Date: Fri, 27 May 2005 14:50:35 -0400

Author: James Frysinger

Subject: Re: TIR Demo


On Friday 27 May 2005 13:39, Bill Alexander wrote:
> 2) I took a 3 lb coffee can and set in on the table. Shined a laser on
> the side, marked that spot, drilled 1/4 inch hole and glued a glass window
> over hole. Then shined laser through the hole to other side, marked spot,
> and drilled 1/8 inch hole.
> Fill can w/ water, and watch laser beam flow into sink. Lots of fun.

I do something similar with a plastic bottle --- flat sided works best. No
hole is needed for the laser light to enter, just a hole on the side the
water stream emerges from.

A cute wrinkle is a trick I picked up at an NSTA "family fun night" show at
the end of a conference week. I collect the "pink" water in a beaker while
running the demo in the dark. The students get to see the beaker before I
start and it is apparently clean and dry. As the demo runs, I comment that
the water appears to be "carrying the red" into the beaker, and indeed the
contents of the beaker glow with a faint pink color. Then I plug the hole and
turn off the laser. When the lights come on the beaker is sitting there
holding pink water! By this time most introductory physics students realize
this is a "trick" and not "physics" but they are usually mystified and often
gasp aloud.

The trick is to put a little sodium bicarbonate in the water in the plastic
bottle before class. Also, "off stage", several drops of phenolphthalein in
alcohol are put in the beaker and the alcohol is allowed to evaporate,
leaving the phenolphthalein behind. Of course, once the basic water comes in
contact with the phenolphthalein it turns pink. The audible gasp when the
lights come on and the beaker of pink water is seen and the ensuing
discussion makes this worth doing.


James R. Frysinger
Lifetime Certified Advanced Metrication Specialist
Senior Member, IEEE

Physics Lab Manager, Lecturer
Dept. of Physics and Astronomy
University/College of Charleston
66 George Street
Charleston, SC 29424
843.953.7644 (phone)
843.953.4824 (FAX)

10 Captiva Row
Charleston, SC 29407
From Fri May 27 15:32:30 2005