Question

The focal length of a convex lens is maximum for which light?
A. Blue light
B. Yellow light
C. Green light
D. Red light

Answer 1

First of all we should be aware of convex lenses and also about focal length for solving this question. For a lens, if the surface is bent outwards, then the lens is known as a convex lens or biconvex lens. The focal length for a given thin lens in air is the distance from the center of the lens to its principal foci which is generally known as focal points of the lens.

Complete step by step answer:
The main use of the convex lens is to converge a straight beam of light. It is generally thicker at the center and is generally thinner at the edges. It is also known as the converging lens.
Uses of Convex lens
The main uses of convex lenses are microscopes, eyeglasses, and magnifying glasses. In the cameras, they are generally used to create real images of objects present at a distance. The way in which we use these lenses generally affects the nature of the images.
The focal length plays an important role in the manufacturing of the lens. The main use of focal length is that it allows us to magnify an object or to make an object appear large, within the frame.
Now, we will solve the above question by using the relation
$f \propto \dfrac{1}{{\left( {\mu - 1} \right)}}\,\,\,\,\,\,\,\,\,and\,also\,from\,\,\,\,\,\,\,\,\,\,\mu \propto \dfrac{1}{\lambda }$
Thus, $f \propto \lambda$
And we know that
${\lambda _r} > {\lambda _v}$
Hence, focal length of convex lens is maximum for red light.

Note: The focal length is generally measured in millimeters. As we know that when the focal length is longer the angle of view is narrower and the magnification is higher. And when the focal length is shorter, then the angle of view is wider and the magnification is lower.