Question

A lens which forms a real image has a focal length 8 cm. Calculate its power.

Answer 1

The lens forms a real image, therefore it is a convex lens.

Focal length of the lens (f) = +8 cm [f for convex lens is +ive]

\[ \text{Applying, } \mathrm{P} = \frac{100}{f \text{(in cm)}} \]

\[ \Rightarrow \mathrm{P} = \frac{100}{8} \]

\[ \Rightarrow \mathrm{P} = + \ 12.5 \ \text{D} \]

Thus, power is +12.5 D. The positive sign shows that lens is convex.