Optics

Plane mirrors

When a light ray meets a smooth mirror, it changes direction according to a definite rule. The angles of incidence and reflection are measured from the normal at the point of incidence.

When a light ray meets a smooth mirror, it changes direction according to a definite rule. The angles of incidence and reflection are measured from the normal at the point of incidence.

Law and application

Quick check

The image in a plane mirror is virtual: reflected rays do not actually meet behind the mirror, but their backward extensions do. The image is as far behind the mirror as the object is in front, and it has the same height. Thus an object 30 cm from the mirror has an image 30 cm behind it, giving an object–image separation of 60 cm. To see a full-length image, the minimum mirror height is half the person’s height, independent of how far the person stands from the mirror.

A virtual image cannot be projected directly onto a screen, yet the eye sees it because reflected rays arrive as if they came from the image.

The image is the object’s reflection across the mirror plane: it lies on the perpendicular through the object. If the object moves 5 cm toward the mirror, its image also moves 5 cm toward it, so object–image separation decreases by 10 cm. The familiar left–right reversal is a perception arising when we compare ourselves with the image; geometrically, the mirror reverses the front–back direction. To locate an image accurately, mark the perpendicular foot and measure equal distances on either side instead of relying on appearance.