Question

The power of a lens in dioptres is given by:

Options

• \[P = \frac{1}{\text{focal length (in metre)}}\]

• \[P = \frac{1}{f^2}\]

• \[P = 2f\]

• \[P = \frac{1}{\text{focal length (in centimetre)}}\]

Answer 1

The power of a lens is \[P = \frac{1}{f(m)}\], where the focal length must be expressed in metres so that the power comes out in dioptres. Using centimetres directly gives an incorrect value.