The electromagnetic spectrum
What you'll learn
- The main groups of the electromagnetic spectrum, in the correct order.
- How wavelength and frequency change from radio waves to gamma rays.
- Why your eyes only detect a small part of electromagnetic radiation.
- How materials and temperature can affect electromagnetic radiation.
Starting point: electromagnetic waves
Light is one type of electromagnetic radiation, but it is not the only type. Radio waves, microwaves, infrared, ultraviolet, x-rays and gamma rays are also electromagnetic radiation.
Electromagnetic radiation
Electromagnetic radiation is energy transferred by electromagnetic waves. These waves can travel through a vacuum, so they do not need particles of matter to move through.
At GCSE, it is useful to remember that all electromagnetic waves are transverse waves. A transverse wave has vibrations at right angles to the direction the wave travels.
All electromagnetic waves travel at the same speed in a vacuum: about 3.0×108 m/s3.0 \times 10^8\ \text{m/s}3.0×108 m/s. In air, their speed is very close to this value.
The wave ideas you need
Before learning the spectrum, make sure these words are secure:
Wavelength and frequency
Wavelength, symbol λ\lambdaλ, is the distance from one point on a wave to the same point on the next wave, for example crest to crest. It is measured in metres (m).
Frequency, symbol fff, is the number of complete waves passing a point each second. It is measured in hertz (Hz).
For waves, speed, frequency and wavelength are linked by:
v=fλv = f\lambdav=fλThis means that if the wave speed stays the same, a larger wavelength means a smaller frequency.
Comparing frequency and wavelength
A microwave and an infrared wave travel through air at the same speed. Infrared has a shorter wavelength than microwaves. Which has the higher frequency?
- Since both waves travel through air at the same speed, use the relationship v=fλv=f\lambdav=fλ to compare them.
- Rearrange the idea as f=vλf=\frac{v}{\lambda}f=λv. If vvv is the same, frequency depends on the size of the wavelength.
- Infrared has the smaller wavelength, so it must have the higher frequency.
The electromagnetic spectrum
The electromagnetic spectrum is the full range of electromagnetic radiation, from the longest wavelengths to the shortest wavelengths.
Continuous spectrum
The electromagnetic spectrum is continuous, meaning there are no real gaps between the different types. The named groups are useful labels, but the changes from one group to the next are gradual.
The main groups, in order, are:
radio waves → microwaves → infrared → visible light → ultraviolet → x-rays → gamma rays
Visible light is only a tiny part of the spectrum. Its colours, in order from longest visible wavelength to shortest visible wavelength, are:
red → orange → yellow → green → blue → indigo → violet
The big pattern
From radio waves to gamma rays, wavelength decreases and frequency increases.
Remembering the order
A common mnemonic is: Raging Martians Invaded Venus Using X-ray Guns.
That gives: radio, microwaves, infrared, visible, ultraviolet, x-rays, gamma rays.
Mixing up wavelength and frequency
Do not say that wavelength and frequency both increase across the spectrum. From radio waves to gamma rays, wavelength gets smaller while frequency gets larger.
What your eyes can detect
Your eyes can only detect a limited range of frequencies: visible light. You cannot see infrared, ultraviolet, microwaves or x-rays, even though they are all electromagnetic waves.
Different visible frequencies are interpreted by your brain as different colours. Red light has the longest wavelength in the visible spectrum, and violet light has the shortest wavelength in the visible spectrum.
How substances affect electromagnetic waves
On the Higher Tier, you should know that different substances can behave differently depending on the wavelength of electromagnetic radiation.
Four ways waves interact with matter
- Absorption: the wave’s energy is taken in by the material.
- Transmission: the wave passes through the material.
- Reflection: the wave bounces off the surface.
- Refraction: the wave changes direction when it enters a different substance because its speed changes.
For example, window glass transmits most visible light, but it absorbs more ultraviolet than visible light. A black surface absorbs visible light strongly, while a shiny surface reflects much more of it.
Materials are wavelength-selective
A material does not have to treat all electromagnetic waves in the same way. It might transmit one wavelength but absorb or reflect another.
