Names for a reflection

The idea

The incident ray falls on the mirror and the reflected ray comes back; the normal is the line at 90° to the mirror at the point of incidence; angles of incidence (i) and reflection (r) are measured from the normal.

Measuring a reflection

A mirror stands upright on a sheet of paper. A torch shines a thin beam along the paper to the mirror, and the beam bounces off. We want to measure the angles.

Which line do you think the angles are measured from?

• A line at 90° to the mirror
• The mirror itself
• The edge of the paper

Name the parts

The torch's ray falls on the mirror and comes back. Tap each part when the goal asks for it.

Measured from the normal

The incident ray falls on the mirror, and the reflected ray comes back. The normal is the line at 90° to the mirror at the point of incidence. The angle of incidence (i) and the angle of reflection (r) are measured from the normal.

• Name | What it is
• Incident ray | The ray that falls on the mirror
• Reflected ray | The ray that comes back from the mirror
• Normal | The line at 90° to the mirror at the point of incidence
• Angle of incidence (i) | Between the normal and the incident ray
• Angle of reflection (r) | Between the normal and the reflected ray

Notes

The incident ray falls on the mirror; the reflected ray comes back. The normal is at 90° to the mirror at the point of incidence. i and r are measured from the normal.

Check yourself

What is the normal?

The angle of incidence is the angle between…

A ray makes an angle of 60° with the surface of the mirror. What is its angle of incidence?

Answer: 30 °

The normal makes 90° with the mirror. The ray is 60° from the mirror, so it is 90° − 60° = 30° from the normal. i = 30°.

• The surface of the mirror. The normal is not the mirror. It is at 90° to the mirror.
• The ray that comes back from the mirror. That is the reflected ray.
• The line at 90° to the mirror at the point of incidence — correct. Yes. It stands straight up from the mirror where the ray hits.
• the incident ray and the normal — correct. Yes. Angles are measured from the normal.
• the incident ray and the mirror. A common mistake. We measure from the normal, not from the mirror.
• the incident ray and the reflected ray. That angle is i and r together. The angle of incidence is from the normal.