🔴 The red end
Red light has the longest wavelength and the lowest frequency of the visible colours. Next to it, just outside the visible region, is infrared. Infrared has a longer wavelength than red light.
The Electromagnetic Spectrum » The Electromagnetic Spectrum
What you'll learn this session
Study time: 30 minutes
Edexcel spec: 3.10, 3.11
- That light is one part of a continuous electromagnetic spectrum
- The seven regions of the spectrum and what they all have in common
- The order of the regions by wavelength and by frequency
- The order of the seven colours of the visible spectrum
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Unlock This CourseLight is part of a bigger family
The light you see with your eyes is a wave. But it is only one small member of a much bigger family of waves. The rest of the family is invisible. Your eyes cannot detect these waves, but they are the same kind of wave as light. Together, the whole family is called the electromagnetic spectrum.
The spec names seven parts of the spectrum: radio, microwave, infrared, visible, ultraviolet, x-ray and gamma ray radiations. Each part is often called a region.
Key terms:
- Electromagnetic wave: a transverse wave that can travel through a vacuum. Light is one example.
- Electromagnetic spectrum: the full, continuous range of electromagnetic waves, from radio waves to gamma rays.
- Continuous spectrum: a spectrum with no gaps in it. Every wavelength is there, and each region blends into the next.
- Free space: a vacuum. This is a space with no particles in it.
- Visible spectrum: the small region of the electromagnetic spectrum that the human eye can detect.
What all electromagnetic waves have in common
The seven regions seem very different. But every wave in the spectrum shares the same features. These are easy marks in the exam, so learn them.
🌊 All are transverse
Remember, in a transverse wave the vibrations are at right angles to the direction the wave travels. This is true for every electromagnetic wave, from radio waves to gamma rays.
🚀 All travel in a vacuum
Electromagnetic waves do not need particles to carry them. That is how light and infrared from the Sun reach the Earth across empty space.
⚡ All have the same speed
In free space, all electromagnetic waves travel at the same speed. This speed is 3 × 108 m/s, which is 300 000 000 metres every second.
The key words in the spec are 'the same speed in free space'. A gamma ray does not travel faster than a radio wave. In a vacuum, all seven regions travel at exactly the same speed.
Here is a way to picture it. The light from a far away star takes years to cross space. That light is a mixture of colours, and the star gives out invisible electromagnetic waves too. They all set off together and they all arrive together, because they all travel at the same speed in free space.
Electromagnetic waves only travel at this speed in a vacuum. They travel more slowly in materials such as glass and water. You will see what that does to light in Refraction of Light.
The seven regions in order
You must know the order of the seven regions. The spec gives the order in terms of decreasing wavelength and increasing frequency. That means the list starts with the longest wavelength and ends with the shortest.
| Order | Region | Wavelength | Frequency | A typical wavelength is about the size of... |
|---|---|---|---|---|
| 1 | Radio waves | longest | lowest | a building or a mountain |
| 2 | Microwaves | a fingernail (about a centimetre) | ||
| 3 | Infrared | the width of a fine hair or smaller | ||
| 4 | Visible light | about half of one thousandth of a millimetre | ||
| 5 | Ultraviolet | smaller still | ||
| 6 | X-rays | an atom | ||
| 7 | Gamma rays | shortest | highest | smaller than an atom |
You do not need to learn the sizes in the last column. They are there to show how huge the range is. What you must learn is the order of the names, and which end has the longest wavelength and which end has the highest frequency.
To draw the spectrum in the exam, draw a long box split into seven parts. Write the names in order from radio waves on the left to gamma rays on the right. Under the box, draw one arrow pointing right labelled 'increasing frequency'. Draw a second arrow pointing right labelled 'decreasing wavelength'.
A memory aid
Take the first letter of each region: R M I V U X G. Then learn a sentence with the same letters:
Raging Martians Invaded Venus Using X-ray Guns.
Radio, Microwaves, Infrared, Visible, Ultraviolet, X-rays, Gamma rays. You can make up your own sentence if it sticks better.
Why shorter wavelength means higher frequency
The two orders always go together. This is not something extra to learn. It follows from the fact that all the waves have the same speed in free space.
Remember from The Wave Equation that wave speed = frequency × wavelength. If the speed is the same for every region, then a wave with a shorter wavelength must have a higher frequency. A wave with a longer wavelength must have a lower frequency. So gamma rays, with the shortest wavelength, have the highest frequency. Radio waves, with the longest wavelength, have the lowest frequency.
