The Electromagnetic Spectrum: Radio to Visible
The electromagnetic spectrum arranges EM waves by frequency or wavelength; from long to short wavelength the main regions are radio waves, microwaves, infrared and visible light (followed by ultraviolet, X-rays and gamma rays), and each region has its own sources, detectors and uses. In brief: radio waves have wavelengths longer than about 1 m, microwaves from about 1 mm to 1 m, infrared from about 700 nm to 1 mm and visible light from about 380 to 750 nm. All travel at c in a vacuum, with c = fλ. Lower-frequency waves carry less energy per photon (E = hf). The regions overlap slightly, and the boundaries are conventions.
Radio, TV, Wi-Fi, mobile phones, remote controls, night-vision cameras and our eyes all use different parts of the same spectrum. This guide covers the low-frequency half of the spectrum (the high-energy half is in the next article), describes the sources, behavior and applications of each region, compares them in a table, works through numerical examples and corrects common mistakes.
What are radio waves?
Radio waves have the longest wavelengths (from about 1 m to hundreds of kilometers) and the lowest frequencies (below 300 MHz). They are produced by alternating currents in antennas and are used for communication: AM radio (about 530 to 1,700 kHz), FM radio and TV (about 88 to 108 MHz and in the VHF/UHF bands), and shortwave. Information is carried by modulating the amplitude (AM) or frequency (FM) of a carrier wave, or digitally. Long waves follow the curvature of the Earth (ground waves), and some are reflected by the ionosphere (sky waves), allowing long-distance reception at night; higher frequencies pass through the ionosphere and travel in straight lines, so satellites and TV rely on the line of sight. Radio astronomers observe radio waves from stars, pulsars and the cosmic microwave background.
What are microwaves?
Microwaves have wavelengths between about 1 mm and 1 m (frequencies of 300 MHz to 300 GHz). They are used for radar, satellite communication, mobile phones (about 0.7 to 3.5 GHz), Wi-Fi (2.4 and 5 GHz), and microwave ovens (2.45 GHz). In an oven, the oscillating electric field makes water molecules, which are polar, rotate and oscillate, and the energy is dissipated by friction between molecules as heat in the food. Microwaves penetrate a few centimeters into food. Metal reflects microwaves, so the oven walls are metal and a mesh in the door keeps the radiation in (the holes are much smaller than the 12 cm wavelength). Radar measures the distance from the time delay of the echo and the speed of the target from the Doppler shift.
What is infrared radiation?
Infrared (IR), wavelengths from about 700 nm to 1 mm, is emitted by all warm objects (thermal radiation); the human body emits mainly at about 9 to 10 µm. Sources: the Sun, flames, heaters, living bodies. Uses: thermal imaging cameras and night vision, remote controls (near infrared, about 940 nm), optical fiber communications (1,310 and 1,550 nm), toasters and grills, physiotherapy lamps, and infrared astronomy (seeing through dust). Infrared is absorbed by greenhouse gases such as carbon dioxide and water vapor, which makes it the key radiation in the climate system: the Earth emits infrared into space. Infrared is felt as heat. Intense infrared can burn the skin or damage the eyes.
What is visible light?
Visible light, from about 380 nm (violet) to 750 nm (red), is the narrow range detected by the human eye. It makes up a large part of the Sun’s output because the eye evolved to use the radiation most abundant at the surface of the Earth, where the atmosphere is transparent. The colors correspond to wavelengths: violet 380 to 450 nm, blue 450 to 495 nm, green 495 to 570 nm, yellow 570 to 590 nm, orange 590 to 620 nm, red 620 to 750 nm. Sources: the Sun, incandescent lamps, LEDs and lasers. Light drives photosynthesis, vision and solar cells.
Regions compared
| Region | Wavelength | Frequency | Typical use |
|---|---|---|---|
| Radio | above 1 m | below 300 MHz | Broadcasting |
| Microwaves | 1 mm to 1 m | 300 MHz to 300 GHz | Radar, Wi-Fi, ovens |
| Infrared | 700 nm to 1 mm | 300 GHz to 430 THz | Thermal imaging, remote controls |
| Visible | 380 to 750 nm | 400 to 790 THz | Vision, lighting |
Worked example: wavelengths and antennas
- FM radio. At 100 MHz, λ = c ÷ f = 3.0 m. A quarter-wave antenna has a length of λ/4 = 0.75 m, which is why FM antennas are about 75 cm long.
- Wi-Fi. At 2.4 GHz, λ = 3.00 × 108 ÷ 2.4 × 109 = 0.125 m = 12.5 cm. At 5 GHz, λ = 6 cm.
- Microwave oven. At 2.45 GHz, λ = 12.2 cm. The mesh holes of about 2 mm are much smaller than this wavelength, so the waves cannot pass, while light (λ ≈ 500 nm) passes through them and lets you look inside.
- Body radiation. Wien’s law at 310 K gives a peak at 2.898 × 10−3 ÷ 310 = 9.3 µm, in the infrared, so thermal cameras are sensitive in the 8 to 14 µm band.
- Photon energies. A photon of FM radio (100 MHz) has E = hf = 6.63 × 10−34 × 108 = 6.6 × 10−26 J = 4.1 × 10−7 eV, far too small to break chemical bonds (a few eV). A photon of green light (550 nm) has 2.3 eV.
- Radar echo. A radar pulse returns from an aircraft after 200 µs: distance = c t ÷ 2 = 3.00 × 108 × 2.0 × 10−4 ÷ 2 = 30 km.
How was the spectrum explored?
William Herschel discovered infrared in 1800 by measuring the temperature beyond the red end of the solar spectrum. Hertz produced radio waves in 1887; Marconi made wireless telegraphy practical in the 1890s. Radar was developed in the 1930s by Robert Watson-Watt and others, and Percy Spencer noticed in 1945 that microwaves from a radar set melted a candy bar in his pocket, which led to the microwave oven. Karl Jansky detected radio waves from the Milky Way in 1932, the beginning of radio astronomy.
Which mistakes are common?
Mistake 1: Thinking that microwaves heat food from the inside out
Microwaves penetrate a few centimeters and heat the outer layers directly. The inside is heated by conduction.
Mistake 2: Confusing infrared with heat itself
Infrared is electromagnetic radiation. Heat is the energy transferred; infrared transfers energy when it is absorbed.
Mistake 3: Believing that radio waves are sound waves
A radio converts the EM signal to an electrical signal and then to sound in a loudspeaker.
Mistake 4: Thinking that the visible spectrum is special in physics
It is a narrow band, defined by the sensitivity of the human eye.