Electromagnetic Spectrum


It is called "electromagnetism" because electricity and magnetism are linked:

electro magnetic field properties
A changing electric field produces a magnetic field,
a changing magnetic field produces an electric field,

changing electric vs magnetic field 

... around and around ... !

So electricity and magnetism are linked in an ongoing dance.

This effect heads off through space at the fastest speed possible: the speed of light.

Here is the full electromagnetic spectrum:


Higher frequency (rate of vibration) has more energy and shorter wavelength.

The spectrum is continuous with no sudden changes or boundaries.

But how waves interact with matter depends on their energy and the type of matter

Example: Our Bodies

So some waves pass right through our bodies, others get reflected or absorbed at different rates.

sun dog

Example: From The Sun

A lot of the the radiation from the Sun gets reflected or absorbed by the atmosphere.


get all the way through:

electromagnetic spectrum block vs pass

Imagine: if our eyes could only see X-rays the sky would be black!

nustar x ray telescope
So telescopes like the NuSTAR X-ray telescope have to be in orbit.


We like to think of the spectrum as having these ranges:

There is disagreement on the exact range values (for example X-Rays from an X-ray tube and Gamma rays from radioactive materials overlap) but here is a handy guide:

    Typical Wavelength   Commonly Defined Range
Radio   meters (m)   Above 10 cm
Microwave   centimeters (cm)   1 mm to 10 cm
Infrared   micrometers (μm)   750 nm to 1 mm
Light   100s of nanometers   380 nm to 750 nm
UV   100 nanometers   10 nm to 380 nm
X-rays   nanometers (nm)   10 pm to 10 nm
Gamma rays   picometers (pm)   below 10 pm

See Units in Equations for more about nanometers, picometers, etc.

Speed of Light

Electromagnetic waves travel at the "Speed of Light" at almost 300,000,000 meters per second (to be exact: 299,792,458 meters per second) in a vacuum.

That is 300 million meters every second, or:

But the speed can be slower ...

Medium Speed
million m/s
Vacuum 300
Ice 228
Water 225
Ethanol 220
Glass 205
Olive oil 204
Diamond 123

At slower speeds the wavelength is shorter for the same frequency.

We can work out the wavelength:

wavelength = speedfrequency

Example: Red Light at a frequency of 4 × 1014

In a vacuum the wavelength is:

3 × 1084 × 1014 = 7.5 × 10-7 =  750 nm

In water the wavelength is:

2.25 × 1084 × 1014 = 5.625 × 10-7 =  562 nm

The wavelength is different but the light stays the same color as the frequency is the same.


Important: the wavelengths mentioned on this page are for a vacuum. Adjust them like above if not.

Wavelengths vs Frequency Activity

Try this: Walk across the room in 5 seconds:

What is the frequency of your steps in each case?

Now try that again but take 20 seconds to cross the room. What happens?


A higher frequency (rate of vibration) has shorter wavelength and more energy.

Example: Which has more energy, light or X-rays?

X-rays have more energy, with frequencies around 1018, compared to light around 1014

Example: Which has more energy, red light or blue light?

Blue light has higher frequency (with shorter wavelengths), so has more energy than red.

light red:lower energy, blue:higher energy


ionizing atom

Energy and Ionization


Gamma rays, x-rays, and some ultraviolet waves have such high energy that they are "ionizing," meaning they can knock electrons out of atoms.


This makes atoms charged and more likely to form new chemical reactions, which can be harmful to our cells, killing them or changing them so they grow out of control and form cancer.


How to remember the spectrum?

It goes: Waves, Infrared, Light, Ultraviolet, Rays

To remember the energy levels, think "waves are slow and soothing, but rays are fast and dangerous".

As Photons

Electromagnetic radiation behaves as waves, but also behaves as packets of energy called Photons.

So it is like a particle and also like a wave. This is called the "wave–particle duality".



Einstein wrote about it:

"It seems as though we must use sometimes the one theory and sometimes the other, while at times we may use either."