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

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Solution

A light ray can pass through a lens from either direction. Therefore, a lens has two principal foci.

For a concave lens, the first focal point is a point F₁ on the principal axis of the lens such that the incident rays of light appearing to meet at it, after refraction from the lens become parallel to the principal axis of the lens.
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The second focal point for a concave lens is a point F₂ on the principal axis of the lens such that the rays of light incident parallel to the principal axis, after refraction from the lens, appear to be diverging from this point.
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Similar questions

Define the term principle foci of a convex lens and illustrate your answer with the aid of proper diagrams.

(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.

Explain the following terms :

Pole, Centre of curvature, Radius of curvature, Principal axis.

Show them on separate diagrams for each of the concave and convex mirrors.

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