question.
Find the focal length of a lens of power $-2.0 \mathrm{D}$. What type of lens is this?

solution:

Given, power, $P=-2.0$ D

Power, $P=\frac{1}{f}$

or $f=\frac{1}{P}=\frac{1}{-2}=-0.5 \mathrm{~m}=-50 \mathrm{~cm}$

The lens is a concave (or diverging) lens.

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