Question

To increase both the resolving power and magnifying power of a telescope

• A. both the focal length and aperture of the objective have to be increased.
• B. the focal length of the objective has to be increased.
• C. the aperture of the objective has to be increased.
• D. the wavelength of light has to be decreased.

Answer 1

The correct option is A both the focal length and aperture of the objective have to be increased.

The resolving power of a telescope,

$R.P=\frac{b}{1.22\lambda }$ (where, b = diameter of objective lens. )

$\therefore$On increasing the aperture of the objective, the resolving power of the telescope will also increase.

The magnifying power of the telescope is,

$m=-\frac{f_0}{f_e}\text{(For normal adjustment)}$
$=-\frac{f_0}{f_e}(1+\frac{f_e}{D})\text{(For near point adjustment)}$

On increasing the focal length of the objective, the magnifying power will also increase.

Hence, option $\left(\text{A}\right)$ is correct.