This note covers the electromagnetic (EM) spectrum: the seven families of waves, what they have in common, how they differ, and how they are used and what harm they can cause. It also covers the difference between analogue and digital signals, which is important in communications.
Key ideas
The order of the spectrum. The EM spectrum is a continuous family of waves. In order of decreasing wavelength (and increasing frequency) the groups are:
radio waves, microwaves, infrared, visible light, ultraviolet, X-rays, gamma rays.
Within visible light, the colours run from red (longest wavelength, lowest frequency) through orange, yellow, green and blue to violet (shortest wavelength, highest frequency).
What all EM waves have in common.
- They are transverse waves.
- They transfer energy and can travel through a vacuum, so no medium is needed.
- In a vacuum (and approximately in air) they all travel at the same speed, 3.0 × 108 m/s. This is the speed of light.
- They can be reflected, refracted and diffracted, and they obey v = f λ.
Because the speed is fixed, a higher frequency means a shorter wavelength. The energy carried by each wave also increases with frequency, which is why gamma rays and X-rays are the most harmful.
Uses and dangers.
- Radio waves: broadcasting radio and television, and communicating with aircraft and satellites. Very low risk at everyday intensity.
- Microwaves: microwave ovens (water molecules in food absorb the energy), mobile phones, Wi-Fi, satellite communication. Danger: internal heating of body tissue.
- Infrared: remote controls, electric grills and toasters, thermal imaging cameras and night vision. Danger: skin burns.
- Visible light: sight, photography, optical fibres. Very bright light, such as a laser, can damage the eye.
- Ultraviolet: detecting security ink on banknotes, sterilising water, sunbeds and fluorescent lamps. Danger: skin burns, skin cancer and eye damage.
- X-rays: imaging bones and in airport baggage scanners. Danger: damages cells and can cause cancer.
- Gamma rays: sterilising medical equipment and food, and treating cancer. Danger: kills cells and causes cancer.
The danger from ultraviolet, X-rays and gamma rays is greater because they carry high energy and can damage or change the molecules in living cells. Protection includes keeping exposure time short, staying further away, and using shielding such as lead for X-rays and gamma rays.
Wave speed in a vacuum. Light and all EM waves cover vast distances in space. The Sun is about 1.5 × 1011 m from the Earth, and light still arrives in a few minutes, using speed = distance ÷ time.
Analogue and digital signals. Information (sound, pictures, data) is carried by waves in two main ways.
- Analogue signals vary continuously. Their amplitude or frequency can take any value in a range. They follow the original sound or picture closely.
- Digital signals have only two values, usually represented by 1 and 0 (on and off, high and low). Information is sent as a sequence of pulses.
All signals weaken (attenuate) and pick up unwanted extra signals (noise) as they travel. Amplifiers boost them. An amplifier on an analogue signal boosts the noise as well, so the noise stays. A digital signal can be cleaned: the receiver only has to decide whether each pulse is a 1 or a 0, so the original pulses can be recreated exactly. Digital signals therefore:
- keep their quality better over long distances,
- can carry more information per second (greater data rate),
- can be stored and processed by computers easily.
Both kinds of signal can be sent using radio waves, microwaves, infrared or light in optical fibres. Optical fibres send digital pulses of light and have low energy loss.
Definitions and formulae
| Term or formula | Meaning / when to use |
|---|---|
| Order of EM spectrum | Radio, microwave, infrared, visible, ultraviolet, X-ray, gamma (left to right: shorter wavelength, higher frequency). |
| Speed of EM waves in a vacuum | 3.0 × 108 m/s, the same for every EM wave. |
| v = f λ | Link between speed, frequency and wavelength. Use v = 3.0 × 108 m/s for EM waves. |
| speed = distance ÷ time | Used for light travel times over long distances. |
| Analogue signal | Signal that varies continuously in amplitude or frequency. |
| Digital signal | Signal with only two states (1 and 0). |
| Noise | Unwanted random variations added to a signal during transmission. |
Worked examples
Example 1 A radio station broadcasts at a frequency of 98.0 MHz. Find the wavelength of the waves. (Speed of EM waves = 3.00 × 108 m/s.)
Convert the frequency: 98.0 MHz = 98.0 × 106 Hz = 9.80 × 107 Hz.
λ = v ÷ f = 3.00 × 108 ÷ 9.80 × 107 = 3.06 m.
Answer: 3.06 m
Example 2 Wi-Fi and microwave ovens use microwaves of frequency 2.45 GHz. Find their wavelength, and say whether this wavelength is longer or shorter than that of visible light.
2.45 GHz = 2.45 × 109 Hz.
λ = v ÷ f = 3.00 × 108 ÷ 2.45 × 109 = 0.122 m (12.2 cm).
Visible light has wavelengths of less than a millionth of a metre, so microwaves have a much longer wavelength.
Answer: 0.122 m, much longer than visible light
Example 3 The Sun is 1.5 × 1011 m from the Earth. How long does light take to reach us? Give your answer in minutes.
t = distance ÷ speed = 1.5 × 1011 ÷ 3.0 × 108 = 500 s.
500 ÷ 60 = 8.33 minutes.
Answer: 8.3 minutes (500 s)
Common errors
- Common error: mixing up the order of the spectrum. A helpful check is that the dangerous ones (ultraviolet, X-rays, gamma) are at the high frequency end.
- Common error: saying that different EM waves travel at different speeds in a vacuum. They all travel at 3.0 × 108 m/s.
- Common error: forgetting to convert MHz or GHz to Hz before using v = f λ.
- Common error: saying that X-rays and gamma rays are dangerous because they are fast. They are dangerous because they carry high energy and ionise or damage cells.
- Common error: saying that digital signals have no noise. Digital signals pick up noise too, but the receiver can recover the original pulses.
Exam tip: When asked about the harm caused by a type of radiation, say what it does to the body (for example, damage to cells or burns) as well as naming the danger.
Check yourself
- Put these in order of increasing frequency: X-rays, radio waves, visible light, infrared.
Answer
Radio waves, infrared, visible light, X-rays. - Green light has a wavelength of 5.0 × 10−7 m. Find its frequency.
Answer
f = v ÷ λ = 3.0 × 108 ÷ 5.0 × 10−7 = 6.0 × 1014 Hz. - Give one use and one danger of ultraviolet radiation.
Answer
Use: sterilising water (or checking banknotes, fluorescent lamps). Danger: skin cancer, skin burns or eye damage. - Explain one advantage of sending information as a digital signal rather than as an analogue signal.
Answer
A digital signal has only two states, so after it has weakened and picked up noise, the original 1s and 0s can be recreated exactly. An analogue signal cannot be fully cleaned because noise changes its continuous values. Digital signals therefore keep their quality over long distances.