Electromagnetic (EM) waves are a family of waves: radio waves, microwaves, infrared, visible light, ultraviolet, X-rays and gamma rays. They look very different in everyday life, but they are all the same kind of wave and differ only in their frequency and wavelength. Unlike sound waves, EM waves do not need a medium, which is why sunlight can cross the vacuum of space to reach the Earth.
The topic is almost entirely recall of properties, order, uses and hazards. It is regularly tested in Paper 1 MCQ and in short-answer Paper 2 questions. These notes cover the five core properties, the order of the seven regions, the use of each region with its reason, the heating and ionising hazards, and calculations with the wave equation.
What these notes cover
- •What are electromagnetic waves?
- •The five core properties
- •What happens when an EM wave meets a surface
- •The spectrum in order
- •Visible light and beams of light
- •Wavelength or frequency: reading the question
- •Uses of each region
- •Linking property to use
- •Hazards of over-exposure
- •Heating effect: microwaves and infrared
- •Ionising effect: ultraviolet, X-rays and gamma rays
- •Common mistakes and exam technique
Key definitions
- Electromagnetic (EM) waves
- A family of waves (radio waves, microwaves, infrared, visible light, ultraviolet, X-rays and gamma rays) that are all the same kind of wave, differ only in frequency and wavelength, and do not need a medium to travel through.
- Transverse wave
- A wave whose oscillations are perpendicular to the direction in which the wave travels (the direction of energy transfer). All EM waves are transverse.
- Ionising radiation
- Ultraviolet, X-rays and gamma rays. Each of these waves carries enough energy to remove electrons from atoms in the material that absorbs it.
- Heating effect
- The hazard of microwaves and infrared: absorbed energy becomes internal energy and raises the temperature of tissue. These waves are not ionising.
- Fluorescence
- When certain chemicals absorb invisible ultraviolet and re-emit the energy as visible light, so they glow.
Key formulas
Wave equation
v = fλ
v is the wave speed in m/s, f is the frequency in hertz (Hz) and λ is the wavelength in metres. All EM waves obey it.
Wave equation for EM waves in vacuum
c = fλ, where c = 3.0 × 10⁸ m/s
c is the speed of light. It is the same for every EM wave in vacuum (or air). Because c is fixed, a higher frequency always means a shorter wavelength.
Frequency from wavelength
f = c ÷ λ
Rearranged wave equation. λ must be in metres before substituting.
Wavelength from frequency
λ = c ÷ f
Rearranged wave equation. f must be in hertz before substituting.
Energy trend across the spectrum
energy carried by the wave ∝ f
Energy increases from radio waves towards gamma rays, so higher frequency means more energy.
Unit conversions
1 MHz = 10⁶ Hz; 1 GHz = 10⁹ Hz; 1 nm = 10⁻⁹ m
The wave equation only works in SI units. Convert to Hz and m first, then substitute.
Worked example
A local radio station broadcasts at a frequency of 93.8 MHz. Calculate the wavelength of this radio wave in vacuum, given that the speed of all electromagnetic waves in vacuum is c = 3.0 × 10⁸ m/s. Give your answer to 2 significant figures.
- 1Convert the frequency to SI units: mega (M) means 10⁶, so f = 93.8 MHz = 93.8 × 10⁶ Hz = 9.38 × 10⁷ Hz.
- 2Rearrange the wave equation for wavelength: v = fλ, so λ = v ÷ f.
- 3Substitute and evaluate: λ = (3.0 × 10⁸) ÷ (9.38 × 10⁷) = 3.198 m.
- 4Round to 2 significant figures: λ ≈ 3.2 m.
- 5Sense check: radio waves sit at the long-wavelength end of the spectrum, so a wavelength of a few metres is what we should expect.
Watch out: Forgetting the 10⁶ in MHz gives an answer of about 3 × 10⁶ m, which is an absurd size and should signal a slip. Convert to Hz before substituting.
Common mistakes
Writing the order in the wrong direction
Wrong: Reciting the mnemonic forwards when asked to arrange ultraviolet, microwaves and visible light in order of increasing wavelength, giving microwaves, visible light, ultraviolet.
Right: Increasing wavelength and increasing frequency are opposite orders. Underline the key word first. In order of increasing wavelength the answer is ultraviolet, visible light, microwaves.
Thinking gamma rays travel fastest
Wrong: Gamma rays travel fastest because they carry the most energy.
Right: All EM waves travel at the same speed in vacuum, 3.0 × 10⁸ m/s. What differs across the spectrum is frequency and wavelength, not speed.
Giving a vague hazard
Wrong: It is dangerous or harmful to health.
Right: State the mechanism. Microwaves and infrared heat water-rich tissue. UV, X-rays and gamma rays are ionising: ionising radiation damages or kills living cells and can cause mutation, leading to cancer.
Questions students ask
What are the 7 regions of the electromagnetic spectrum in order?
In order of increasing frequency (decreasing wavelength), the regions are radio waves, microwaves, infrared, visible light, ultraviolet, X-rays and gamma rays. For increasing wavelength, reverse the list so that gamma rays come first. Radio waves have the longest wavelength and lowest frequency, and gamma rays have the shortest wavelength and highest frequency.
Do all electromagnetic waves travel at the same speed?
Yes. All EM waves travel at 3.0 × 10⁸ m/s in vacuum, so a radio wave is exactly as fast as a gamma ray. In glass or water they slow down, and different frequencies slow by different amounts, which is why a prism disperses white light.
Which electromagnetic waves are ionising?
Ultraviolet, X-rays and gamma rays are ionising because they carry enough energy to remove electrons from atoms. Visible light and everything below it (infrared, microwaves and radio waves) is non-ionising. Ionising radiation damages or kills living cells and can cause mutation, leading to cancer.
How do you answer 'state one use and explain why' for EM waves?
Name the use, then give the property of the region that makes it suitable and link it to the outcome. For example, microwaves are used in microwave ovens because microwaves of a particular wavelength are strongly absorbed by water molecules in food, heating the water and cooking the food. Restating the use without a property earns only the first mark.
In the full PDF
- •Labelled diagram of the spectrum with typical wavelengths for each region, plus ray diagrams of parallel, converging and diverging beams
- •The full uses table for all seven regions, giving the reason each region suits its job
- •The hazards-at-a-glance table with protective measures for microwaves, infrared, ultraviolet, X-rays and gamma rays
- •Further worked examples: frequency of red light, X-ray frequency, green light from nm, thermal-imaging alarm, microwave oven door, radiographer's lead screen and sunscreen
- •The remaining wrong-vs-right misconception corrections and the three-step routine for any EM spectrum question
- •Spectrum mnemonics and end-of-section summaries