--- title: "What is the electromagnetic spectrum?" subtitle: "Year 9 Physics · Wave and particle models · Lesson 5" --- # What is the electromagnetic spectrum? **Name:** {{field-24}} **Date:** {{field-15}} **Class:** {{field-15}} > **Key question:** A sunburn, a microwave oven, a TV remote and a hospital X-ray all use the same kind of wave. What makes them so different? :::teacher-note **In one sentence:** Students describe light and the electromagnetic spectrum as transverse waves differing in frequency and wavelength, and link frequency to energy and use (AC9S9U04, node 4). **Students should be able to say:** I can say what makes light and X-rays the same kind of wave. I can say how wavelength, frequency and energy change from radio to gamma. I can match a wave to a use and a risk. **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 **Microwaves and X-rays are both electromagnetic waves. Which has more energy in each wave?** ☐ X-rays: a higher frequency means more energy ☐ You can't compare them ☐ They are the same, because they travel at the same speed ☐ Microwaves: they can cook food My prediction, and why: {{lines-3}} ::: :::teacher-note **What happens:** X-rays. Energy goes up with frequency, and X-rays have a frequency millions of times higher than microwaves, or more. A microwave oven cooks food because it is powerful, not because each wave has a lot of energy. ::: ## Explore Try it on the board in the tutorial. Was your prediction right? What was different? {{lines-2}} ## Explain: the wave model of light :::keep-together ### Transverse **Light is a transverse wave. What does that mean?** ☐ The wave needs particles to travel through ☐ The vibration is along the direction of travel, like sound ☐ The vibration is at right angles to the direction the wave travels ::: :::teacher-note **Answer:** The vibration is at right angles to the direction the wave travels Transverse: like a rope shaken sideways, the vibration is across the direction of travel. Electromagnetic waves are vibrating electric and magnetic fields, so they need no particles. ::: :::keep-together ### Same speed **Radio waves and gamma rays travel through a vacuum. How do their speeds compare?** ☐ Gamma rays are much faster ☐ Radio waves are faster because they are longer ☐ They are the same: 300 000 km/s ::: :::teacher-note **Answer:** They are the same: 300 000 km/s All electromagnetic waves travel at the same speed in a vacuum, the speed of light. They differ in frequency and wavelength, not speed. ::: :::keep-together ### Wavelength and energy **Going from radio to gamma, what happens to the wavelength and the energy?** ☐ The wavelength gets shorter and the energy smaller ☐ The wavelength gets longer and the energy greater ☐ The wavelength gets shorter and the energy greater ::: :::teacher-note **Answer:** The wavelength gets shorter and the energy greater Speed is fixed, so a higher frequency means a shorter wavelength, and a higher frequency means more energy in each wave. ::: :::keep-together ### Uses **Match each wave to a use.** A. Sterilising water and checking banknotes B. Imaging bones and checking luggage C. Treating cancer and sterilising equipment D. TV remotes and thermal cameras E. Broadcasting to radios and TVs F. Cooking food, Wi-Fi and mobile phones {{columns 60% 40%}} {{rows 1}} | Part | Letter | |---|---| | `Radio waves` | | | `Microwaves` | | | `Infrared` | | | `Ultraviolet` | | | `X-rays` | | | `Gamma rays` | | ::: :::teacher-note **Answer:** Radio waves → E. Broadcasting to radios and TVs; Microwaves → F. Cooking food, Wi-Fi and mobile phones; Infrared → D. TV remotes and thermal cameras; Ultraviolet → A. Sterilising water and checking banknotes; X-rays → B. Imaging bones and checking luggage; Gamma rays → C. Treating cancer and sterilising equipment ::: :::keep-together ### Danger **Why are UV, X-rays and gamma rays more dangerous to living cells than radio waves?** ☐ Each wave carries a lot more energy, enough to damage cells ☐ They are brighter ☐ They travel faster ::: :::teacher-note **Answer:** Each wave carries a lot more energy, enough to damage cells High-frequency waves carry much more energy in each wave, enough to damage the molecules in cells. Low-frequency waves such as radio don't. ::: :::typical ## Elaborate: choosing a wave :::keep-together ### Microwave or X-ray **Why does a mobile phone use microwaves and not X-rays?** ☐ X-rays travel too slowly ☐ Microwaves are visible ☐ Microwaves carry information well and are safe at low power, but X-rays would damage cells ::: :::teacher-note **Answer:** Microwaves carry information well and are safe at low power, but X-rays would damage cells A communication technology needs waves that carry signals and are safe to use all around us. High-energy X-rays would be dangerous. ::: ::: :::wider ## Elaborate: sun safety :::keep-together **Why does Australian sun safety advice focus on UV, even on cool, cloudy days?** ☐ UV has enough energy per wave to damage skin cells, and you can't feel it as heat ☐ UV is the hottest part of sunlight ☐ Clouds make UV stronger ::: :::teacher-note **Answer:** UV has enough energy per wave to damage skin cells, and you can't feel it as heat UV is invisible and not felt as heat, but each wave carries enough energy to damage skin cells. That's why shade, clothing, a hat and sunscreen matter on any bright day. ::: ::: :::typical ## Elaborate: Slip, Slop, Slap :::keep-together ### What helped **Did it help Australians take up sun safety?** Groups: Helped adoption, Not a factor - Strong evidence linking UV exposure to skin cancer: {{field-15}} - A short, catchy message with cheap, easy actions: {{field-15}} - 'No hat, no play' rules in schools: {{field-15}} - The UV Index in daily weather reports: {{field-15}} - UV feeling hot on the skin, so people notice it: {{field-15}} ::: :::teacher-note **Answer:** Strong evidence linking UV exposure to skin cancer → Helped adoption; A short, catchy message with cheap, easy actions → Helped adoption; 'No hat, no play' rules in schools → Helped adoption; The UV Index in daily weather reports → Helped adoption; UV feeling hot on the skin, so people notice it → Not a factor ::: :::keep-together ### Evidence is not enough **Scientists knew UV causes skin cancer well before 1981, but sun-safe habits spread only after the campaigns began. What does that show?** ☐ Campaigns only work when the science is brand new ☐ The science changed in 1981 ☐ Sunscreen wasn't on sale in Australia before 1981 ☐ Evidence alone isn't enough: science is taken up when the message is clear, the actions are easy and people keep hearing it from sources they trust ::: :::teacher-note **Answer:** Evidence alone isn't enough: science is taken up when the message is clear, the actions are easy and people keep hearing it from sources they trust Adoption depends on more than evidence: cost, convenience, trust, repetition and rules all matter. Australia still has one of the highest skin cancer rates in the world, so the campaigns continue. ::: ::: ## Evaluate: look back Write one sentence: "At first I thought… Now I think… because…" **Revision summary** - Light is a **transverse wave** and one part of the **electromagnetic spectrum**: radio, microwave, infrared, visible, ultraviolet, X-ray, gamma. - All travel at the **same speed** in a vacuum (300 000 km/s), with no medium needed. - Radio to gamma: **shorter wavelength, higher frequency, more energy per wave**. - Frequency decides use and risk: low-energy waves carry signals; high-energy waves can damage cells. ## Exit ticket :::keep-together **A student says X-rays are dangerous because they are faster than light. Correct them in a sentence.** {{lines-3}} ::: :::teacher-note **Answer:** X-rays travel at the same speed as light. They are dangerous because each wave has a high frequency and carries a lot of energy, enough to damage cells. :::