What is an electromagnetic wave?
A changing electric field makes a changing magnetic field, and a changing magnetic field makes a changing electric field. Together they travel as an electromagnetic wave. This idea was worked out by James Clerk Maxwell in the 1860s; Heinrich Hertz first made and detected radio waves in 1887.
EM waves are made when charges accelerate (for example, electrons moving up and down an aerial) or when electrons in atoms jump between energy levels.
Properties shared by all EM waves
- They are transverse: the fields wiggle at right angles to the direction of travel.
- They need no medium; they cross the vacuum of space (that is how sunlight reaches us).
- In a vacuum they all travel at c = 3 × 10⁸ m/s (300,000 km every second).
- They carry energy from a source to an absorber.
- They can be reflected, refracted (bent when they change speed in a new material) and absorbed.
The seven types in order
From longest wavelength (lowest frequency, lowest energy) to shortest wavelength (highest frequency, highest energy):
- Radio waves: λ from about 1 mm up to many km
- Microwaves: about 1 mm to 30 cm
- Infrared: about 700 nm to 1 mm
- Visible light: about 400 nm (violet) to 700 nm (red)
- Ultraviolet: about 10 nm to 400 nm
- X-rays: about 0.01 nm to 10 nm
- Gamma rays: shorter than about 0.01 nm
(1 nm = 10⁻⁹ m.) The groups merge into each other; there are no sharp edges.
Our eyes detect only the visible part. Within it, red has the longest wavelength and violet the shortest: red, orange, yellow, green, blue, indigo, violet. An object looks red because it reflects red light and absorbs the other colours.
The wave equation: c = fλ
wave speed = frequency × wavelength, or c = f λ
- c in metres per second (m/s), f in hertz (Hz = waves per second), λ in metres (m).
- Since c is fixed in a vacuum, a higher frequency means a shorter wavelength.
- Rearranged: f = c ÷ λ and λ = c ÷ f.
Remember prefixes: kHz = 10³ Hz, MHz = 10⁶ Hz, GHz = 10⁹ Hz.
When an EM wave enters glass or water it slows down. Its frequency stays the same, so its wavelength gets shorter, and it bends (refraction) if it enters at an angle.
Uses and dangers
- Radio: radio and TV broadcasts, mobile communication. Made by oscillating currents in an aerial; they can also make a current in a receiving aerial. Very low risk.
- Microwaves: satellite links, Wi-Fi, cooking (water molecules absorb them and heat up). Risk: internal heating of body tissue at high power.
- Infrared: heaters, toasters, remote controls, thermal cameras, optical fibre data. Risk: skin burns.
- Visible: seeing, photography, optical fibres. Very bright light (lasers, looking at the Sun) can damage the eyes.
- Ultraviolet: energy-saving lamps, sterilising water, detecting fake notes, sun tanning. Risk: sunburn, early skin ageing, skin cancer, eye damage. Use sunscreen and sunglasses.
- X-rays: images of bones (they pass through soft tissue but are absorbed by bone), airport security. Risk: ionising, can damage cells; staff stand behind lead screens.
- Gamma: killing cancer cells (radiotherapy), sterilising medical tools and food. Risk: ionising, can cause mutations and cancer.
Ionising radiation has enough energy to remove electrons from atoms. UV (at its high-energy end), X-rays and gamma are ionising. Risk depends on the type, the dose and the time of exposure.
Try it
Point a TV remote at your phone camera and press a button: the camera can see the infrared flash that your eyes cannot. Next, put the remote behind a sheet of paper, then behind a steel plate, and see which one blocks the signal. Then slide through the 7 families in the 3D and predict, before you look, whether each one is ionising.
Key formulas and definitions
- c = f λ
- f = c ÷ λ
- λ = c ÷ f
- c = 3 × 10⁸ m/s (in a vacuum)
- Order (long λ → short λ): radio, microwave, infrared, visible, ultraviolet, X-ray, gamma
- Higher f → shorter λ → more energy
Worked examples
1. An FM station broadcasts at 100 MHz. Find the wavelength.
f = 100 MHz = 1 × 10⁸ Hz. λ = c ÷ f = 3 × 10⁸ ÷ 1 × 10⁸ = 3 m.
2. A microwave oven uses waves of λ = 0.12 m. Find the frequency.
f = c ÷ λ = 3 × 10⁸ ÷ 0.12 = 2.5 × 10⁹ Hz = 2.5 GHz.
3. Green light has λ = 500 nm. Find its frequency.
500 nm = 5 × 10⁻⁷ m. f = 3 × 10⁸ ÷ 5 × 10⁻⁷ = 6 × 10¹⁴ Hz.
4. An AM radio wave has f = 600 kHz. Find λ.
f = 6 × 10⁵ Hz. λ = 3 × 10⁸ ÷ 6 × 10⁵ = 500 m.
5. Light takes about 500 s to reach Earth from the Sun. Estimate the distance.
distance = speed × time = 3 × 10⁸ × 500 = 1.5 × 10¹¹ m (150 million km).
6. A wave's frequency is doubled in a vacuum. What happens to its wavelength and speed?
Speed stays 3 × 10⁸ m/s. Since c = fλ is fixed, λ halves.
Common mistakes
- Thinking gamma rays travel faster than radio waves. In a vacuum all EM waves have the same speed.
- Getting the order backwards. Radio has the LONGEST wavelength and LOWEST frequency; gamma the shortest and highest.
- Forgetting to convert MHz, GHz or nm before using c = fλ.
- Calling sound an EM wave. Sound is a longitudinal wave that needs a medium.