Question

The chromatic aberration in Huygen's eyepiece is corrected using $f_1=3f_2$ and separation between the lenses is

• A. $f_2$
• B. $\frac{3}{2}{f}_2$
• C. $1.2f_2$
• D. $2f_2$

Answer 1

Answer: (D)

$2f_2$

One of the lenses is converging, and the other is diverging. Let $f_1$ be the focal length of the converging lens.

For achromatic aberration, the combination of the lenses should be such that the resultant beam from the combination is parallel to the incident.

The converging lens converges the beam at a distance of $f_1=3f_2$ from itself. The diverging lens should be kept such that the focus of it lies on this point of converges so that it diverges it to become parallel.

Thus the distance between the lenses=$3f_2-f_2=2f_2$