
Understanding
the Inverse Square Law
 Light
 Magnetism
 Gravity
 Sound
The closer you are, the brighter the light looks!
The stronger the magnet seems!
The harder gravity pulls you!
The louder the sound is!

The Inverse Square Law
We have all noticed that
far off lights seem small and dim, and that distant voices are
hard to hear. When we are next to a light source we receive all
its brightness, and even very small sounds are audible when we
are next to their source.Brightness and loudness grow less as
our distance from them grows, and they do so in a way that scientists
can measure. Try this activity to get a feeling for what scientists
found out.
Our important measure is one.
It might be one city block or one mile or one AU (the distance
from earth to the sun). When we get to two, the same thing always
happens: the same amount of light energy or sound or magnetism
or gravity that we had at one now has to be spread
across (2 * 2) times as much area. Therefore light looks fainter
and sound intensity drops away. Before we start, let's model
the fading away with peanut butter. Watch!
How Many Slices of Bread 
This slice gets all the peanut butter!

Each slice gets only a quarter of a spoonful of peanut butter 
One ninth of a spoonful !

Just
a
little
dab
will
do
it!

How Much Peanut Butter 




Distance 
1 
2 
3 
4 
Multiply the Distance by itself 
1 * 1 = 1 slice 
2 * 2 = 4 slices 
3 * 3 = 9 slices 
4 *4 = 16 slices 
HandsOn Activity
Materials for each person
or group 
0 one inch squares (with stickon
backing)
an object to represent the light source (a big sequin)
one half sheet of tag board (8.5 x 11 inches)/2 
1 inch pipe cleaner
10 inch pipe cleaner
scissors 
1. Measure your pipe cleaner. Mark it at one inch intervals.
Cut it to make pieces that are one inch long, two inches long,
three inches long, and four inches long.
2. Take a pipe cleaner and the one inch squares that are in
your packet. Use the pipe cleaner and the squares to lay out
a square like this. Do not remove the paper squares from their
backing yet.
Example:





This is a pipe cleaner.
It is 3 inches long. 
Put three one inch squares
along the pipe cleaner. 
Turn the pipe cleaner.
Put squares along it. 
Fill in the spaces to make
a big square. 
Now you have a group of squares
(an area) that we call three squared. 
3. Do this with all your pipe cleaners. You will have four
squares of different sizes.
4. Now find the piece of tagboard that most closely fits each
set of squares. Remove the squares from their backing and stick
them onto the tagboard. There will be some tagboard with no squares
on it in each.
5. Your tagboard will look like this: 


6. Fold the part without squares on it so that
your squares will stand up like this: 

7. Put your little sun marker down on your table.
8. Put the different lengths of pipe cleaners
around your sun, like this: 


9. Now put the little groups of squares
down beside the matching lengths of pipe cleaners, like this: 




__1__
(4 * 4 )
__1__
( 3 * 3 )
__1__
( 2 * 2 )
__1__
( 1 * 1 )

You can see how you can use
the distance measurement to make the square of the distance.
The number of smaller squares tell you how many parts the original
light (or sound, or magnetic force or gravity) has been spread
over now. Scientists express this as a fraction, dividing the
light on the original square by the numbers of squares in the
square of the distance.
© 2000. Elizabeth Anne Viau. All rights reserved.
eviau@earthlink.net
World Builders Web Site: http://curriculum.calstatela.edu/courses/builders/
