Question

Geometric Optics

1) What is the function of the lens in the formation of a real image? Why don't the rays from the object form an image without the lens?

2) How are focusing power and focal length of a lens related?

Answer 1

1. Function of a lens is to converge or diverge the rays of light. For example, a convex lens converges a set parallel ray to a single point. On the contrary, a concave lens diverges a set parallel ray.

A point object emits light rays which diverges at all angles in the space. Now, to get a ream image of that point object we have to collect some of those rays emitted by the point object to a single point. If we can cast this point to the screen, a real image will be formed. Now, a lens' function is it helps us to converge the rays coming from object on the screen i.e., to form a real image.

As a light ray travels in a straight line, diverging rays can never converge by itself without a convex lens in its path. That's why rays from the object can't form an image without the lens.

2. If the focusing power of a lens is P and focal length of that lens be f, then

That is, power of a lens is inversely proportional with the focal length of a lens. If f is in meters, P will be in Diopters.