Plankton Projector

1. Screw the adjustable clamp onto the ring stand using the clamp holder.
2. Remove the syringe tip and draw a few milliliters of the plankton-containing water into the syringe.
3. Clamp the syringe, nozzle pointing down, into the adjustable clamp. Put the shallow container underneath to catch drips (see photo below).

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4. Prop up the screen a foot or two away from the ring-stand setup.
5. Gently depress the plunger of the syringe so that a drop of water hangs from the tip of the syringe, but doesn’t fall (see photo below).
   

Look carefully at the hanging droplet. Can you see any moving organisms in it?

Shine the laser pointer through the water droplet towards the screen (see photo below). You will have to move the laser pointer around a bit, experimenting with the angle, to get a good projection. Can you see the shadows of living creatures on the screen now?

While still shining the laser pointer through the water droplet, have a partner move the screen closer to the droplet, or farther away. Notice how the shadows on the screen change.

Watch an individual shadow of a swimming animal. Does the shadow stay the same size, or does it change size?

Use a hand lens to observe a single organism in the water drop. Can you tell which direction the organisms are swimming? See if you can determine whether the image of the organisms you see on the screen is right side up or upside down.

A drop of water functions much like the lens in a projector, spreading the laser beam into a wide cone of light and revealing the shadows of the opaque organisms swimming within the water drop.

The light rays travel from the laser in a parallel beam, but are bent together by the convex surface of the water droplet. This bending, called refraction, happens both as the light rays enter and exit the droplet. Converged by the lens-like droplet, the rays come to a sharp focal point, crossing there and then spreading out again, and continuing to spread as they travel. It is this crossing of the rays that causes your organisms to appear upside down—though it may be hard to tell since they are often quite acrobatic (see diagram below).

As you reduce the distance between the screen and the water drop, you should notice that the shadows get smaller and sharper. You can see a similar phenomenon if you use a flashlight to make a shadow of your hand near a wall. Move your hand closer to the wall, and the shadow gets smaller.

You may also notice the shadow of an individual animal change size, even when you don’t move the screen. When the animal is closer to the laser side of the water drop, its shadow will be smaller. As the animal swims toward the screen side of the water drop, its shadow will get bigger.

If brine shrimp larvae or seawater are unavailable, try looking through droplets of pond or lake water. What creatures can you find?

There’s another interesting light phenomenon you can notice here: bands of dark and light stripes surrounding the swimming creatures. You may also see concentric circles of alternating shadow and light around specks of debris that are much smaller than the creatures. These are diffraction patterns, caused by interference between light waves. Diffraction occurs when light bends slightly as it passes by the edges of objects.

This Snack and the corresponding Exploratorium exhibit were based on the following article:
Planinsic, Gorazd. "Water-drop projector." The Physics Teacher 39, no. 2 (2001): 76-79.