Reflection of Waves

Reflection occurs when a wave hits a boundary and bounces back. The wave stays in the same medium.

The law of reflection

$\text{angle of incidence} = \text{angle of reflection}$

Both angles are measured between the ray and the normal (the line perpendicular to the surface at the point of incidence), not between the ray and the surface.

Key terms

• Incident ray: the ray approaching the surface.
• Reflected ray: the ray leaving the surface.
• Normal: a line perpendicular to the surface at the point where the ray hits.
• Angle of incidence ($i$): the angle between the incident ray and the normal.
• Angle of reflection ($r$): the angle between the reflected ray and the normal.

Reflection at a plane (flat) mirror

• The image is virtual (cannot be projected on a screen).
• The image is the same size as the object.
• The image is the same distance behind the mirror as the object is in front.
• The image is laterally inverted (left and right are swapped).

Worked example

A ray of light hits a plane mirror at 35$^\circ$ to the normal. What is the angle of reflection?

By the law of reflection: angle of reflection = 35$^\circ$.

Common errors and how to correct them

Measuring angles from the surface instead of the normal. Always draw the normal first, then measure both angles from the normal.

Stating the image in a plane mirror is “real.” A plane mirror always produces a virtual image. Real images are formed by converging lenses or curved mirrors and can be projected on a screen.

How to apply this in an exam

Draw the normal first (dotted line, perpendicular to the surface). Mark the angle of incidence and angle of reflection from the normal. If asked about a plane mirror image, state all four properties: virtual, same size, same distance, laterally inverted.