Question

Under what condition in Telescope does the brightness of image increase $M^{-}$ times?

• A. $\frac{F_{objective}}{f_{eyepiece}}=\frac{D}{d}$
• B. $\frac{F_{objective}}{f_{eyepiece}}\times \frac{D}{d}$
• C. $\frac{F(f+D)}{fd}=\frac{D}{d}$
• D. None of these

Answer 1

The correct option is A $\frac{F_{objective}}{f_{eyepiece}}=\frac{D}{d}$

For viewing clear image through eyepiece, focal length of eyepiece is adjusted so that maximum light passes through it. Increasing the brightness means increasing the focal length.

Therefore, magnification of telescope $M=\frac{F_{objective}}{f_{eyepiece}}=\frac{D}{d}$