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A plane wavefront AB travelling parallel to the principal axis is incident on a concave mirror. What is the shape of the reflected wavefront just after reflection from the mirror?

1. A converging spherical wavefront
2. A diverging spherical wavefront
3. A plane wavefront
4. A cylindrical wavefront with no curvature

Correct answer: A converging spherical wavefront

Solution

A plane wavefront corresponds to a parallel beam of rays travelling along the axis. A concave mirror is a converging mirror: it reflects parallel rays so that they all meet at the principal focus. Since the reflected rays are converging toward a point (the focus), the wavefront perpendicular to them must be a spherical surface that is shrinking toward that focus, i.e. a converging (concave-toward-the-focus) spherical wavefront.

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