--- title: "Wave or particle?" subtitle: "Year 9 Physics · Wave and particle models · Lesson 7" --- # Wave or particle? **Name:** {{field-24}} **Date:** {{field-15}} **Class:** {{field-15}} > **Key question:** Is light a wave or a stream of particles? Scientists argued for 300 years. The answer turned out to be: the question is a bit too simple. :::teacher-note **In one sentence:** Students evaluate the wave and particle models against phenomena from the unit, say where each succeeds and fails, and explain that models are tools with domains of usefulness (AC9S9U04, node 5, an integration node). **Students should be able to say:** I can say which model explains each phenomenon best. I can say where a model stops working. I can explain why models are tools, not copies of reality. **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 **One scientist says 'light is a wave'. Another says 'light is a stream of particles'. What is the best way to think about this?** ☐ Scientists don't really know anything about light ☐ Each model is a tool: it explains some things well and others badly, so we use the one that suits the question ☐ One of them is right and the other is wrong ☐ It depends on which scientist you ask My prediction, and why: {{lines-3}} ::: :::teacher-note **What happens:** Each model is a tool. The wave model explains reflection, refraction and the spectrum. The particle idea (photons) explains some effects the wave model can't. Scientists use whichever helps with the question in hand. ::: ## Explain: where each model works :::keep-together ### Sort **Which model explains it best?** Groups: Wave model, Particle model - An echo: sound bouncing back off a cliff: {{field-15}} - Heat passing along a metal spoon: {{field-15}} - Light bending as it goes into glass: {{field-15}} - Warm water rising in a pot: {{field-15}} - A higher note having a shorter wavelength: {{field-15}} - A balloon charged by rubbing: {{field-15}} ::: :::teacher-note **Answer:** An echo: sound bouncing back off a cliff → Wave model; Heat passing along a metal spoon → Particle model; Light bending as it goes into glass → Wave model; Warm water rising in a pot → Particle model; A higher note having a shorter wavelength → Wave model; A balloon charged by rubbing → Particle model ::: :::keep-together ### Space **The particle model explains conduction well. Why can't it explain how the Sun's energy reaches Earth?** ☐ The Sun is too hot ☐ Energy can't cross space ☐ Space is a vacuum, with no particles to pass the energy on; the wave model of light needs no medium ::: :::teacher-note **Answer:** Space is a vacuum, with no particles to pass the energy on; the wave model of light needs no medium Conduction and convection need matter. Electromagnetic waves carry energy across empty space, so the wave model is the useful one here. ::: :::keep-together ### Photons **Light shining on some metals knocks electrons out, but only if the light's frequency is high enough, however bright it is. A wave model has trouble with this. What does it suggest?** ☐ Light also behaves as packets of energy (photons), each with an energy set by its frequency ☐ The metals are faulty ☐ Light isn't really a wave at all ::: :::teacher-note **Answer:** Light also behaves as packets of energy (photons), each with an energy set by its frequency Light behaves like a wave when it bends and spreads, and like a stream of photons when it hands energy to single electrons. Neither model alone covers everything. ::: :::keep-together ### Models are tools **Which statement about scientific models is best?** ☐ A model that fails once should be thrown away completely ☐ A good model is true in every situation ☐ A model is a tool with a domain of usefulness, not a literal description of reality ::: :::teacher-note **Answer:** A model is a tool with a domain of usefulness, not a literal description of reality We keep a model for as long as it helps us explain and predict, and we know where it stops working. ::: :::typical ## Elaborate: evaluate :::keep-together **Sound travels through air as a wave, but air is also made of particles. How can both models be right?** ☐ One of them is wrong ☐ They describe the same thing at different levels: the particle model says what air is made of, the wave model describes the pattern that travels through it ☐ Sound is a wave in space and particles in air ::: :::teacher-note **Answer:** They describe the same thing at different levels: the particle model says what air is made of, the wave model describes the pattern that travels through it The wave model of sound is built on the particle model of air: the wave is the pattern of squashing and stretching in the particles. Which one you use depends on the question. ::: ::: :::wider ## Elaborate: photons :::keep-together **Bright red light cannot knock electrons out of a metal, but a dim ultraviolet lamp can. Using photons, why?** ☐ Ultraviolet is brighter than red light ☐ Red light travels more slowly ☐ Each ultraviolet photon carries enough energy to free an electron; a red photon doesn't, however many arrive ::: :::teacher-note **Answer:** Each ultraviolet photon carries enough energy to free an electron; a red photon doesn't, however many arrive An electron absorbs one photon at a time. Only a photon with enough energy (high enough frequency) can free it, so many low-energy photons together can't do what one high-energy photon can. ::: ::: ## Evaluate: look back Write one sentence: "At first I thought… Now I think… because…" **Revision summary** - The **particle model** explains conduction, convection and static charge; the **wave model** explains sound, reflection, refraction and the electromagnetic spectrum. - Light behaves like a wave when it bends and spreads, and like **photons** when it gives energy to single electrons. - A scientific **model is a tool** with a domain of usefulness, not a literal picture of reality. ## Exit ticket :::keep-together **Give one phenomenon where the wave model is more useful than the particle model, and one where it is the other way round.** {{lines-3}} ::: :::teacher-note **Answer:** Wave: sound, reflection or refraction of light. Particle: conduction of heat, static charge. Any one of each, with a reason. :::