CATALYTIC REACTIONS?

If you have a photo or other graphic that reflects an aspect of life (including laboratory life) or a quotation that scientists mightappreciate that would be fit to print in the space to the right, why not

send it to us via e-mail:

<catalyst@nih.gov>; fax: 402-4303; or mail: Building 2, Room 2E26.

Also, we welcome "letters to the editor" for publication and your reactions to anything on the Catalyst pages.

In Future Issues...

Myopathy Genetics in the Middle East

NCCAM Fellows Find Varied Homes

OBSSR Turns 10

Kids' Catalyst: Shadow Casting

Overhead 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 matter?
We 're going to find out. Here's what you'll need:

1) A projector or another focused light source—such 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)

4)  Measuring tape

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 shadow?

Think of this experiment the next time you hear about an eclipse—when the sun blocks some or all of the moon, or vice versa—and 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 its shadow?

So now you need not fall asleep waiting for a presentation to start. Have fun playing with your own shadow!

—Jennifer White