Laser Projection Microscope

By Sean Michael Ragan
Category: Science
Time Required: 1 hour
Difficulty: Easy

Update: The earliest publication of this idea I am now aware of is:

“Water-drop projector”

Gorazd Planinsic, Phys. Teach. 39, 76 (2001), DOI:10.1119/1.1355162

This paper can be freely downloaded here. A formula for calculating the approximate magnifying power of the projector based on the radius of the droplet, the refractive index of the liquid, and the distance to the screen is given on p. 20.

Inspired by this page over at Teravolt.org, this project is near the top of the list when it comes to getting the most bang for your buck. Whether you’re a scientist yourself, a science educator, or just, you know, a bright curious monkey, this is one of those you really have to try for yourself.

The tricky part is getting everything aligned just right–the laser, the projection surface, the hanging drop of water–but this simple stand from junk-box odds and ends makes it much, much easier. The laser and syringe are mounted to broom clips on supermagnet bases which allow for easy positional adjustments, but hold strongly enough to keep everything positioned once you’ve got it tuned right. You can even pick the whole thing up and move it to another surface without perturbing the alignment.

Parts
/
Tools

Heavy mounting base, about 6 inches long (1) I used an old marble trophy base.
Corner brace, galvanized steel, 6' legs (1)
Laser source, 25mW output power or better (1) I used a green 30mW laser from Wicked Lasers. A continuous on/off switch is preferable to a momentary switch, but be aware that modifying a laser with a momentary switch for continuous operation makes it considerably more hazardous to operate, because it won't turn off when you drop it.
Syringe with blunt needle, 10 mL (1) Mine came out of an old inkjet cartridge refill kit.
broom clips, small (4) Test fit against your laser source and syringe.
Magnets rare earth, ring form with countersunk hole (4) I used Magcraft part NSN0587.
Machine screw flat head, 10/24 x 1/2' (2)
Machine screw, flat head, 10/24 x 3/8' (2)
Washer flat, #8 (16)
Hex nut, 10/24 (4)
Liquid sample (1 mL) I used stagnant water from a drainage ditch.
Mounting tape, double-sided foam (6 in) I used SM 114/DC Scotch mounting tape, 1' x 50'.
Bottle cap (1) to catch drips

Steps

Step #1: Mount corner brace

Use scissors to cut a strip of mounting tape to fit one side of the corner brace.
Adhere it to one side of the corner brace.
Remove the protective film from the other side of the tape, and affix it to the base.

Conclusion

Update: The earliest publication of this idea I am now aware of is:

"Water-drop projector"

Gorazd Planinsic, Phys. Teach. 39, 76 (2001), DOI:10.1119/1.1355162

An improvement on this design might incorporate a "heavy duty corner brace" having a triangular third side, such as the Stanley #755565, with the laser attached to the vertical, triangular side instead of to the horizontal base, to allow greater adjustability in the up/down projection angle.

Also, it has been suggested that it may be possible to focus blue and red lasers on the same drop, simultaneously, and achieve a 3D projection when viewed through red/blue 3D glasses. My experience with this monochrome projector suggests that getting everything aligned correctly for such a system would be challenging, but certainly not impossible.

Finally, I am curious as to the effect that changing the laser color may or may not have on the quality of the image. As soon as I get my hands on red and/or blue lasers in an appropriate power range, I will try that experiment and post my results.