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# What is the electromagnetic spectrum?

Year 9 Physics · Wave and particle models

<!-- Speaker note: 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). About 26 minutes (Engage and Explore 8 min · Explain 11 min · Evaluate 3 min · Typical adds 3 min, Wider about 2 min). -->

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## Today

**We are learning to** light is a transverse wave, and it is one small part of a family of electromagnetic waves: radio, microwave, infrared, visible light, ultraviolet, X-rays and gamma rays. They all travel at the same speed in a vacuum (300 000 km/s) and differ in frequency and wavelength. Higher frequency means a shorter wavelength and more energy, which decides what each is used for and how careful we must be with it.

- 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.

<!-- 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 How does current flow? and comes before How does light bend? The pattern is the point: wavelength down, frequency up, energy up. The wave at the bottom of the board is a schematic with a few cycles per band: the real wavelengths span about fifteen powers of ten. Figures are typical and rounded: radio 3 m (100 MHz), microwave 12 cm (2.5 GHz), infrared 10 µm, visible 500 nm, UV 100 nm, X-ray 1 nm, gamma 1 pm. Be clear that 'more energy per wave' is different from 'more powerful': a strong radio transmitter delivers more power than a dim UV lamp, but each radio wave carries little energy. Teacher: In your own words: the labelled line of seven bands. -->

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## Why it matters

> 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?

<!-- Speaker note: Sound needed particles to carry it. Light doesn't. This lesson meets the wave model for light and for its whole family. -->

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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

**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

<!-- Speaker 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. Take two or three predictions before anyone runs it. -->

<!-- Speaker note: Teacher: tally the predictions. Ask the 'microwaves' group what makes an oven powerful. -->

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## Explore it

{{embed: https://studio.learningwaypoints.com/tutorials/_playground-board/index.html?id=deck%2Fscience%2Fphysics-9%2Flight%2Fstep-2&code=widget%3A+wave%0Amode%3A+spectrum%0Aband%3A+visible%0A&fresh=1}}

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

<!-- Speaker note:  -->

<!-- Speaker note: Teacher: ask what pattern they notice from Radio to Gamma. -->

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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?

* 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.

<!-- 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>

## 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

* Answer: The vibration is at right angles to the direction the wave travels

<!-- Speaker note: 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. -->

<!-- Speaker note: Teacher: the energy question and the uses match 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>

## 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

* Answer: They are the same: 300 000 km/s

<!-- Speaker note: All electromagnetic waves travel at the same speed in a vacuum, the speed of light. They differ in frequency and wavelength, not speed. -->

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## 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

* Answer: The wavelength gets shorter and the energy greater

<!-- Speaker note: Speed is fixed, so a higher frequency means a shorter wavelength, and a higher frequency means more energy in each wave. -->

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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>

## Uses

**Match each wave to a use.**

| | |
|---|---|
| `Radio waves` | A. Sterilising water and checking banknotes |
| `Microwaves` | B. Imaging bones and checking luggage |
| `Infrared` | C. Treating cancer and sterilising equipment |
| `Ultraviolet` | D. TV remotes and thermal cameras |
| `X-rays` | E. Broadcasting to radios and TVs |
| `Gamma rays` | F. Cooking food, Wi-Fi and mobile phones |

* 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

<!-- Speaker note:  -->

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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>

## 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

* Answer: Each wave carries a lot more energy, enough to damage cells

<!-- Speaker note: 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. -->

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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: choosing a wave

{{embed: https://studio.learningwaypoints.com/tutorials/_playground-board/index.html?id=deck%2Fscience%2Fphysics-9%2Flight%2Fstep-4&code=widget%3A+wave%0Amode%3A+spectrum%0Aband%3A+visible%0A&fresh=1}}

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

<!-- Speaker note: What this step is for: Choosing a band for a job from wavelength, frequency and risk applies the pattern to real communication and medical technologies. -->

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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>

## 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

* Answer: Microwaves carry information well and are safe at low power, but X-rays would damage cells

<!-- Speaker note: A communication technology needs waves that carry signals and are safe to use all around us. High-energy X-rays would be dangerous. -->

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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: sun safety

**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

* Answer: UV has enough energy per wave to damage skin cells, and you can't feel it as heat

<!-- Speaker note: 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. What this step is for: Linking the wave model to UV damage explains protective behaviour through the wave property that matters, frequency. -->

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## What helped

**Did it help Australians take up sun safety?**

Groups: Helped adoption · Not a factor

Strong evidence linking UV exposure to skin cancer · A short, catchy message with cheap, easy actions · 'No hat, no play' rules in schools · The UV Index in daily weather reports · UV feeling hot on the skin, so people notice it

* 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

<!-- Speaker note:  -->

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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>

## 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

* 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

<!-- Speaker note: 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. -->

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## 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.

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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 student says X-rays are dangerous because they are faster than light. Correct them in a sentence.**

* 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.

<!-- 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: How does light bend? uses the wave model to explain reflection and refraction. In class: a prism or a CD to show visible light spreading; a TV remote seen through a phone camera (infrared); UV beads that change colour in sunlight; discuss local sun-safety rules. -->