KS3 · Physics

Refracting light

A pencil in a glass of water looks snapped at the waterline. Pull it out: perfectly straight. Nothing happened to the pencil — something happened to the light.

Refraction · one ray, two surfaces

Follow a ray through a glass block
glassairairnormal

View: 1 · Arriving. Showing 2 layers: Glass block, Arriving

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step through the journey →

The ray travels through the air in a straight line and hits the glass at an angle. The dashed line is the normal: drawn at 90° to the surface, exactly where the ray arrives.

Tap each step to follow the ray. Tap the parts to find out what they are.

Predict, then check

Look at the ray as it leaves the block. Commit before you peek.

A ray goes into a glass block with parallel sides and comes out of the far side. Compared with the ray that went in, which way does the ray that comes out travel?

Transmitted or refracted?

Getting through isn't the same as turning

When light meets a material, some of it may be reflected, some absorbed and some transmitted (passed through). For each situation, decide: is the light transmitted, refracted, both, or neither?

  • A Transmitted (gets through)
  • B Refracted (changes direction)
  1. Light hits a glass window at an angle and passes through
  2. Light hits a glass block straight on, along the normal
  3. Light hits a brick wall
  4. Light shines at an angle from the air into a pond
  5. Light hits a thick black T-shirt
  6. A torch shines straight down into still water, along the normal

Your turn to draw it

Tracing a ray through a glass block

You shine a narrow ray from a ray box at a rectangular glass block, so it hits the top surface at an angle and comes out of the opposite side. How do you record its path on paper?

  1. Shine the narrow ray at the block, then mark dots along the ray going in, and along the ray coming out of the far side. Take care: the ray box can get hot.Dots on the paper are your evidence of where the light really went.
  2. missing step
Which line is step 2?

The broken pencil, explained

?

Reason it through

Why does a pencil standing in a glass of water look broken at the water's surface?

Link 1 of 4

First link · your turn

Light from the part of the pencil under the water travels up towards the surface. What happens to it when it crosses from the water into the air?

2
Locked — reveal the link above first
3
Locked — reveal the link above first
4
Locked — reveal the link above first

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

What you need to know

  • A medium is the material light is travelling through, such as air, water or glass.
  • Refraction means light changing direction as it passes into a new medium at an angle to the normal.
  • The normal is an imaginary line at 90° to the surface where the ray meets it, drawn dashed.
  • A ray changes direction only at the boundary between two media; inside one medium it travels in a straight line.
  • Refraction can make an object appear somewhere it isn't — a virtual image.

The big picture

Refraction is light changing direction when it passes from one material into another at an angle to the normal. The turn happens only at the boundary — towards the normal going into glass or water, away from it coming out — and in between the light travels in straight lines. Because our eyes trace light back in straight lines, refraction can make things appear where they aren't.

Key points

1When light hits a material, some may be reflected, some absorbed and some transmitted. Transparent materials like glass and water transmit most of it; opaque materials reflect or absorb most of it.
2Transmission means light gets through; refraction means it changes direction. A ray that hits a surface along the normal is transmitted without being refracted.
3Going from air into glass or water, a ray turns towards the normal. Coming back out into the air, it turns away from the normal.
4The greater the angle between the incoming ray and the normal, the more the ray changes direction.
5A ray passing through a block with parallel sides comes out travelling in its original direction, but shifted sideways.
6Your eye traces light back in a straight line, so after refraction an object appears where the light seems to come from — a pencil in water looks broken, and water looks shallower than it is.

Worked example

Problem

Two rays of light travel from the air into a pond. Ray A hits the water with a small angle between it and the normal. Ray B hits the water with a large angle between it and the normal. Describe what happens to each ray as it enters the water, and say which changes direction more.

⚠ Watch out

Drawing the ray curving gradually through the glass. Light only changes direction at a boundary — inside the block it goes in a straight line, so draw it with a ruler from the point where it enters to the point where it leaves.

🧠

Memory hook

IN goes IN, OUT goes OUT: going INto glass, the ray leans IN towards the normal; coming OUT, it swings OUT, away from it.

✓

Check yourself

From memory: a ray goes from air into a glass block at an angle. Which way does it turn going in, what path does it take inside, and which way does it turn coming out?

Flashcards

(13)
What is refraction?
Light changing direction when it passes into a new medium at an angle to the normal.
What is the normal?
An imaginary line at 90° to the surface, at the point where the ray meets it. It's drawn as a dashed line.
Where along its path does a ray change direction?
Only at the boundary between two media. Inside one medium it travels in a straight line — it never curves inside a block.
Which way does a ray turn going into glass, and coming out?
Into glass: towards the normal. Out into air: away from the normal.
What happens to a ray that hits glass along the normal?
It is transmitted without changing direction — it gets through but is not refracted.
How does the angle to the normal affect refraction?
The greater the angle between the incoming ray and the normal, the more the ray changes direction.
Transmission vs refraction — what's the difference?
Transmission means light gets through a material. Refraction means it changes direction. Light can be transmitted without being refracted.
What three things can happen when light hits a material?
It can be reflected, absorbed or transmitted (passed through) — often some of each.
Transparent vs opaque materials?
Transparent materials, like glass and water, transmit most of the light that hits them. Opaque materials reflect or absorb most of it.
How does a ray leave a block with parallel sides?
Travelling in its original direction — parallel to the ray that went in — but shifted to one side.
Why does a pencil in water look broken?
Light from the underwater part turns as it leaves the water. Your eye traces it back in a straight line, to a place the pencil isn't.
What is a virtual image?
An image you see where light only seems to come from, rather than where the object really is.
How do you trace a ray's path through a glass block?
Shine a narrow ray from a ray box, mark dots along it, draw the normals, and join the dots with a ruler. The ray box can get hot.

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 30 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.