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# How does light bend?

Year 9 Physics · Wave and particle models

<!-- Speaker note: Students use the wave model to explain reflection and refraction: wave fronts bounce at equal angles, and slow down and bend towards the normal in a denser material (AC9S9U04, node 4; builds on AC9S5U03). About 25 minutes (Engage and Explore 8 min · Explain 10 min · Evaluate 3 min · Typical adds 4 min, Wider about 2 min). -->

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## Today

**We are learning to** the wave model explains two things you can see. In reflection, wave fronts bounce off a surface and the angle of reflection equals the angle of incidence. In refraction, light slows down in a denser material such as glass or water, so the wave fronts bunch up and the direction bends towards the normal; light going straight in at 90° does not bend.

- I can say why light bends when it goes into glass.
- I can say what happens to a ray that goes straight in.
- I can use the law of reflection.

<!-- Speaker note: Before class: A concept tutorial for introducing or revising the idea. Set the width with `?width=minimum`, `typical` (default) or `wider`. It follows What is the electromagnetic spectrum? and comes before Wave or particle? Students met reflection and refraction as rules for rays in Year 5. Here the wave model gives the reason. The blue lines are wave fronts (crests); the orange arrows are rays, at right angles to the fronts. Board notes: the wave fronts' spacing in the second material is the incident spacing divided by the refractive index (1.33 water, 1.5 glass, 2.4 diamond); the ray's angle follows Snell's law. The light is a single colour, so dispersion isn't shown. Partial reflection at the glass is drawn as a dotted line. The index and Snell's law are not named for students. Teacher: In your own words: the labelled ray diagram with fronts. -->

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## Why it matters

> A straw in a glass of water looks snapped at the surface. The straw is fine. What is the light doing?

<!-- Speaker note: The last lesson said light is a wave. Here the wave model explains two things you can see every day. -->

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<span style="background:#dbeafe; color:#1a1a1a; border:1.5px solid #3b82f6; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">💬 Discussion</span>

## Engage: predict

**A ray of light goes from air into glass at an angle. Which way does it bend?**

- Towards the normal (the line at right angles to the glass)
- It doesn't bend
- Away from the normal
- It depends on the colour of the glass

<!-- Speaker note: What happens: Towards the normal. Light travels more slowly in glass than in air. The wave fronts bunch up and swing round so that the direction of travel is closer to the normal. Take two or three predictions before anyone runs it. -->

<!-- Speaker note: Teacher: tally the predictions. Ask the 'away' group which end of the front gets into the glass first. -->

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<span style="background:#fef3c7; color:#1a1a1a; border:1.5px solid #f59e0b; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✍️ Activity</span>

## Explore it

{{embed: https://studio.learningwaypoints.com/tutorials/_playground-board/index.html?id=deck%2Fscience%2Fphysics-9%2Fbending-light%2Fstep-2&code=widget%3A+wave%0Amode%3A+refract%0Amedium%3A+glass%0Aangle%3A+40%0A&fresh=1}}

[Open this step in the tutorial](https://studio.learningwaypoints.com/tutorials/science/physics-9/bending-light/?full=step-2#step-2)

<!-- Speaker note:  -->

<!-- Speaker note: Teacher: ask what happened to the gap between fronts. -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## What happened?

* Towards the normal. Light travels more slowly in glass than in air. The wave fronts bunch up and swing round so that the direction of travel is closer to the normal.

