The order of the electromagnetic spectrum

By the end you can list the seven regions of the electromagnetic spectrum in order, give an approximate wavelength range for each, and state the wavelength range the human eye can detect.

Electromagnetic waves are all the same kind of wave. We split them into regions because they are produced and detected in different ways, and because our eyes happen to respond to only a tiny slice.

The regions, in order of decreasing wavelength, are radio waves, microwaves, infrared, visible light, ultraviolet, X-rays and gamma rays.

Because $c = f\lambda$ and $c$ is fixed, frequency runs the other way: radio waves have the lowest frequencies and gamma rays the highest.

The ranges below are approximate. Real sources do not check which box they belong in, so the boundaries are fuzzy and the regions overlap slightly. Learn the order and the approximate sizes, not a precise cut-off.

Region	Approximate free-space wavelength
Radio waves	longer than $0.1\ \text{m}$
Microwaves	$0.1\ \text{m}$ to $1\ \text{mm}$
Infrared	$1\ \text{mm}$ to $700\ \text{nm}$
Visible light	$700\ \text{nm}$ to $400\ \text{nm}$
Ultraviolet	$400\ \text{nm}$ to $10\ \text{nm}$
X-rays	$10\ \text{nm}$ to $0.01\ \text{nm}$
Gamma rays	shorter than $0.01\ \text{nm}$

The numbers look messy until you notice they are mostly powers of ten:

$1\ \text{mm} = 10^{-3}\ \text{m}$
$1\ \text{nm} = 10^{-9}\ \text{m}$ , so $700\ \text{nm} = 7.0 \times 10^{-7}\ \text{m}$
$0.01\ \text{nm} = 10^{-11}\ \text{m}$

Doing that conversion first makes the comparisons easy. A microwave at $1\ \text{mm}$ is $10^{-3}\ \text{m}$ and a visible wave at $1\ \text{nm}$ is $10^{-9}\ \text{m}$ , so the microwave is $10^{-3} \div 10^{-9} = 10^{6}$ times longer — a million times. Powers of ten keep you honest; guessing from the units alone is where mistakes creep in.

Put another way, the whole visible band from $400$ to $700\ \text{nm}$ spans less than one factor of two in wavelength, while the spectrum as a whole spans more than ten powers of ten.

In wavelength order, longest first:

Radio, Microwave, Infrared, Visible, Ultraviolet, X-ray, Gamma.

One way to hold it: Run My Incredible Vehicle Under X-ray Goggles. Silly, which is exactly why it sticks.

A second check that needs no mnemonic: work outwards from visible light. Infrared sits just beyond the red end (infra means below), and ultraviolet just beyond the violet end (ultra means beyond). Everything else falls into place around them.

Where the memory aids stop helping: they give the order, not the ranges. You still need the approximate boundaries in the table.

Which list gives the regions of the electromagnetic spectrum in order of decreasing wavelength?

The human eye detects which range of free-space wavelengths?

In order of decreasing wavelength, the spectrum runs radio, , infrared, , ultraviolet, X-rays and gamma rays.

The order of the electromagnetic spectrum

One spectrum, seven names

Read the ranges as powers of ten