### You should know¶

$$ \large M = \frac{f_o}{f_e} $$

where

$f_o$: focal length of the objective lense in a refractor or of the primary mirror in a reflector

$f_e$: focal length of the eyepiece

### Solution¶

The separation between two lenses is 100 cm.

Since the eye-piece lense has a focal length of 20 cm, the focal length of the objectiec lense is 80 cm.

The angular magnification is

$$ \large M = \frac{f_o}{f_e} = \frac{80 \ \text{cm}}{20 \ \text{cm}} = 4 $$### Answer¶

(A) 4