The Electromagnetic Spectrum
The Electromagnetic Spectrum
The electromagnetic (EM) spectrum is a continuous range of waves, all of which travel at the speed of light in a vacuum (\(3 \times 10^8\) m/s). All EM waves are transverse and can travel through a vacuum — they do not need a medium.
The seven types of EM wave in order of decreasing wavelength (increasing frequency) are: radio waves, microwaves, infrared, visible light, ultraviolet, X-rays, and gamma rays. Use the mnemonic: Running Men In Vests Usually X-ray Giraffes.
All EM waves share the same basic properties: they transfer energy, travel at \(3 \times 10^8\) m/s in a vacuum, and obey the wave equation \(v = f\lambda\). Since speed is constant, waves with higher frequency have shorter wavelength.
Uses and Dangers
Each type of EM wave has specific uses that depend on its properties.
Radio waves have the longest wavelengths and are used for television, radio broadcasts, and communication. Different wavelengths are used for different purposes — long wave can diffract around hills, while short wave can be reflected off the ionosphere for long-distance communication.
Microwaves are used in satellite communication, mobile phone signals, and microwave ovens. In ovens, microwaves cause water molecules to vibrate, heating the food.
Infrared radiation is emitted by all warm objects. It is used in remote controls, thermal imaging cameras, cooking, and optical fibre communication. Too much infrared can cause skin burns.
Visible light is the only part of the EM spectrum we can see. It is used in photography, fibre optic communication, and vision. It ranges from red (longest wavelength) to violet (shortest).
Ultraviolet (UV) causes skin to tan and is used in fluorescent lamps, security marking, and detecting forged banknotes. Overexposure causes sunburn and increases the risk of skin cancer.
X-rays can pass through soft tissue but are absorbed by bone and metal. They are used in medical imaging and airport security scanners. Overexposure can cause cell damage and cancer.
Gamma rays have the highest frequency and shortest wavelength. They are emitted by radioactive nuclei and used in cancer treatment (radiotherapy), sterilising medical equipment, and as tracers in medical imaging. They are the most penetrating and can cause serious cell damage.
The danger of EM radiation generally increases with frequency. Radio waves and microwaves pose low risk. UV causes sunburn and skin cancer. X-rays and gamma rays can damage cells and DNA, potentially causing cancer and mutations.
Emission and Absorption of Infrared
All objects emit and absorb infrared radiation. The hotter an object, the more infrared it emits. Dark, matt surfaces are the best emitters and absorbers of infrared. Light, shiny surfaces are the worst emitters and absorbers (they are good reflectors instead).
Fill a Leslie cube (a metal cube with four different surfaces: shiny black, matt black, shiny white, matt white) with hot water. Use an infrared detector at the same distance from each face. The matt black surface emits the most infrared; the shiny white surface emits the least.