Laser Microscope Lesson

Build a Homemade Laser Microscope

Today you'll discover the microorganisms in pond water using a laser and a paperclip. Using basic principles of optics, you'll be able to build a homemade laser microscope that uses projection and magnification to see tiny critters swimming in your water sample.

Key Concepts

Here's how it works: by shining a laser though a drop of water, we can see the shadows of objects inside the water. It's like playing shadow puppets, only we're using a highly concentrated laser beam instead of a flashlight.

If you're wondering how a narrow laser beam spreads out to cover a wall, it has to do with the shape of the water droplet. Water has surface tension, which makes the water want to curl into a ball shape. But because water's heavy, the ball stretches a little. This makes the water a tear-drop shape, which makes it act like a convex lens, which magnifies the light and spreads it out like this:

Experiment & Video

Here's how to make your laser microscope:

Materials: 

• red or green laser (watch video for laser tips)
• large paperclip
• rubber band
• stack of books
• white wall
• pond water sample (or make your own from a cup of water with dead grass that's been sitting for a week on the windowsill)

Steps

1. Using pliers, bend your paper into the shape shown in the video.
2. Use your rubber band to attach the paperclip into position on the laser.
3. Bend the wire so that when you turn the laser on, the beam goes right through the loop at the end.
4. Dip the loop (not the laser!) into your water sample.
5. Turn off the lights and project your laser image onto the wall.

Questions to Ask

1. Does this work with other clear liquids?
2. What kind of lens occurs if you change the amount of surface tension by using soapy water instead?
3. Does the temperature of the water matter? What about a piece of ice?
4. Does this work with a flashlight instead of a laser?