Question

what is the focal length of a convex lens having power +0.5D?

Answer 1

Heya
Here is your answer 
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We know that power is equal to $\frac{1}{f}$

So focal length = $\frac{1}{p}$

that is $\frac{1}{0.5}$

but 0.5 is equal to $\frac{1}{2}$

So focal length is = $\frac{1}{1/2}$

that is 1 * 2 = 2

So the focal length is  2 ____________________________________________________________

Hope it helps! ^_^