Question

If the effective focal length of Huygen’s eye piece is $f_H$, then the length of eye piece is :

• A. $\frac{3f_H}{4}$
• B. $\frac{4f_H}{3}$
• C. $\frac{2f_H}{3}$
• D. $\frac{3f_H}{2}$

Answer 1

The correct option is B $\frac{4f_H}{3}$

Two lenses of Huygen's eyepiece are of focal length $f$ and $3f$.

Then, effective focal length of Huygens eyepiece $=f_H$ $=\frac{3f}{2}$

Length of eyepiece $=2f=2\times \frac{2f_h}{3}=\frac{4f_H}{3}$