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For plane mirror focal length is _________ m.

Asked in GUJCET 2024 · Plane mirrors

Answer: (4) ∞

Step-by-step solution

Given: a plane mirror.

Idea: a plane mirror is a spherical mirror whose surface has been flattened out completely, so its radius of curvature is infinite.

For any mirror f=R/2, and with R→∞ the focal length is infinite too.

A parallel beam reflected off a flat surface stays parallel: it never comes to a focus at any finite distance.

The mirror formula agrees: 1/v+1/u=1/f=0 gives v=-u, the image as far behind the mirror as the object is in front.

Why the other options are wrong

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