The spectrum is continuous, so there are no sharp lines between the regions. The longest microwaves blend into the shortest radio waves, and so on along the spectrum. The seven names are labels that people have given to parts of one unbroken range.
The colours of the visible spectrum
Visible light sits in the middle of the spectrum, between infrared and ultraviolet. It is a very narrow region, but it is the only one our eyes can detect. Inside it, different wavelengths are seen as different colours. You need to know the seven colours in order.
| Colour | Wavelength | Frequency |
|---|---|---|
| Red | longest | lowest |
| Orange | ||
| Yellow | ||
| Green | ||
| Blue | ||
| Indigo | ||
| Violet | shortest | highest |
This is the order of the colours in a rainbow. A well known memory aid is Richard Of York Gave Battle In Vain: Red, Orange, Yellow, Green, Blue, Indigo, Violet.
🔮 The violet end
Violet light has the shortest wavelength and the highest frequency of the visible colours. Next to it, just outside the visible region, is ultraviolet. Ultraviolet has a shorter wavelength than violet light.
Notice that the colour order fits the order of the whole spectrum. Going from radio waves to gamma rays, you pass through the visible region from red to violet.
How the invisible neighbours were found
In 1800 the astronomer William Herschel used a prism to split sunlight into its colours. He put a thermometer just beyond the red end, where no light could be seen. Its temperature still went up. He had discovered infrared. A year later Johann Ritter tested the region just beyond the violet end with a chemical that goes dark in light. It went dark there too, even faster than in violet light. He had discovered ultraviolet. You will not be asked for these names or dates, but the story shows that the spectrum carries on beyond both ends of what we can see.
Worked example
Question: Here are four regions of the electromagnetic spectrum: ultraviolet, microwaves, gamma rays, infrared. (a) Put them in order of increasing frequency. (b) State which of the four has the longest wavelength.
Step 1: Write out the full order from the memory aid: Radio, Microwaves, Infrared, Visible, Ultraviolet, X-rays, Gamma rays. This list goes from lowest frequency to highest frequency.
Step 2: Pick out the four regions in the question, keeping them in the same order.
(a) microwaves, infrared, ultraviolet, gamma rays.
(b) The lowest frequency goes with the longest wavelength, so the answer is microwaves.
Common mistakes
- Saying gamma rays or X-rays travel faster. In free space every electromagnetic wave travels at the same speed. Only the wavelength and the frequency are different.
- Getting the two orders the wrong way round. Check which order the question wants. Radio waves have the longest wavelength but the lowest frequency.
- Swapping infrared and ultraviolet. Infrared sits next to red. Ultraviolet sits next to violet.
- Leaving out 'in free space' or 'in a vacuum'. The waves only all travel at the same speed in a vacuum, so put those words in your answer.
- Calling the waves longitudinal. All electromagnetic waves are transverse.
Exam-style question
The list shows some of the regions of the electromagnetic spectrum in order.
radio waves | microwaves | P | visible light | ultraviolet | Q | gamma rays
(a) State the names of regions P and Q. [2 marks]
(b) State which region in the list has the longest wavelength. [1 mark]
(c) State two properties that are the same for all electromagnetic waves. [2 marks]
(d) Visible light is made up of seven colours. (i) State the colour of visible light that has the highest frequency. [1 mark] (ii) State the colour that comes between yellow and blue in the visible spectrum. [1 mark]
(e) Explain why microwaves have a lower frequency than ultraviolet waves when both travel in free space. [2 marks]
Model answer
(a) P is infrared (1). Q is X-rays (1).
(b) Radio waves (1).
(c) Any two from: they are all transverse waves (1); they all travel at the same speed in free space (a vacuum) (1); they can all travel through a vacuum (1).
(d) (i) Violet (1). (ii) Green (1).
(e) Both waves travel at the same speed in free space (1). Microwaves have a longer wavelength, and wave speed = frequency × wavelength, so their frequency must be lower (1).
Exam tip
Before you answer any question like this, write the seven first letters R M I V U X G in the margin and label the left end 'longest wavelength, lowest frequency'. Then parts like (a), (b) and (e) become a matter of reading along your own list, and you will not mix up the two ends.