Reflecting light from mirrors

Plane mirror

Pick a distance, then tap the other buttons

The cat is 20 cm in front of the mirror, so its image appears 20 cm behind it — 40 cm from the cat.

The cat gives out light in all directions. Some of it hits the mirror, which reflects it, and some of that reflected light reaches the eye. The eye can’t tell the light bounced — it follows the rays back in straight lines (the dashed lines) to where they seem to come from.

Reading a ray diagram

Tap a part to see what it means

Before you trust a ray diagram you need to know its parts. Tap each one.

Predict, then check

Commit to an answer before you reveal anything.

A ray of light hits a plane mirror. It makes an angle of 20° with the surface of the mirror itself. What is the angle of reflection?

Test the law: does r depend on i?

The order really matters here — step through it.

1. 1Draw the mirror lineOn paper, draw a line for the mirror and mark a point on it. This will be the point of reflection.+
2. 2Mark the anglesPut the centre of the protractor on the point, with its 90° line along the mirror line. Mark every 10° from zero, then join each mark to the point. You now have the normal and a set of incident-ray lines.+
3. 3Place the mirrorPut the mirror with its reflecting surface on the mirror line. For a glass mirror the reflecting surface is the back surface.+
4. 4Shine the beamDirect the ray box beam along one incident line at a time. A ray box has a bulb inside and a narrow slit, so it gives a thin beam that stands for one ray.+
5. 5Mark the reflected beamFor each incident line, mark the centre of the reflected beam in two places with a sharp pencil.+
6. 6Join and measureRemove the mirror and join the two marks with a ruler. Measure each angle of reflection with the protractor, again measuring to the normal.+
7. 7RecordWrite each angle of incidence and angle of reflection in a results table you drew before you started.+

What you need to know

• Light travels in straight lines, and a ray shows the direction it travels. A mirror reflects each ray so that the angle of reflection equals the angle of incidence.
• Both angles are measured to the normal — the dashed line at 90° to the mirror — not to the mirror itself.
• The image in a plane mirror is virtual: it appears directly opposite the object, the same distance behind the mirror as the object is in front.
• The image is the right way up but laterally inverted (flipped left to right).

The big picture

A plane mirror reflects light so that the angle of reflection equals the angle of incidence, both measured to the normal. Your eyes trace the reflected rays back in straight lines, so the image appears directly opposite the object and as far behind the mirror as the object is in front. It is a virtual image: upright, but laterally inverted.

⚠ Watch out

Measuring the angles to the mirror instead of to the normal. Always draw or find the normal first — the dashed line at 90° to the mirror — and measure the angle of incidence and the angle of reflection from it.

Memory hook

Angles live on the Normal: i and r both start at the dashed line, never at the mirror.

Check yourself

Sketch a mirror, the normal, an incident ray and its reflected ray, and label i and r. Then say where the image of an object 50 cm from the mirror appears.