What is the electromagnetic spectrum? Radio waves to gamma rays

The electromagnetic spectrum is the whole range of electromagnetic radiation, sorted by wavelength: radio waves, microwaves, infrared, visible light, ultraviolet, X-rays and gamma rays. They are all the same thing, waves of electric and magnetic fields traveling at the speed of light. What differs is the wavelength, and with it the energy each photon carries, which decides what the radiation can do to matter.

In short

• Radio, microwaves, infrared, visible light, ultraviolet, X-rays and gamma rays are one phenomenon at different wavelengths. Visible light is a thin slice in the middle.
• Shorter wavelength means higher frequency and more energy per photon. Above about 10 electronvolts per photon, radiation can knock electrons off atoms: it is ionizing.
• Earth's air blocks most of the spectrum and lets a few windows through, mainly visible light and some radio, which is why X-ray telescopes have to fly in space.

What is electromagnetic radiation?

Electromagnetic radiation is a traveling disturbance of electric and magnetic fields, each one's changes regenerating the other, so it needs no material to move through. It is a wave, with a wavelength and a frequency, and in a vacuum it always moves at the speed of light, 299,792,458 m/s. Because speed equals frequency times wavelength, a longer wave has a lower frequency and a shorter one a higher frequency. It is also delivered in photons, packets of energy: a photon's energy is E = h × f, the Planck constant times the frequency. Visible light is the part your eyes happen to detect.

A handy shortcut: a photon's energy in electronvolts (eV) is about 1,240 divided by its wavelength in nanometers. An electronvolt is the energy one electron gains crossing 1 volt, about 1.6 × 10⁻¹⁹ joules, the natural unit for things that happen to single atoms and molecules.

What are the bands of the spectrum?

The bands are named for how they were discovered and used, not by any sharp physical boundary, so the edges below are conventions and sources draw them slightly differently. X-rays and gamma rays overlap: the names often say where the radiation came from, electrons or atomic nuclei, rather than its energy.

Band	Wavelength	Frequency	Energy per photon	Everyday uses and sources
Radio	longer than 1 m	below 300 MHz	below about 1 millionth of an eV	AM and FM radio, broadcast TV, radio astronomy
Microwaves	1 mm to 1 m	300 MHz to 300 GHz	about 1 millionth to 1 thousandth of an eV	Phones, Wi-Fi, microwave ovens, radar, GPS
Infrared	700 nm to 1 mm	300 GHz to 430 THz	about 0.001 to 1.8 eV	Heat from warm objects, TV remotes, thermal cameras, fiber-optic links
Visible light	about 380 to 700 nm	430 to 790 THz	about 1.8 to 3.3 eV	Sight, photosynthesis
Ultraviolet	10 to 380 nm	790 THz to 30 PHz	about 3.3 to 120 eV	Sunburn, vitamin D, sterilizing lamps, black lights
X-rays	about 0.01 to 10 nm	30 PHz to 30 EHz	about 120 eV to 120,000 eV	Medical and airport imaging, crystallography
Gamma rays	shorter than about 0.01 nm	above 30 EHz	above about 120,000 eV	Radioactive decay, cancer radiotherapy, sterilizing food and equipment

(MHz is a million hertz, GHz a billion, THz a trillion, PHz a thousand trillion and EHz a million trillion.)

The range is staggering. The longest radio waves are longer than a country is wide; the shortest gamma rays are smaller than an atomic nucleus. Across the whole table, the energy of one photon changes by a factor of more than a billion billion. The NASA Science tour of the spectrum gives a page to each band.

What makes some radiation ionizing?

Radiation is ionizing when a single photon carries enough energy to knock an electron off an atom or molecule, breaking chemical bonds and, in living tissue, damaging DNA directly. The threshold isn't sharp, because different atoms hold their electrons with different strengths, but it is usually put at about 10 eV per photon; the U.S. Federal Communications Commission uses that figure, as Wikipedia's article on ionizing radiation notes. That falls in the far ultraviolet, near 124 nm. X-rays and gamma rays are well above it.

A worked example: Wi-Fi against ultraviolet

The energy per photon is E = h × f, with h = 6.626 × 10⁻³⁴ joule-seconds.

