Microwaves used by a satellite link have a wavelength of 0.030 m. They travel at 3.0 × 108 m/s. What is their frequency?
- Rearrange v = fλ: f = v ÷ λ.
- f = 3.0 × 108 ÷ 0.030.
- f = 1.0 × 1010 Hz.
Answer: 1.0 × 1010 Hz
Electromagnetic waves: properties, uses and hazards
Physics · Waves. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.
The idea
Electromagnetic waves are transverse waves that transfer energy from a source to an absorber. They need no medium, and in a vacuum they all travel at the same speed. They form a continuous spectrum, split into seven groups by wavelength and frequency. Our eyes detect only visible light.
Refraction is the change of direction when a wave crosses a boundary at an angle. It happens because the wave speed changes. The part of each wave front that crosses first changes speed first, so the wave front turns.
Short wavelength waves have high frequency and carry the most energy. Ultraviolet, X-rays and gamma rays can be ionising, which makes them hazardous.
Rules to remember
- Order from long to short wavelength: radio, microwaves, infrared, visible, ultraviolet, X-rays, gamma rays. Frequency increases in the same order.
- All travel at 3.0 × 108 m/s in a vacuum; v = fλ applies.
- Slowing down at a boundary bends the ray towards the normal; speeding up bends it away. Frequency does not change.
- Radio waves are produced by oscillations in electrical circuits; when absorbed by an aerial they create an alternating current of the same frequency.
- Ultraviolet: premature skin ageing and skin cancer. X-rays and gamma rays: ionising, can cause gene mutation and cancer.
- Uses: radio (TV and radio), microwaves (satellites, cooking), infrared (heaters, cooking, infrared cameras), visible (fibre optics).
- Uses: ultraviolet (energy-efficient lamps, sun tanning), X-rays and gamma rays (medical imaging and treatment).
Common mistake
Students say the frequency changes when light enters glass. Frequency is fixed by the source; it is the speed and the wavelength that change.
Worked example
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