The electromagnetic spectrum, explained
Electromagnetic waves range from radio waves kilometres long to gamma rays smaller than an atom. They all travel at the speed of light, and each type has its own uses and dangers.
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What electromagnetic waves are
Electromagnetic (EM) waves are transverse waves: the oscillations are at right angles to the direction the wave travels. Unlike sound or water waves, they do not need a medium, so they can travel through a vacuum. That is how light and heat reach us from the Sun across empty space. All EM waves travel at the same speed in a vacuum, about 3 × 108 m/s, the speed of light.
EM waves transfer energy from a source to an absorber. A microwave oven transfers energy to food; the Sun transfers energy to the Earth. They form a continuous spectrum, grouped into seven types by wavelength and frequency.
The order of the spectrum
From longest wavelength and lowest frequency to shortest wavelength and highest frequency:
- Radio waves
- Microwaves
- Infrared
- Visible light (red, orange, yellow, green, blue, indigo, violet)
- Ultraviolet
- X-rays
- Gamma rays
A popular way to remember the order: Rich Men In Vegas Use eXpensive Gadgets. As you move along the list, wavelength gets shorter, frequency gets higher, and the energy carried by each wave increases. Higher energy is why the waves at the gamma end are the most dangerous.
Wavelength and frequency
All EM waves obey the wave equation v = fλ, where v is wave speed in m/s, f is frequency in hertz and λ is wavelength in metres. Because v is the same for every EM wave in a vacuum, frequency and wavelength are inversely related: if one doubles, the other halves.
Example 1: radio wavelength
A radio station broadcasts at 100 MHz. Find the wavelength.
- f = 100 × 106 Hz. λ = v/f = 3 × 108/1 × 108.
- λ = 3 m.
Example 2: microwave frequency
Microwaves in an oven have a wavelength of 12 cm. Find their frequency.
- λ = 0.12 m. f = v/λ = 3 × 108/0.12.
- f = 2.5 × 109 Hz, or 2.5 GHz.
Example 3: visible light
Green light has a wavelength of 5.5 × 10−7 m. Find its frequency.
- f = 3 × 108/5.5 × 10−7 ≈ 5.5 × 1014 Hz.
Uses of each type
- Radio waves: television and radio broadcasting. Long wavelengths can diffract around hills and travel long distances.
- Microwaves: satellite communications and mobile phones (they pass through the atmosphere), and cooking (water molecules in food absorb them).
- Infrared: electric heaters, cooking, thermal imaging cameras, TV remote controls and optical fibres.
- Visible light: vision, photography and fibre optic communication.
- Ultraviolet: energy-efficient lamps, sun tanning, security marking and detecting forged bank notes, and sterilising water.
- X-rays: medical imaging of bones and airport security scanners. X-rays pass through soft tissue but are absorbed by bone and metal.
- Gamma rays: sterilising medical equipment and food, and treating cancer with radiotherapy.
Dangers
The risk from EM waves increases with frequency, because higher-frequency waves carry more energy.
- Microwaves and infrared can heat body tissue and cause burns at high intensities.
- Ultraviolet can damage skin cells, causing sunburn and premature ageing, and increases the risk of skin cancer. It can also damage the eyes.
- X-rays and gamma rays are ionising radiation. They can knock electrons from atoms, damaging DNA and causing mutations that may lead to cancer.
The size of the risk depends on the type of radiation and the dose. Medical staff using X-rays stand behind lead screens or leave the room, and the dose to patients is kept as low as possible. The risk is measured in sieverts (Sv), usually millisieverts (mSv).
Infrared and temperature
All objects emit and absorb infrared radiation, whatever their temperature. The hotter an object is, the more infrared it emits in a given time. An object at constant temperature absorbs radiation at the same rate as it emits it. Matt black surfaces are the best absorbers and emitters, while shiny white surfaces are the poorest, which is why the infrared required practical compares surfaces of different colours.
How EM waves interact with materials
When an EM wave meets a material, it can be reflected, absorbed or transmitted, and its speed can change, causing refraction. What happens depends on the wavelength and the material. Glass transmits visible light but absorbs much ultraviolet; bone absorbs X-rays while soft tissue transmits them. This difference is exactly what makes many uses possible.
Separate Physics students also learn that radio waves can be produced by oscillations in electrical circuits, and that when radio waves are absorbed they can create an alternating current with the same frequency, which is how an aerial receives a signal. Changes in atoms and in the nuclei of atoms can generate or absorb EM waves across a wide frequency range; gamma rays originate from changes in the nucleus.
Common mistakes
- Mixing up the order, especially ultraviolet and infrared on either side of visible light.
- Saying higher frequency waves travel faster; all EM waves travel at the same speed in a vacuum.
- Forgetting to convert MHz, GHz, cm or nm into standard units before using v = fλ.
- Calling microwaves or radio waves ionising; only the high-frequency end (some UV, X-rays and gamma) is ionising.
Practice questions
Try each one before opening the answer.
Q1. Which EM wave has the longest wavelength?
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Q2. Which type of EM radiation is used in thermal imaging cameras?
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Q3. Find the wavelength of a 300 MHz radio wave.
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Q4. Why are X-rays useful for imaging broken bones?
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Q5. Give one danger of ultraviolet radiation and one way to reduce it.
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Frequently asked questions
Do I need to learn the actual wavelengths?
You need the order and the general idea that wavelengths range from about 104 m for radio waves to about 10−15 m for gamma rays. Exact values for each type are not usually required.
Is the electromagnetic spectrum in Combined Science?
Yes. It is part of the waves topic in both Combined Science and separate Physics, though separate Physics goes into more detail.
Key takeaways
- EM waves are transverse and travel at 3 × 108 m/s in a vacuum.
- Order: radio, microwaves, infrared, visible, UV, X-rays, gamma.
- Shorter wavelength means higher frequency and more energy.
- Use v = fλ with standard units.
- UV, X-rays and gamma are the most harmful; X-rays and gamma are ionising.