Question

A lens of 80cm f.l. Is used as the objective for a 2 lens terrestrial telescope. What focal length lens is used to achieve a power of 50?

Answer 1

A lens of 80cm f.l. is used as the objective for a 2 lens terrestrial telescope. What focal length lens is used to achieve a power of 50?

Telescope magnification is:

$$M = \frac{F}{f}$$

where $F$ is the focal length of the objective lens and $f$ is the focal length of the eyepiece.

We can find the focal length of the eyepiece:

$$f = \frac{F}{M} = \frac{80\,\text{cm}}{50} = 1.6\,\text{cm}$$

Answer: $f = 1.6\,\text{cm}$