Question

Define 1 dioptre of power of a lens. [2 mark]

Answer 1

Dioptre is the unit of measurement of the optical power of an optical instrument, like a lens or a mirror. [0.5 Marks]
Power is equal to the reciprocal of the focal length. So, 1 doptre can be written as:
$1D=\frac{1}{1m}$ [0.5 Marks]

In other words, 1 dioptre is defined as the power of a lens of focal length 1 metre. [1 Mark]