--- marp: true theme: default html: true --- # Wave or particle? Year 9 Physics · Wave and particle models --- ## Today **We are learning to** a model is a tool, not a photograph. The particle model explains conduction, convection and static charge; the wave model explains sound, reflection, refraction and the electromagnetic spectrum. Light shows both wave-like and particle-like behaviour, so neither model alone explains it all: each is useful for some phenomena and weak for others. - 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. --- ## Why it matters > 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. --- 💬 Discussion ## Engage: predict **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 --- ✅ Check ## What happened? * 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. --- ✅ Check ## Sort **Which model explains it best?** Groups: Wave model · Particle model An echo: sound bouncing back off a cliff · Heat passing along a metal spoon · Light bending as it goes into glass · Warm water rising in a pot · A higher note having a shorter wavelength · A balloon charged by rubbing * 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 --- ✅ Check ## 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 * Answer: Space is a vacuum, with no particles to pass the energy on; the wave model of light needs no medium --- ✅ Check ## 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 * Answer: Light also behaves as packets of energy (photons), each with an energy set by its frequency --- ✅ Check ## 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 * Answer: A model is a tool with a domain of usefulness, not a literal description of reality --- ✅ Check ## Elaborate: evaluate **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 * 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 --- ✅ Check ## Elaborate: photons **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 * Answer: Each ultraviolet photon carries enough energy to free an electron; a red photon doesn't, however many arrive --- ## 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. --- ✅ Check ## Exit ticket **Give one phenomenon where the wave model is more useful than the particle model, and one where it is the other way round.** * Wave: sound, reflection or refraction of light. Particle: conduction of heat, static charge. Any one of each, with a reason.