Optical power (also referred to as dioptric power, refractive power, focusing power, or convergence power) is the degree to which a lens, mirror, or other optical system converges or diverges light. It is equal to the reciprocal of the focal length of the device, $\mathrm{P}=\frac{1}{\mathrm{f}}$

${\mathrm{P}}_{\mathrm{objective}}=0.5 \mathrm{D}$

${\mathrm{f}}_{\mathrm{objective}}=2$

${\mathrm{P}}_{\mathrm{eye}-\mathrm{piece}}=20 \mathrm{D}$

${\mathrm{f}}_{\mathrm{eye}-\mathrm{piece}}=0.05$

The magnifying power $\left(\mathrm{M}\right)$ of an astronomical telescope is given by:

$\mathrm{M}=\frac{ \mathrm{focal} \mathrm{length} \mathrm{of} \mathrm{objective}}{\mathrm{focal} \mathrm{length} \mathrm{of} \mathrm{eye}-\mathrm{piece}}$

$\mathrm{M}=\frac{2}{0.05}$

$\mathrm{M}=\frac{200}{5}$

$\mathrm{M}=40$