Question

An achromatic combination of lenses is formed by joining…
A. 2 convex lenses
B. 2 concave lenses
C. 1 convex and 1 concave
D. 1 convex and 1 plane mirror

Answer 1

Recall the definition of chromatic aberrations. To eliminate the chromatic aberrations two lenses of different focal lengths are used. Also, the nature of the lenses should be different because the combination of these two lenses is such that the chromatic aberrations formed by one lens is balanced by the aberrations of another lens.

Complete answer:
To answer this question, let’s see what are chromatic aberrations? We know that when the light passes through the glass or prism, due to dispersion, the white light gets separated into colored wavelengths. Therefore, the lens cannot focus these deviated light rays at the same focus. This failure of the lens to focus the light rays at the same focus is known as chromatic aberrations. Chromatic aberrations can be resolved by combining two lenses together.

Let’s define achromatic lens as the combination of two different types of lens of different focal lengths such that the image formed by the combination of these two lenses is free from chromatic aberrations. To do so, we combine one convex lens and one concave lens and place them next to each other. The combination of these two lenses is such that the chromatic aberrations formed by one lens is balanced by the aberrations of another lens.

The concave lens is generally formed of flint glass which has higher dispersion and the convex lens is formed of crown glass which has low dispersion.

So, the correct answer is option C.

Note: We cannot combine the two lenses of the same nature because the image formed by the combination will have addition of chromatic aberrations from both the lenses. Also, we cannot combine a spherical lens with a plane mirror because the focal length of the plane mirror is infinite.