Define the term principle foci of a convex lens and illustrate your answer with the aid 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 convex lens, the first focal point is a point F₁ on the principal axis of the lens such that the rays of light starting from it or passing through it, after refraction through lens, become parallel to the principal axis of the lens
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The second focal point for a convex lens is a point F₂ on the principal axis such that the rays of light incident parallel to the principal axis, after refraction from the lens, pass through it.
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