Question

When a beam of white light parallel to the principal axis is refracted through a convex lens, then the rays converge to the principal axis. If a screen is placed at a sufficient distance from the focus-point perpendicular to the principal axis then the image on the screen will be.

• A. A sharp white point
• B. A sharp violet point or red point
• C. A circular spot whose edges are violet and central portion red
• D. A circular spot whose edges are red and central portion violet

Answer 1

Answer: (C)

A circular spot whose edges are violet and central portion red

A lens will not focus different colors in exactly the same place because the focal length depends on refraction and the index of refraction for blue light (short wavelengths) is larger than that of red light (long wavelengths). This phenomenon is called chromatic aberration.

The image will be like this:

A circular spot will be formed whose edges are violet and the central portion red.