Question 1

Why is the focal length of a spherical mirror equal to half its radius of curvature?

Accepted Answer

The principal focus F is midway between the pole P and the centre of curvature C. Since R is the magnitude of PC, the magnitude of the focal length is R/2, although f is negative for a concave mirror and positive for a convex mirror.

Question 2

What happens when an object is placed between the focus and pole of a concave mirror?

Accepted Answer

The reflected rays diverge after striking the mirror. Their backward extensions meet behind the mirror and form a virtual, upright and magnified image, which cannot be intercepted by a screen.

Question 3

How are real and virtual images distinguished using the Cartesian sign convention?

Accepted Answer

A real image forms in front of the mirror and has v < 0, while a virtual image forms behind it and has v > 0. For a real object with u < 0, m = -v/u is negative for a real inverted image and positive for a virtual upright image.

Question 4

What do the three principal rays show in a concave mirror?

Accepted Answer

A ray parallel to the principal axis is drawn through F after reflection, a ray directed toward F is drawn parallel after reflection, and a ray directed through C retraces its path. Their paraxial forward or backward extensions locate the image predicted by the mirror formula.

Question 5

Why can a screen intercept a real image but not a virtual image?

Accepted Answer

The paraxial reflected rays physically meet at a real image position, so a screen perpendicular to the principal axis can receive them. For a virtual image, the outgoing rays diverge and only their backward extensions meet, so no screen can intercept the image.

Spherical Mirror Lab — Concave & Convex Mirrors in 3D

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