sun dog


Radiation is energy moving through space or matter.

Radiation can be:


Radiation is emitted from a source, transmitted to a destination, where it can be absorbed.

radiation Sun emit, transmit, absorb at Earth
Here the Sun is the source
and the Earth is the destination


Radiation can also be scattered when hitting particles along the way, as well as reflected, refracted and diffracted (read up on those topics to learn more).

light beam
Here we see light (a type of radiation) being scattered by the atmosphere.

Example: Microwave Oven

microwave magnetron, stirrer, pie absorbs
the magnetron emits microwaves which get reflected off a stirrer and the sides
until absorbed by food.


We can measure radiation intensity at any location as power per area: usually Watts per square meter (W/m2).

As it spreads out on its journey, the intensity gets less.

light intensity

Inverse Square

In fact the intensity changes as the Inverse Square of the distance.

brightness decreases by the square of the distance Inverse Square: when one value decreases as the square of the other value.

Example: Light and distance

The further away we are from a light, the less bright it is.

inverse square law: distance=1 area=1 intensity=1, distance=2 area=4 intensity=0.25, distance=3 area=9 intensity=0.111...

The brightness decreases as the square of the distance.



As radiation gets absorbed it loses intensity.

Materials vary in how much they absorb different types of radiation.

feet fire

When energy is absorbed the material may:

Ionizing Radiation

ionizing atom

High energy radiation is ionizing, meaning it can knock electrons out of atoms.


Losing an electron makes the atom charged and more likely to form new chemical reactions which can be harmful to our cells.

The cells can die or change so they grow out of control and form cancer.


But medicine uses ionizing radiation to help us, such as with X-rays, or as a way to kill cancer cells.

And we live with a small amount of ionizing radiation from the world around us every day.

Ionizing radiation can come from

Electromagnetic Radiation

Gamma rays and X-rays and some ultraviolet waves have enough energy to be ionizing.


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 absorb

So ultraviolet is the only ionizing radiation that reaches us at ground level.

Radioactive Materials

nucleus unstable


Some atoms have too much energy.

They will randomly decay to a more stable state by sending out radiation.

We say those atoms are radioactive (they can actively emit radiation!)

The radiation emitted has high energy, is ionizing and can be of 3 types (α, β and γ):

alpha beta gamma from nucleus

Alpha Particles (α)

Alpha particles have two protons and two neutrons in the form of an atomic nucleus.

They are highly charged, fairly heavy and slow. So they are easily stopped by a sheet of paper or even a few centimeters of air.

Beta Particles (β)

Beta particles are high energy electrons.

They have 100 times more penetrating power than alpha particles, but can be stopped by a few millimeters of aluminium or a centimeter or so of water.

Gamma rays (γ)

Gamma rays are high frequency electromagnetic waves.

They need thick lead or lots of water to stop them.

Most will go straight through our bodies. But when they hit atoms in our body they can harm us.

nuclear power station

Neutron Radiation

There is also neutron radiation, which occurs in nuclear reactions such as thermonuclear explosions or inside nuclear reactors.


Typical sources of radiation are

Irradiation and Contamination

Irradiation is when we are exposed to radiation (usually meaning ionizing radiation)

Contamination is when we breath in or swallow radioactive material

Irradiation may happen for only a short time, but contamination is inside our body and keeps us irradiated for a long time.

Radiation Doses

We get small amounts of radiation all the time from the world around us. And sometimes a little extra when we have an X-ray.

The health effect of ionizing radiation is measured in Sieverts (Sv).

1 Sv is a very large and dangerous dose, so we tend to talk about:

Some typical values:

Source Typical Dose As Sieverts
Dental X-Ray 10 µSv 0.000 01
Chest X-ray 20 µSv 0.000 02
Mammogram 400 µSv 0.000 4
Typical yearly background 2 mSv 0.002
Head CT Scan 3 mSv 0.003
Full CT Scan 30 mSv 0.03
Severe radiation poisoning 2 Sv 2
Fatal acute dose 5 Sv 5

x ray hand
X-ray of a hand on a computer mouse

Stay safe!