Revision summary

One spectrum, two orderings

Increasing frequency, decreasing wavelength: radio waves, microwaves, infrared, visible light, ultraviolet, X-rays, gamma rays.

Increasing wavelength, decreasing frequency: gamma rays, X-rays, ultraviolet, visible light, infrared, microwaves, radio waves. Within visible light, red is at the longer-wavelength end and violet at the shorter-wavelength end.

Shared properties

All electromagnetic waves are transverse and travel at the same speed in vacuum. They can travel without a material medium; sound cannot.

Frequency and wavelength

c = fλ; λ = c/f; f = c/λ. For the examples here, c = 3.0 x 10⁸ m/s. Higher frequency means shorter wavelength at the same speed.

Prefix reminders

1 MHz = 10⁶ Hz; 1 GHz = 10⁹ Hz; 1 nm = 10^-9 m. Convert to compatible units before substitution.

Applications to recall and explain

Radio waves

Radio and television communication; Astronomy; RFID tags.

Microwaves

Mobile phones; Microwave ovens; Satellite television.

Infrared

Infrared remote controllers; Intruder alarms; Thermal imaging.

Visible light

Photography; Optical fibres in medicine; Optical fibres in telecommunications.

Ultraviolet

Sunbeds; Bank-note authentication; Disinfecting water.

X-rays

Medical radiology; Security screening; Industrial defect detection.

Gamma rays

Sterilising food; Detecting cancer; Treating cancer.

For any application, identify the source, the radiation's path or absorption, and the detector or intended effect. A passive infrared alarm receives radiation; a remote emits a coded signal. An X-ray transmission image uses an external source; a gamma-tracer image records emissions from a tracer inside the body. Some optical-fibre links use visible light, while many use infrared.

Heating and ionising effects

• Heating: absorption raises internal energy and can raise temperature. Excessive heating can damage tissue.
• Ionisation: radiation removes electrons from atoms or molecules. The resulting changes can damage cells and DNA. X-rays and gamma rays are ionising examples.
• Ultraviolet: some sufficiently energetic UV is ionising, but not all UV is. UV can harm skin and eyes through molecular damage; sunbed exposure increases skin-cancer risk.
• Exposure conditions: the amount absorbed, duration, radiation type and tissue exposed matter. An application is not evidence that every exposure is harmless.

Keep the radiation distinct from what a device produces or displays: a radio receiver makes sound through a loudspeaker, and a thermal image uses assigned display colours for detected infrared information.