# The Electromagnetic Spectrum

> Radio, microwaves, infrared, visible light, ultraviolet, X-rays and gamma rays. One family of waves with very different jobs.

From HSC Physics on WhizzNotes, free study notes for NSW students: https://whizznotes.com/physics/electromagnetic-spectrum/. Updated 2 November 2026.
Year 11 and Year 12 boxes go deeper than the Year 10 text around them.

### Story time: One family, seven jobs
On a normal Tuesday, Daniel's family used the whole electromagnetic spectrum without noticing.

His mum listened to the morning news on the **radio** in the car. Daniel warmed his leftover pasta in the **microwave**. His little brother changed the TV channel with the **infrared** remote. Everyone saw it all happen using **visible light**. At the beach after school, they put on sunscreen against **ultraviolet** rays. That evening, Daniel's grandad had an **X-ray** on his sore wrist. And the hospital down the road used **gamma rays** to kill cancer cells and sterilise equipment.

Seven very different jobs. But all seven are the same kind of wave, just with different wavelengths.

**In short:** The electromagnetic spectrum is the family of waves that all travel at the speed of light: radio, microwaves, infrared, visible light, ultraviolet, X-rays and gamma rays. From radio to gamma the wavelength gets shorter and the frequency and energy get higher, and UV, X-rays and gamma can damage cells.

## What are electromagnetic waves?

**Electromagnetic (EM) waves** are made of changing electric and magnetic fields. All EM waves:

- are **transverse** waves
- can travel through a **vacuum** (empty space). They do not need a medium
- travel at the **same speed** in a vacuum: the **speed of light**

$$c = 3.0 \times 10^8 \text{ m/s}$$

- carry **energy**
- can be reflected, refracted and absorbed

The only real difference between them is their **wavelength** (and so their **frequency**).

## The spectrum, in order

From **longest wavelength** (lowest frequency, lowest energy) to **shortest wavelength** (highest frequency, highest energy):

| Wave | Typical wavelength | Uses | Dangers |
|---|---|---|---|
| **Radio waves** | 1 m to many km | Radio, TV broadcasts, communication | Very low risk |
| **Microwaves** | 1 mm to 1 m | Cooking, mobile phones, Wi-Fi, satellites, radar | Heating of body tissue in large doses |
| **Infrared (IR)** | about 0.001 mm | Remote controls, heaters, thermal cameras, night vision | Burns |
| **Visible light** | 400 to 700 nm | Seeing, photography, optical fibres | Very bright light damages eyes |
| **Ultraviolet (UV)** | 10 to 400 nm | Sun tanning, sterilising water, security marks, makes vitamin D | Sunburn, skin cancer, eye damage |
| **X-rays** | about 0.01 to 10 nm | Medical images of bones, airport security | Can damage cells and cause cancer |
| **Gamma rays** | less than 0.01 nm | Killing cancer cells, sterilising medical tools, food | Most dangerous: damages cells and DNA |

### Exam tip: Memory trick
**R**ich **M**en **I**n **V**egas **U**se **X**-ray **G**lasses

Radio, Microwaves, Infrared, Visible, Ultraviolet, X-rays, Gamma.

### Key idea: The pattern
Going from radio towards gamma:
- wavelength gets **shorter**
- frequency gets **higher**
- energy gets **higher**
- the waves become **more dangerous** to living cells

Speed does **not** change. In a vacuum they all travel at $c$.

## Ionising radiation

**Ultraviolet**, **X-rays** and **gamma rays** carry enough energy to knock electrons out of atoms. This is called **ionising radiation**. It can damage DNA inside cells, which can cause mutations and cancer.

Protection:
- **Sunscreen**, hats and sunglasses block UV. Australia has one of the highest skin cancer rates in the world, so "Slip, Slop, Slap, Seek, Slide" really matters.
- Radiographers stand behind **lead screens** when taking X-rays.
- Gamma sources are stored in thick **lead or concrete** containers.

### Watch out: Radio waves are not sound waves
A radio receives **radio waves** (electromagnetic, transverse, travel at $3 \times 10^8$ m/s, can cross space). It then turns the signal into **sound waves** (longitudinal, need air, about 340 m/s). They are completely different kinds of wave.

## Using the wave equation

All EM waves obey $c = f\lambda$.

### Worked example: A radio station
An FM radio station broadcasts at 100 MHz ($1.0 \times 10^8$ Hz). What is the wavelength?

$$\lambda = \frac{c}{f} = \frac{3.0 \times 10^8}{1.0 \times 10^8} = 3 \text{ m}$$

**Mind map**

```text
EM spectrum
  All EM waves
    Transverse
    Travel in a vacuum
    Speed c = 3 × 10⁸ m/s
  Low energy end
    Radio: communication
    Microwaves: cooking, phones
    Infrared: heat, remotes
  Middle
    Visible light
    ROY G BIV
  High energy end
    Ultraviolet: sunburn
    X-rays: bones
    Gamma: cancer treatment
  Pattern
    Shorter wavelength
    Higher frequency
    More energy, more danger
```

### Year 11: How the different waves are made and absorbed
- **Radio waves** are made by charges oscillating in an antenna.
- **Infrared** is given off by all warm objects as their particles vibrate.
- **Visible and UV** come from electrons in atoms dropping to lower energy levels.
- **X-rays** are made when fast electrons slam into a metal target.
- **Gamma rays** come from changes inside atomic nuclei (radioactive decay).

Earth's **atmosphere** lets visible light and most radio waves through, but blocks most UV (the ozone layer), X-rays and gamma rays. That is why X-ray telescopes have to be in space.

### Year 12: Maxwell's prediction
In the 1860s, James Clerk Maxwell combined the laws of electricity and magnetism and found they predicted waves travelling at:

$$c = \frac{1}{\sqrt{\mu_0 \varepsilon_0}} \approx 3.0 \times 10^8 \text{ m/s}$$

This matched the measured speed of light exactly. Maxwell concluded that light **is** an electromagnetic wave. Heinrich Hertz then made radio waves in the lab in 1887, confirming the prediction. The constant speed of light for every observer later became the starting point of Einstein's special relativity.

## Quick summary

- EM waves are transverse, travel through a vacuum, and all move at $c = 3 \times 10^8$ m/s.
- Order (long to short wavelength): **radio, microwaves, infrared, visible, ultraviolet, X-rays, gamma**.
- Shorter wavelength means higher frequency and more energy.
- UV, X-rays and gamma are **ionising** and can damage cells.
- Every type has important uses, from Wi-Fi to cancer treatment.
- $c = f\lambda$ links frequency and wavelength for every EM wave.

## Quiz

Try these questions from the chapter quiz.

1. Which EM wave has the longest wavelength?
   - A. Radio waves
   - B. Microwaves
   - C. X-rays
   - D. Gamma rays
   - Answer: A. Radio waves. Radio waves are at the long wavelength end of the spectrum.

2. Which EM wave has the highest frequency?
   - A. Radio waves
   - B. Visible light
   - C. Ultraviolet
   - D. Gamma rays
   - Answer: D. Gamma rays. Gamma rays have the shortest wavelength and highest frequency.

3. In a vacuum, compared with radio waves, X-rays travel:
   - A. Faster
   - B. Slower
   - C. At the same speed
   - D. Not at all
   - Answer: C. At the same speed. All EM waves travel at c in a vacuum.

The whole chapter quiz is on the page, for signed-in students: https://whizznotes.com/physics/electromagnetic-spectrum/#quiz