Question

When the length of an astronomical telescope tube increases its magnifying power :

• A. decreases
• B. increases
• C. does not change
• D. may increase or decrease

Answer 1

The correct option is A decreases

Length of an astronomical telescope.
$L=f_0+\frac{f_{e}D}{f_e+D}$
$\Rightarrow L-f_0=\frac{f_{e}D}{f_e+D}$

so, as L increases $\frac{f_{e}D}{f_e+D}$ increases

and $m=-\frac{f_0(f_e+D)}{f_{e}D}$ (In astronomical telescope)
So, as $\frac{f_{e}D}{f_e+D}$ increases m decreases.