projectors aren't just for those wonderful (Wake up!) classroom presentations.
The next time you re waiting for your teacher to set up a lesson, you
might want to do a little side experiment of your own.
Few can resist making shadows of different shapes with their
hands when they see a blank screen. (Adults want to. They just don't
most of the time.) If you're sitting in the front of the
class, further away from the projector, the shadow you cast is much
smaller than that of someone in the back, who is much closer to the
light source. Your little shadow doesn't have a chance!
How much larger is the shadow of the classmate sitting behind
you? Two rows back? Three rows back? How much does the size of the classmate
We 're going to find out. Here's what you'll need:
A projector or another focused light sourcesuch as a flashlight
2) Round objects. I happened to have a baseball, a golf
ball, and a basketball lying around, but if you don't have similar props,
you can create round disks of different sizes with your compass (homemade
or otherwise . . . . remember how to do that?) and can tape them to
a ruler or a pencil
3) A piece of paper taped to the wall the shadow will be
cast on (and a volunteer to trace the shadow that will be cast)
First, measure from the wall to the light source. With one
person holding the golf ball and one person holding the baseball, make
their shadows the same size. How far away do they have to be from each
other in order to cast shadows of the same size?
The golf ball is much smaller than the baseball, but it
can cast just as large a shadow when it's closer to the light source.
But how much closer? Calculate the distance with your measurements on
the floor, write them down, and then compare that with the diameter
of the objects. Now try with different sizes. If one ball is twice as
large as the other, does it need to be twice as far to create the same
Think of this experiment the next time you hear about an
eclipsewhen the sun blocks some or all of the moon, or vice versaand
how we can calculate the size and movement of an object based on its
shadow dynamics. How much of a difference does a small change make in
So now you need not fall asleep waiting for a presentation
to start. Have fun playing with your own shadow!