--- title: "How does light bend?" subtitle: "Year 9 Physics · Wave and particle models · Lesson 6" --- # How does light bend? **Name:** {{field-24}} **Date:** {{field-15}} **Class:** {{field-15}} > **Key question:** A straw in a glass of water looks snapped at the surface. The straw is fine. What is the light doing? :::teacher-note **In one sentence:** 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). **Students should be able to say:** 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. **Width:** the unmarked parts are the floor every student does (Engage, Explore, Explain, Evaluate). Typical and Wider sections add contexts, not difficulty. Students work in the online tutorial alongside this sheet. ::: ## Engage :::keep-together **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 My prediction, and why: {{lines-3}} ::: :::teacher-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. ::: ## Explore Try it on the board in the tutorial. Was your prediction right? What was different? {{lines-2}} ## Explain: reflection and refraction :::keep-together ### 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 ::: :::teacher-note **Answer:** It slows down, so the wave fronts change direction 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. ::: :::keep-together ### 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 ::: :::teacher-note **Answer:** It slows down but doesn't change direction The whole wave front reaches the glass at the same moment and slows together, so it doesn't turn. ::: :::keep-together ### 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 ::: :::teacher-note **Answer:** 35° on the other side of the normal The angle of reflection equals the angle of incidence, measured from the normal. ::: :::keep-together ### 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 ::: :::teacher-note **Answer:** Light from the part under water bends as it leaves the water, so it seems to come from a different place 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. ::: :::typical ## Elaborate: mirror, glass, diamond :::keep-together ### 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 ::: :::teacher-note **Answer:** Light is slower in diamond than in glass The more the light slows down, the more the wave fronts bunch up and the more the direction changes. ::: ::: :::wider ## Elaborate: fibre optics :::keep-together **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 ::: :::teacher-note **Answer:** They carry signals as light, which has a high frequency and so can carry a lot of information 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. ::: ::: ## 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. ## Exit ticket :::keep-together **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?** {{lines-3}} ::: :::teacher-note **Answer:** It is slower in water than in air. The change of speed turns the wave fronts and so the ray. :::