<!-- Speaker note: Reveal after running. Ask who predicted it, and what made them think so. -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## Why it bends

**Why does light bend as it goes from air into glass at an angle?**

- It slows down, so the wave fronts change direction
- It speeds up, so the wave fronts change direction
- The glass pushes it sideways

* Answer: It slows down, so the wave fronts change direction

<!-- Speaker note: One end of each wave front enters the glass first and slows down while the other end is still travelling faster in the air. The front swings round, like a line of marchers when one end slows. -->

<!-- Speaker note: Teacher: why it bends and the law of reflection are the success criteria. -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## Straight in

**A ray goes straight into glass along the normal (angle of incidence 0°). What happens?**

- It bends away from the normal
- It slows down but doesn't change direction
- It bends towards the normal

* Answer: It slows down but doesn't change direction

<!-- Speaker note: The whole wave front reaches the glass at the same moment and slows together, so it doesn't turn. -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## Mirror

**A ray hits a mirror at 35° to the normal. At what angle does it reflect?**

- 35° on the other side of the normal
- 55°
- It depends on the mirror

* Answer: 35° on the other side of the normal

<!-- Speaker note: The angle of reflection equals the angle of incidence, measured from the normal. -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## The straw

**Why does a straw in water look bent at the surface?**

- Light from the part under water bends as it leaves the water, so it seems to come from a different place
- Water makes light go faster
- The water bends the straw

* Answer: Light from the part under water bends as it leaves the water, so it seems to come from a different place

<!-- Speaker note: Light leaving the water speeds up and bends away from the normal. Your brain assumes it travelled in a straight line, so the underwater part seems shifted. -->

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<span style="background:#fef3c7; color:#1a1a1a; border:1.5px solid #f59e0b; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✍️ Activity</span>

## Elaborate: mirror, glass, diamond

{{embed: https://studio.learningwaypoints.com/tutorials/_playground-board/index.html?id=deck%2Fscience%2Fphysics-9%2Fbending-light%2Fstep-4&code=widget%3A+wave%0Amode%3A+refract%0Asurface%3A+mirror%0Aangle%3A+40%0A&fresh=1}}

[Open this step in the tutorial](https://studio.learningwaypoints.com/tutorials/science/physics-9/bending-light/?full=step-4#step-4)

<!-- Speaker note: What this step is for: Comparing a mirror with glass, then glass with diamond, shows reflection as a bounce, refraction as a bend, and the amount of bending as the change in speed. -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## Why more

**Diamond bends light more than glass. What does that tell you about the speed of light in diamond?**

- It is the same: only the angle changes
- Light is slower in diamond than in glass
- Light is faster in diamond

* Answer: Light is slower in diamond than in glass

<!-- Speaker note: The more the light slows down, the more the wave fronts bunch up and the more the direction changes. -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## Elaborate: fibre optics

**If light inside glass hits the surface at a very shallow angle, it can't get out: it reflects back in. This is why light can run along a glass fibre for kilometres. What makes fibre optic cables useful?**

- They carry signals as light, which has a high frequency and so can carry a lot of information
- They are cheaper than copper wire
- Light moves faster in glass than in air

* Answer: They carry signals as light, which has a high frequency and so can carry a lot of information

<!-- Speaker note: Light waves vibrate hundreds of trillions of times a second, so they can carry far more information than radio waves or electric currents in wires. What this step is for: Total internal reflection is the limit of refraction; fibre optics shows how an idea from the wave model carries the internet. -->

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## Evaluate: look back

Write one sentence: "At first I thought… Now I think… because…"

**Revision summary**

- **Reflection**: the angle of reflection equals the angle of incidence, both measured from the normal.
- **Refraction**: light slows down in a denser material, so the wave fronts bunch up and the direction bends **towards the normal**. Going out, it bends away.
- Light going straight in (along the normal) slows down but doesn't bend.
- The more the light slows, the more it bends.

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## Exit ticket

**A ray of light goes from air into water and bends towards the normal. What does that tell you about the speed of light in water?**

* It is slower in water than in air. The change of speed turns the wave fronts and so the ray.

<!-- Speaker note: Students answer on paper or a mini whiteboard before the reveal. Anyone who cannot answer needs the main idea again next lesson. Where this leads: Next: Wave or particle? weighs up the wave and particle models. In class: a laser pointer through a tank of water, a clear block or a semicircle; a coin in a cup that appears when water is poured in; a mirror and a ruler for the law of reflection. -->