Why speed changes matter
Also for Higher Tier, you need to explain the effects of electromagnetic waves travelling at different speeds in different substances.
When a wave enters a new substance, its frequency stays the same, but its speed may change. Since v=fλv=f\lambdav=fλ, a change in speed causes a change in wavelength.
If a wave enters the new substance at an angle, the speed change can make it change direction. This is refraction.
Different wavelengths can have slightly different speeds in the same substance. This is why a prism can split white light into different colours: the colours refract by different amounts.
Calculating wavelength in glass
A visible light wave has frequency 5.0×1014 Hz5.0 \times 10^{14}\ \text{Hz}5.0×1014 Hz. Its speed in air is 3.0×108 m/s3.0 \times 10^8\ \text{m/s}3.0×108 m/s and its speed in glass is 2.0×108 m/s2.0 \times 10^8\ \text{m/s}2.0×108 m/s. Find its wavelength in air and in glass.
- Use v=fλv=f\lambdav=fλ, rearranged to λ=vf\lambda=\frac{v}{f}λ=fv.
- In air: λair=3.0×108 m/s5.0×1014 Hz=6.0×10−7 m\lambda_{\text{air}}=\frac{3.0 \times 10^8\ \text{m/s}}{5.0 \times 10^{14}\ \text{Hz}}=6.0 \times 10^{-7}\ \text{m}λair=5.0×1014 Hz3.0×108 m/s=6.0×10−7 m.
- In glass: λglass=2.0×108 m/s5.0×1014 Hz=4.0×10−7 m\lambda_{\text{glass}}=\frac{2.0 \times 10^8\ \text{m/s}}{5.0 \times 10^{14}\ \text{Hz}}=4.0 \times 10^{-7}\ \text{m}λglass=5.0×1014 Hz2.0×108 m/s=4.0×10−7 m.
- The frequency is unchanged, but the speed is lower in glass, so the wavelength is shorter in glass.
Speed in vacuum versus speed in materials
All electromagnetic waves have the same speed in a vacuum. In materials such as glass, water or plastic, their speeds can be lower and can depend on wavelength.
Radiation emitted by bodies
If you are taking Separate Physics, you also need this idea: all bodies emit radiation. At everyday temperatures, objects mainly emit infrared radiation.
Intensity and wavelength distribution
Intensity means how strong the emitted radiation is. The wavelength distribution describes how much radiation is emitted at different wavelengths.
The temperature of a body affects the radiation it emits. A hotter body emits radiation with greater intensity and with a peak at a shorter wavelength.
This is why very hot objects can glow red, then orange-white, as their emitted radiation shifts into the visible part of the spectrum.
Constant temperature and power balance
For Separate Physics Higher Tier, you should also know the power-balance idea.
Power balance
Power is the rate of energy transfer, measured in watts (W). For a body to stay at a constant temperature, it must radiate the same average power as it absorbs.
So, for constant temperature:
Pabsorbed=PradiatedP_{\text{absorbed}} = P_{\text{radiated}}Pabsorbed=PradiatedIf a body absorbs more power than it radiates, its temperature increases. If it radiates more power than it absorbs, its temperature decreases.
Checking power balance
A body absorbs 120 W of radiation and radiates 95 W.
- Compare the absorbed power and radiated power: 120 W is greater than 95 W.
- Find the net power gain: 120 W minus 95 W gives 25 W gained.
- Since the body is gaining energy each second, its temperature will increase.
- It will only be at constant temperature when the average power radiated becomes equal to the average power absorbed.
In the exam
- When asked for the order of the spectrum, write the full sequence carefully: radio, microwaves, infrared, visible, ultraviolet, x-rays, gamma rays.
- If comparing wavelength and frequency, state the link clearly: as wavelength decreases, frequency increases.
- For material questions, use the correct word: absorb, transmit, reflect or refract. Do not just say “the wave is affected”.
Check yourself
- Can you write the electromagnetic spectrum in order from radio waves to gamma rays?
- Which visible colour has the longest wavelength, and which has the shortest?
- What must be true about absorbed power and radiated power if a body is at constant temperature?
