(a) Explain with the help of a diagram, why the concave lens is also called a diverging lens. (b) Define the principal focus of a concave lens.

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Solution

a)
Concave lenses are thinner at the middle. Rays of light that pass through the lens are spread out (they diverge). A concave lens is a diverging lens. When parallel rays of light pass through a concave lens the refracted rays diverge so that they appear to come from one point called the principal focus.

b)When parallel rays of light pass through a concave lens the refracted rays diverge so that they appear to come from one point called the principal focus.

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Q. Why is a convex lens called a converging lens and a concave lens called a diverging lens?

Q. Define the principal focus of a convex lens and that of a concave lens.

Define the term principal foci of a concave lens and show them with the help of proper diagrams.

(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 the convex lens has a real focus or a virtual focus. (c) List some things that convex lens and concave mirror have in common.

A concave lens is called a diverging lens because the incident rays are bent ________.

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