(a) With the help of a labelled diagram explain how a convex lens converges a beam of parallel light rays. Mark the principal axis, optical centre, principal focus and focal length of the convex lens on the diagram.(b) State whether convex lens has a real focus or a virtual focus.(c) List some things that convex lens and concave mirror have in common.

(a) Suppose that a beam of light rays parallel to one another and also to the principal axis of the convex lens when falls on the lens the incident rays pass through the lens and get refracted according to the laws of refraction.

 All the rays, after passing through the lens, converge at the same point $F$, on the other side of the lens. The point $F$ is known as the principal focus of the convex lens.

(b) A convex lens has a real focus since all the light rays actually pass through the focus.

(c) Some things that a convex lens and a concave mirror have in common are as follows:

1. Both converge parallel rays of light coming from infinity (parallel to the principal axis) at the focus $(F)$.

2. Both always form a real and inverted image of an object, except in one case.

3. Both form a virtual, erect, and magnified image when the object is placed within the focus, i,e between the pole $P$ and focus $F$ of a convex mirror, and between the optical centre $C$ and focus $F$, of a convex lens.

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Updated on: 10-Oct-2022

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