A refracting telescope is adjusted for normal relaxed eye observation (final image at infinity). Its objective lens has focal length $f_o=120\text{cm}$ and its eyepiece has focal length $f_e=12\text{cm}$. Calculate the angular magnification $M$ of the telescope.

Step-by-step solution

1. For a telescope focused at infinity, the angular magnification is given by $M=-\frac{f_o}{f_e}$.
2. Substitute the given focal lengths: $M=-\frac{120\text{cm}}{12\text{cm}}=-10$.
3. The negative sign indicates that the final image is inverted with respect to the object.
4. Thus the magnitude of magnification is $10$.

Concept tested: Telescope angular magnification

Published 12 September 2026 · How this solution was prepared and checked