Light Waves: Reflection by Plane Mirrors

Reflection is when light bounces off a surface, like your face in a mirror or sunlight on a calm lake. We use mirrors to see images because reflected light travels to our eyes.

Key Terms

• Normal: an imaginary line at right angles (90°) to the surface at the point where the light hits.
• Angle of incidence (i): the angle between the incoming ray and the normal.
• Angle of reflection (r): the angle between the reflected ray and the normal.

Law of Reflection

For a plane (flat) mirror, the law is simple:

$$i = r$$

This means the incoming angle equals the outgoing angle. Think of a ball bouncing off a wall at the same angle.

Measuring Angles Correctly

• Always measure from the normal, not from the mirror surface.
• Use a protractor to check that $i$ and $r$ are equal.

Images in a Plane Mirror

The image you see in a flat mirror has these features:

• Same size as the object.
• Same distance behind the mirror as the object is in front.
• Virtual (light appears to come from behind the mirror, but does not actually pass through that point).
• Laterally inverted (left–right reversed).

How to Draw the Image (Ray Construction)

1. Draw the mirror as a straight line. Add the normal at the point where a ray hits.
2. Mark the object. Measure its distance to the mirror. Place the image the same distance behind the mirror on a line perpendicular to the mirror.
3. Draw two rays from the top of the object to the mirror; reflect them using $i = r$. Extend the reflected rays back behind the mirror with dotted lines; they meet at the image.

Common Misconceptions

• Angles are measured from the normal, not from the mirror surface.
• The image is not “on” the mirror; it appears behind it (virtual).
• Mirrors swap left and right, not top and bottom.

Real-World Connections

Periscopes, kaleidoscopes, and road safety reflectors all rely on predictable reflection from flat or angled mirrors.