Revision notes for AQA GCSE Physics Types of electromagnetic waves. Open the guide for explanations and worked examples. Written against the AQA GCSE Physics (8463) specification, so the content matches what's examinable rather than general Physics background.
Types of electromagnetic waves
What you'll learn
- What electromagnetic waves are and why they are transverse waves.
- The order of the electromagnetic spectrum from radio waves to gamma rays.
- How wavelength and frequency change across the spectrum.
- How electromagnetic waves transfer energy from a source to an absorber.
Starting point: what is a wave?
A wave is a way of transferring energy from one place to another without transferring matter all the way from the source to the absorber.
For example, light from a lamp transfers energy to your eye, but bits of the lamp do not travel into your eye.
Definition
Source and absorber
A source is where the wave comes from. An absorber is the object or material that takes in energy from the wave.
Electromagnetic waves
Electromagnetic waves include light, radio waves, microwaves, X-rays and several other types of wave. They can travel through a vacuum, which means empty space. This is why light from the Sun can travel through space to Earth.
Definition
Electromagnetic wave
An electromagnetic wave is a transverse wave that transfers energy from the source of the wave to an absorber.
Unlike sound waves, electromagnetic waves do not need particles of air, water or solids to travel through. They can travel through air, but they can also travel through space.
Electromagnetic waves are transverse
A transverse wave is a wave where the oscillations are at right angles to the direction the wave travels.
Definition
Transverse wave
In a transverse wave, the vibrations or oscillations are perpendicular to the direction of energy transfer.
The diagram shows a simple way to picture a transverse electromagnetic wave. The wave travels from the source to the absorber, while the oscillation is up and down.
Key Idea
Energy transfer
Electromagnetic waves transfer energy from a source to an absorber. When the waves are absorbed, the absorber gains energy.
Example
Explaining energy transfer by infrared radiation
An electric heater is switched on and a student feels warmer nearby. Explain how energy is transferred to the student.
-
Identify the source: the electric heater emits infrared radiation, which is an electromagnetic wave.
-
Identify the absorber: the student’s skin absorbs some of the infrared radiation.
-
Apply the energy-transfer idea: the infrared waves transfer energy from the heater to the skin, increasing the internal energy of the skin, so the student feels warmer.
Wavelength and frequency
Electromagnetic waves are grouped by their wavelength and frequency.
Definition
Wavelength
Wavelength, symbol λ\lambdaλ, is the distance from one point on a wave to the same point on the next wave, such as crest to crest. It is measured in metres (m).
Definition
Frequency
Frequency, symbol fff, is the number of waves passing a point each second. It is measured in hertz (Hz).
For waves, the relationship between speed, frequency and wavelength is:
v=fλv = f\lambdav=fλwhere vvv is wave speed in metres per second (m/s), fff is frequency in hertz (Hz), and λ\lambdaλ is wavelength in metres (m).
For electromagnetic waves in a vacuum, the speed is about:
3.0×108 m/s3.0 \times 10^8 \text{ m/s}3.0×108 m/sAt GCSE, electromagnetic waves are also treated as travelling at the same speed through air.
Key Idea
Wavelength and frequency are linked
For electromagnetic waves in a vacuum or air, the speed is the same. So if frequency increases, wavelength decreases.
Example
Calculating the frequency of a microwave
A microwave has a wavelength of 0.12 m in air. Calculate its frequency using v=3.0×108 m/sv = 3.0 \times 10^8 \text{ m/s}v=3.0×108 m/s.
-
Rearrange the wave equation to make frequency the subject:
f=vλf = \frac{v}{\lambda}f=λv -
Substitute the wave speed and wavelength:
f=3.0×108 m/s0.12 mf = \frac{3.0 \times 10^8 \text{ m/s}}{0.12 \text{ m}}f=0.12 m3.0×108 m/s -
Calculate the answer and include the unit:
f=2.5×109 Hzf = 2.5 \times 10^9 \text{ Hz}f=2.5×109 Hz
Common Mistake
Long wavelength does not mean high frequency
Long wavelength and high frequency are opposites for electromagnetic waves in air or a vacuum. Radio waves have the longest wavelengths and the lowest frequencies; gamma rays have the shortest wavelengths and the highest frequencies.
