Reflection & Plane Mirrors

You’ll be able to

State and apply the law of reflection using angles measured from the normal
Distinguish specular from diffuse reflection
Describe the properties and location of an image in a plane mirror

The law of reflection

When light strikes a smooth surface it bounces off following one clean rule: the angle of incidence equals the angle of reflection, both measured from the normal — the line drawn perpendicular to the surface at the point of contact. The incoming ray, the reflected ray, and the normal all lie in the same plane. Measuring from the normal (not the surface) is the convention that makes every optics formula consistent, so build the habit now.

Specular vs. diffuse, and plane-mirror images

A smooth surface like a mirror gives specular reflection — parallel rays stay parallel, forming a clear image. A rough surface gives diffuse reflection — rays scatter in all directions, which is why you see a wall but no reflection in it. A plane (flat) mirror forms an image that is virtual (light does not actually pass through it), upright, the same size as the object, and located as far behind the mirror as the object is in front. Left and right appear swapped, but the image is never magnified.

Law of reflection

θ_incidence = θ_reflection (both measured from the normal)

A ray hitting the mirror at angle θ to the surface makes an angle (90° − θ) to the normal, and reflects at that same angle on the other side of the normal.

Worked example

A light ray strikes a plane mirror, making a 30° angle with the mirror surface. Find the angle of reflection (measured from the normal).

1.Angles in optics are measured from the normal, not the surface.
2.The ray is 30° from the surface, so it is 90° − 30° = 60° from the normal — that is the angle of incidence.
3.The law of reflection: angle of reflection = angle of incidence = 60°.

Answer: 60° from the normal. (Beware: 30° is the angle from the surface, not from the normal.)

Checkpoint

A light ray strikes a plane mirror at 30° to the mirror’s surface. What is the angle of reflection, measured from the normal?

Watch out

Always measure angles from the normal, never from the surface. A ray “grazing” a mirror at a small angle to the surface actually has a large angle of incidence (near 90°). Mixing up the reference line is the single most common reflection error.

Checkpoint

An object stands 2.0 m in front of a plane mirror. Where is the image, and what kind is it?

On the exam

Memorize the plane-mirror image: virtual, upright, same size, equal distance behind. It is the reference point you compare curved-mirror and lens images against later in the unit.

Answer the 2 checkpoints as you read.