A refracting telescope consists of an objective lens of focal length $f_{o} = 100 cm$ and an eyepiece of focal length $f_{e} = 10 cm$ . The final image is formed at infinity. What is the angular magnification $M$ of the telescope?

medium MCQ 3 marks Class 12 · Physics · Ray Optics and Optical Instruments

Answer

Option (c)

M = 10

The correct answer is Option (c) $M = 10$ — Hence the correct option is the one stating $M = 10$ .

Step-by-step solution

For a telescope adjusted for normal eye (final image at infinity), the angular magnification is $M = - \frac{f _{o}}{f _{e}}$ .
Substitute the given focal lengths: $M = - \frac{100}{10} = - 10$ .
The negative sign indicates that the image is inverted; the magnitude of magnification is $10$ .
Hence the correct option is the one stating $M = 10$ .

Common mistake: Using

M = \frac{f _{e}}{f _{o}}

instead of the correct ratio, which gives the reciprocal value.

Concept tested: Angular magnification of a refracting telescope