1. A Wi-Fi signal at 2.4 GHz: E = 6.626 × 10⁻³⁴ × 2.4 × 10⁹ = 1.6 × 10⁻²⁴ joules. Dividing by 1.6 × 10⁻¹⁹ joules per eV gives about 0.00001 eV, ten millionths of an electronvolt.
2. Ultraviolet sunlight at 300 nm, the UV-B that causes sunburn: E ≈ 1,240 ÷ 300 = 4.1 eV.
3. A medical X-ray photon at 0.02 nm: E ≈ 1,240 ÷ 0.02 = 62,000 eV.

A Wi-Fi photon carries about a millionth of the 10 eV needed to ionize anything, and ultraviolet about 400,000 times more energy per photon than Wi-Fi. The point that matters is that brightness can't make up the gap: a stronger signal sends more photons, not more energetic ones, and a billion photons each too weak to break a bond still break none. This is the photoelectric effect's lesson, and it is why a radio transmitter can't give anyone radiation sickness however powerful it is.

Non-ionizing doesn't mean harmless. UV-B photons, at about 4 eV, sit below the ionizing threshold but are absorbed directly by DNA and can fuse neighboring parts of it, one of the main ways sunlight causes skin cancer. Microwaves and radio waves can heat tissue if they are intense enough; that is how a microwave oven works, at power levels far above any phone or router. In a 2020 question-and-answer page, the World Health Organization says tissue heating is the main way radio waves interact with the body, and that, after much research, no adverse health effect has been causally linked to exposure to wireless technologies. The questions people raise about phone networks are taken up in the note on 5G.

Why does the atmosphere let only some of it through?

Earth's air absorbs most of the spectrum. NASA's introduction to the spectrum names water vapor, carbon dioxide and ozone as the main absorbers. Each absorbs where its molecules can take in a photon's energy: by shifting electrons for ultraviolet, by vibrating for infrared, by rotating for some microwaves.

What gets through falls into a few windows:

• Visible light and nearby wavelengths pass almost untouched. This is also where the Sun's output is strongest, and the band animals' eyes evolved to use.
• Radio waves from about a centimeter to around ten meters pass through, which is why radio telescopes can work from the ground. Longer waves bounce off the electrically charged upper layer of the atmosphere, the ionosphere, which lets shortwave radio reach around the world.
• Ultraviolet is mostly blocked. NASA says UV-C is almost completely absorbed and about 95% of UV-B is absorbed by ozone. What reaches the ground is mainly UV-A plus some UV-B.
• X-rays and gamma rays are absorbed high up, so telescopes for them have to orbit above the air.
• Infrared is patchy. Water vapor and carbon dioxide absorb large parts of it. That absorption, of the infrared that Earth's warm surface gives off, is the greenhouse effect.

The pattern of windows has shaped life and technology alike: eyes tuned to the visible window, a planet warmed by infrared absorption, and an ozone layer whose thinning in the 1980s was a concern precisely because it is the shield against ultraviolet.

What do people get wrong about the spectrum?

The most common confusion is the word "radiation", which covers both the spectrum here and the particles from radioactive decay. In physics it means any energy that travels outward, and most of it, from sunlight to the heat off a stove, is harmless at ordinary levels. A second is picturing different kinds of rays as different substances. Radio waves and gamma rays are the same thing with wavelengths differing by a factor of trillions or more, and the boundaries between bands are human labels. A third is thinking stronger means more dangerous in every band. Below the ionizing threshold, intensity matters for heating; above it, what matters is the energy of each photon and the total dose.

Every band carries energy from where it is made to where it is absorbed. That is how the Sun warms Earth, how a phone reaches a tower, and how a hospital sees through skin.

Check yourself

• Without looking back: what is the one property that differs between radio waves, visible light and gamma rays?
• Why can't a very powerful Wi-Fi router ionize atoms, while a weak X-ray source can?
• Why do astronomers put X-ray telescopes in orbit but build radio telescopes on the ground?

Sources

• Tour of the Electromagnetic Spectrum — NASA Science
• Introduction to the Electromagnetic Spectrum — NASA Science
• Ultraviolet Waves — NASA Science
• Radiation: 5G mobile networks and health — World Health Organization, 2020
• Ionizing radiation — Wikipedia
• Electromagnetic spectrum — Wikipedia