Waves you can’t see
Visible light is just a tiny slice of a much larger family of waves called electromagnetic waves. Radio waves, microwaves, X-rays and more are all the same kind of wave, differing only in wavelength and frequency.
What are electromagnetic waves?
An electromagnetic wave is made of electric and magnetic fields that vibrate at right angles to each other and to the direction of travel. They:
- are transverse waves
- don’t need a medium; they can travel through space
- all travel at the speed of light: about 300 000 km/s in a vacuum
- carry energy
James Clerk Maxwell predicted these waves in the 1860s. Heinrich Hertz first produced and detected radio waves in 1887; the unit of frequency, the hertz, is named after him.
The electromagnetic spectrum
The electromagnetic spectrum arranges these waves from longest wavelength (lowest frequency and energy) to shortest wavelength (highest frequency and energy).
| Type | Wavelength | Example source | Uses |
|---|---|---|---|
| Radio waves | Longer than about 30 cm (up to kilometres) | Radio transmitters, stars | Radio, TV, cell phones, MRI |
| Microwaves | About 1 mm – 30 cm | Microwave ovens, satellites | Cooking, Wi-Fi, radar, GPS |
| Infrared | About 700 nm – 1 mm | Warm objects, the Sun | TV remotes, thermal cameras, heat lamps |
| Visible light | 400–700 nm | The Sun, bulbs | Sight, photography, fibre optics |
| Ultraviolet | About 10–400 nm | The Sun, black lights | Sterilizing, vitamin D, detecting counterfeit money |
| X-rays | About 0.01–10 nm | X-ray tubes | Medical and dental imaging, airport security |
| Gamma rays | Shorter than about 0.01 nm | Radioactive materials, exploding stars | Cancer treatment, sterilizing equipment |
Memory trick (long to short): Red Martians Invaded Venus Using X-ray Guns.
Each type up close
Radio waves
Radio waves can be longer than a football field. They carry radio, TV and cell signals. Radio telescopes detect radio waves from galaxies and pulsars.
Microwaves
Microwave ovens use waves that make water molecules in food vibrate, heating the food. Wi-Fi, Bluetooth, GPS and weather radar also use microwaves.
Infrared
All warm objects give off infrared. You feel it as heat from a fire or the Sun. Thermal cameras show warm objects glowing, which helps firefighters find people in smoke. TV remotes send infrared signals.
Visible light
The only part of the spectrum human eyes can detect. It ranges from red (longest) to violet (shortest).
Ultraviolet
UV light causes sunburn and tans, helps your skin make vitamin D, and makes some materials glow under black lights. Bees can see UV patterns on flowers that are invisible to us. The ozone layer blocks most of the Sun’s harmful UV.
X-rays
X-rays pass through soft tissue but are absorbed by bones and teeth, making them useful for medical images. Wilhelm Röntgen discovered X-rays in 1895; one of his first images was of his wife’s hand, showing her bones and ring.
Gamma rays
The most energetic waves. They come from radioactive materials, nuclear reactions and violent events in space like supernovas. Doctors aim gamma rays at tumours to destroy cancer cells.
Energy and safety
The higher the frequency, the more energy each wave carries.
| Lower energy (non-ionizing) | Higher energy (ionizing) |
|---|---|
| Radio, microwaves, infrared, visible light | High-energy UV, X-rays, gamma rays |
| Generally safe at normal levels | Can damage DNA and cells |
That’s why:
- you wear sunscreen to block UV
- X-ray technicians stand behind shields and patients wear lead aprons
- radioactive materials are handled carefully
Cell phones and Wi-Fi use non-ionizing microwaves, which don’t have enough energy to damage DNA.
Seeing the universe
Astronomers use the whole spectrum. Different types of waves reveal different things: radio shows cold gas, infrared shows dusty star nurseries (the James Webb Space Telescope sees mainly infrared), and X-rays show super-hot gas around black holes.
Common mistakes
- Thinking only visible light is "light." All EM waves are the same kind of wave.
- Thinking radio waves are sound waves. They’re electromagnetic; radios convert them into sound.
- Thinking all radiation is dangerous.
Try it yourself
- Point a TV remote at a phone camera and press a button. Can you see the infrared light?
- Make a poster of the EM spectrum with one use and one danger for each type.
- Rank by energy: X-rays, radio waves, visible light, infrared, gamma rays.
(Answer to 3: radio < infrared < visible < X-rays < gamma)
Quick recap
- Electromagnetic waves travel through space at the speed of light.
- The spectrum runs from radio waves to gamma rays.
- Shorter wavelength means higher frequency and more energy.
- Each type has important uses in communication, medicine and science.
- High-energy waves like X-rays and gamma rays can damage cells.