KS3 · Physics

Reflected images in mirrors

Wave at a mirror and your reflection waves back from behind the glass — where there is only wall. So how is your eye being fooled? Follow the light.

Physics · Light

Where is your reflection, really?
mirrorin frontbehindobject

View: 1 · Light hits the mirror. Showing 1 layer: Light from the object

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Light scattered by the object spreads out in all directions. Some of it hits the mirror and bounces off. The mirror makes no light of its own — it only sends back light that reaches it.

Your eye is about to be fooled. Watch how.

Predict, then check

Picture three parallel rays of light, side by side, heading towards a mirror.

The same three parallel rays hit a flat mirror and a curved mirror. Which one gives a clear reflection?

What do you think?

Why does mirror writing look back to front?

You write HELLO on a card, turn it round and hold it up to a mirror. In the reflection the letters are back to front — but they are not upside down.

Which explanation is closest to what you think right now?
How sure are you?

Two kinds of image

Virtual image — a reflection in a plane mirrorvsReal image — from a projector or a pinhole camera

A reflection in a mirror is not the only kind of image. Compare it with the picture a projector throws on a screen.

Focus

Where the image is

Virtual image — a reflection in a plane mirror

Behind the mirror, where the rays only seem to come from

Real image — from a projector or a pinhole camera

Where rays of light really meet, or hit a screen

The insight

A virtual image is where light appears to come from; a real image is where light actually arrives.

Does light travel from the image to your eye?

Virtual image — a reflection in a plane mirror

No — the light comes from the object, via the mirror

Real image — from a projector or a pinhole camera

Yes — some light travels from the picture on the screen into your eyes

Can you catch it on a screen?

Virtual image — a reflection in a plane mirror

No — a sheet of paper behind the mirror stays blank

Real image — from a projector or a pinhole camera

Yes — that is exactly what a projector screen does

How the rays are drawn

Virtual image — a reflection in a plane mirror

Dashed lines behind the mirror (virtual rays)

Real image — from a projector or a pinhole camera

Solid lines — the light really travels along them

Your turn to draw it

Draw the ray diagram for a mirror

A small object (a point) sits in front of a plane mirror, and an eye is looking into the mirror. Put the steps of the ray diagram in order — choose each missing step.

  1. Draw the mirror as a straight line. Mark the eye, and mark the object as a dot in front of the mirror.
  2. missing step
Which line is step 2?

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

How a flat mirror puts a picture of you somewhere no light has ever been.

What you need to know

  • An image is a picture of something that is not the real thing. The image you see in a mirror is called a reflection — and "reflection" is also the word for what a mirror does to light.
  • Mirrors are not luminous. They do not make light or copies; they reflect light scattered by objects, and some of it enters your eyes.
  • Law of reflection: angle of reflection = angle of incidence. Both are measured from the normal, a line at 90° to the mirror where the ray hits.
  • Your eyes trace reflected rays back in straight lines, so the light seems to come from behind the mirror.
  • Plane mirror image: same size, upright, laterally inverted, and as far behind the mirror as the object is in front.
  • A mirror image is virtual: no light travels from it to your eye. A real image (projector, pinhole camera) forms where light actually meets or hits a screen.

The big picture

You see a reflection because light from an object bounces off a mirror into your eyes. Your eyes assume the light travelled in straight lines, so they trace it back to a point behind the mirror. That is where the image appears: in a plane mirror it is the same size, upright, and as far behind the mirror as the object is in front. It is a virtual image, so no light really comes from it and it cannot be caught on a screen.

Key points

1Light travels from the object to the mirror, then to your eye — never out of your eye.
2Angles of incidence and reflection are always equal and are measured from the normal, not from the mirror.
3Flat (plane) mirrors reflect parallel rays at the same angle as each other, giving a clear image; curved mirrors reflect them at different angles and distort it.
4You only see an object's reflection if light from it can bounce off the mirror into your eyes, so people in different places see different things in the same mirror.
5Reflections look back to front (laterally inverted) because the object had to be turned round to face the mirror, not because the mirror flips left and right.
6In ray diagrams, virtual rays behind the mirror are dashed because light never really travelled along them.

Worked example

Problem

Priya stands 1.5 m in front of a plane mirror. (a) Where is her image? (b) How far is Priya from her image? (c) She walks 0.5 m towards the mirror. How far is she from her image now?

⚠ Watch out

Putting the image ON the mirror, or measuring angles from the mirror's surface. The image is behind the glass, as far back as the object is in front, and angles of incidence and reflection are always measured from the normal.

🧠

Memory hook

Same size, same way up, same distance back — and nobody home. The image is where the light SEEMS to come from, not where it came from.

✓

Check yourself

Explain in three steps why your reflection seems to be behind the mirror. Then say why it could never show up on paper held behind the mirror.

Flashcards

(14)
What is an image?
A picture of something that is not actually the real thing. The image you see in a mirror is called a reflection.
Does a mirror give out its own light?
No. Mirrors are not luminous. They reflect light that has been scattered by objects, and some of it enters your eyes.
What is the normal?
An imaginary line drawn at 90° to the mirror at the point where a ray hits it.
State the law of reflection.
The angle of reflection equals the angle of incidence, on the other side of the normal.
What are the angles of incidence and reflection measured from?
From the normal — not from the surface of the mirror.
Why does your reflection seem to be behind the mirror?
Your eyes can't tell the light changed direction. They trace the reflected rays back in straight lines, which meet behind the mirror.
Describe the image in a plane mirror (size, way up, position).
Same size as the object, upright, and just as far behind the glass as the object is in front.
Why can people in different places see different things in the same mirror?
You only see an object's reflection if light from it can hit the mirror and reach YOUR eyes, and that depends on where you are.
Why does a flat mirror give a clear image and a curved one a distorted image?
A flat (plane) mirror reflects parallel rays at the same angle as each other; a curved mirror reflects them at different angles.
What does "laterally inverted" mean?
Flipped from side to side, like mirror writing — but not upside down.
Why does writing held up to a mirror look back to front?
You turned the writing round to face the mirror. The mirror just reflects what faces it — it doesn't swap left and right.
What is a virtual image?
An image where the light only seems to come from. No light travels from it to your eyes, so it can't be caught on a screen.
Give two examples of a real image.
The picture from a projector, and the image in a pinhole camera — both formed where light actually arrives.
Why are some lines in a mirror ray diagram dashed?
They are virtual rays: they show where the light seems to come from, but light never really travelled along them.

Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.

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How this lesson was checked. This KS3 Physicslesson was published through Lightbulb Learning's human-designed editorial process — the educational standards, accuracy rules and publication checks it must pass were authored and approved by Philip Halpin. It passed subject-specific assessment, automated educational checks and technical publication verification before going live (publication checks completed 29 September 2026). Published pages are monitored, human spot-checking is ongoing across the lesson library, and anything found wrong is corrected or withdrawn. How our lessons are made and checked. Spotted a mistake? Email hello@lightbulblearning.co and we'll review it.