The electromagnetic spectrum
The electromagnetic spectrum is the full range of electromagnetic waves.
Definition
Electromagnetic spectrum
The electromagnetic spectrum is the continuous range of electromagnetic waves, arranged by wavelength or frequency.
The spectrum is called continuous because there are no real gaps between the types. The names, such as “microwaves” and “infrared”, are useful group labels.
Going from long wavelength to short wavelength, the order is:
- Radio waves
- Microwaves
- Infrared
- Visible light
- Ultraviolet
- X-rays
- Gamma rays
Going in the same direction, the frequency goes from low frequency to high frequency.
Tip
Remembering the order
A useful mnemonic is: Raging Martians Invaded Venus Using X-ray Guns.
That gives: Radio, Microwave, Infrared, Visible, Ultraviolet, X-rays, Gamma rays.
Visible light is only a small part
Your eyes only detect visible light, so you only see a small range of the electromagnetic spectrum.
Visible light itself goes from red to violet:
- Red light has the longest wavelength within visible light.
- Violet light has the shortest wavelength within visible light.
- The colours in between include orange, yellow, green, blue and indigo.
Key Idea
Your eyes are limited detectors
There are many electromagnetic waves around you that your eyes cannot detect, including radio waves, microwaves, infrared, ultraviolet, X-rays and gamma rays.
Same speed in a vacuum or air
All electromagnetic waves travel at the same speed through a vacuum. That means gamma rays are not faster than radio waves in space — they just have a much higher frequency and a much shorter wavelength.
Through air, GCSE Physics treats them as travelling at the same speed too.
Common Mistake
The same-speed rule has a condition
The statement “all electromagnetic waves travel at the same speed” is for a vacuum or air. In other materials, such as glass or water, electromagnetic waves can slow down, and some types may be absorbed strongly.
Examples of energy transfer
You should be able to give examples showing that electromagnetic waves transfer energy.
| Type of electromagnetic wave | Example of energy transfer |
|---|---|
| Radio waves | Energy is transferred from a transmitter to a radio aerial. |
| Microwaves | Energy is transferred to food in a microwave oven. |
| Infrared | Energy is transferred from a heater or the Sun to your skin. |
| Visible light | Energy is transferred from a lamp to your eyes or to a solar cell. |
| Ultraviolet | Energy is transferred from the Sun to skin or to fluorescent materials. |
| X-rays | Energy is transferred through soft tissue to a detector; some is absorbed by bone. |
| Gamma rays | Energy is transferred from radioactive sources to materials or tissues that absorb them. |
Notice that the same basic pattern appears every time:
source → electromagnetic wave → absorber
Example
Choosing the correct wave group from wavelength and frequency
A wave has a very short wavelength and a very high frequency. Another wave has a very long wavelength and a very low frequency. Decide which end of the electromagnetic spectrum each wave belongs to.
-
Use the spectrum rule: moving from radio waves to gamma rays, wavelength decreases and frequency increases.
-
Match the very short wavelength and very high frequency wave to the gamma-ray end of the spectrum.
-
Match the very long wavelength and very low frequency wave to the radio-wave end of the spectrum.
Exam technique
In the exam
-
If asked for the order of the spectrum, write Radio, Microwave, Infrared, Visible, Ultraviolet, X-rays, Gamma rays and check that radio is the long-wavelength end.
-
If asked about energy transfer, name the source, the electromagnetic wave, and the absorber.
-
If using v=fλv = f\lambdav=fλ, make sure wavelength is in metres (m), frequency is in hertz (Hz), and wave speed is in metres per second (m/s).
Self review
Check yourself
- Which electromagnetic wave has a longer wavelength: microwaves or ultraviolet?
- Why can light from the Sun travel through space to Earth?
- What happens to frequency as wavelength decreases across the electromagnetic spectrum?
You've reached the end
Test yourself on this topic, or move on to the next guide.
How was this guide?