The concept of "half-life" does not
appear in some foundation GCSE exams.
Check with your teacher if you're not sure which level of exam you'll
be taking.

If you point a Geiger counter at a radioactive
substance for a period of time, you'll notice that the reading
on the meter decreases as you watch. This is shown on the
graph.

The radioactivity from some substances dies
away very fast - perhaps in a few microseconds. Others take
thousands of years before you'll notice that the radioactivity
had decreased at all.

In theory, every
radioactive substance should stay slightly radioactive for
ever - the graph should never actually fall to zero.

This means that we can't usefully talk about the "life"
of a radioactive source.

Instead, we use the idea of "half-life".

This
is the time it takes for the radioactivity
to fall by half.

This sequence shows what would
happen with an imaginary radioactive substance.

Use the buttons to step through
it >>

Notice that the radioactivity in this example
falls by half every 2 hours.

So we say that this imaginary substance has
a half-life of 2 hours.

The count rate coming from a
radioactive source depends on how many unstable
atoms it contains.

That's the number of un-decayed
atoms.

If the count rate has
fallen by half, it means the number of unstable atoms has
fallen by half.

A typical exam question may be "A
radioactive substance has a half-life of 2 hours. How much of the
substance will remain after 6 hours have passed?"

Here's how to do it - simply
count on your fingers.

First
finger: after one half-life (2 hours), half
of the substance is left.

Second
finger: after two half-lives (another 2 hours),
half of that is left, so we're down to a quarter.

Third
finger: after three half lives (that's 6 hours)
half of that is left, so it's down to an eighth
of what we started with.

So the answer to the exam question
is:for a substance
with a half-life of 2 hours, 1/8 of the original atoms
will remain after 6 hours.

Note that this also means that 7/8 of the atoms
will have decayed in that time.

Take another look at the graph sequence above, you'll see
that after 6 hours the activity has fallen from 8,000 to 1,000;
i.e. it's fallen to 1/8 of the starting value.

That's the key to solving half-life questions - count on
your fingers, saying "half", "quarter",
"eighth", "sixteenth", "thirty-second",
"sixty-fourth"... etc.

At GCSE level you're
unlikely to meet a question that needs more than 5 half-lives,
and any questions you're asked will always involve whole numbers
of half-lives (which makes life much easier!)