Question

An achromatic combination of lens is formed by joining

• A. two convex lenses
• B. two concave lens
• C. one convex lens and one concave lens
• D. one convex lens and one plain mirror

Answer 1

Answer: (C)

one convex lens and one concave lens

Answer 2

For an achromatic combination of lenses, there should be
$\frac{\omega_{1}}{f_{1}} + \frac{\omega_{2}}{f_{2}} = 0$
$\Rightarrow \frac{\omega_{1}}{f_{1}} = - \frac{\omega_{2}}{f_{2}}$
Here, one focal length is positive and another one is negative i.e., one lens is convex and another is concave lens.