Question

The focal length of convex lens depends upon:
This question has multiple correct options
A. Refractive index of lens material
B. Radii of curvature of lens
C. Refractive index of the surrounding medium
D. Position of the object

Answer 1

Hint : The focal length of a lens is a point where the parallel lines passing through the lens meet after converging. It depends on how much the lens is able to bend the light rays passing through it and how spherical is the lens.

Detailed step by step solution:
The convex lens has the property of converging the light rays passing through it. We can define the focal length for this lens as the distance from the lens on the principal axis at which the parallel rays passing through the lens meet after converging. This can be visualized as follows:

Now the focal length of a lens depends on the fact that how much the lens can converge the light rays passing through it.
Therefore, the refractive index of the lens material affects the focal length because the refractive index tells us the amount of bending of light when it passes from one medium to another. The refractive index of the surrounding medium makes no difference as long as the medium is the same on the left and right side of the lens.
The radii of curvature tell us how much the surface of the lens is curved. The larger curvature or smaller radius of curvature means that the light passing through the lens would be bent at larger angles leading to decrease in the focal length of the lens.
The position of the object does not affect the focal length. In fact, the nature of the image obtained depends on the position of the object with respect to the focus.

Note: The definition of focal length has a little difference when we talk about the concave lens which diverges the light rays passing through it. The focal length of a concave lens is defined as the point where the parallel lines passing through the lens appear to meet after passing through the lens as can be seen in the following diagram.