This page shows a nice illustration of reraction of light as it passes between media of different densities (second picture). It looked like a great little “experiment” for my seven-year-old daughter who is very interested in science and math.
When we tried this, though, we were unable to obtain any noticeable bending of the pencil of light. Any suggestions to “make it work”.
Thanks.
I’m not sure about this experiment. It won’t look bent to you because the light reaching your eyes is bending too!
You could always put a coin at the bottom of the jar of water, then get a pencil, sit to the side, aim at coin (from outside the water), push forward to hit coin. First one to do it gets a sweetie 
Some suggestions
[ul]
[li]Make sure you’re using something that creates a small column of light. A regular flashlight will have too much dispersion, which will wash out the experiment. If you have a laser pointer, that will work nicely.[/li][li]Use as wider and deeper bowl, 6-12 inches wide and deep.[/li][li]Mix in something to make the water more occluded, like lots of salt. That’ll bring out the light beam’s path.[/li][/ul]
Hope this helps. I love to see children interested in science. It’s really a plus when the experiments work.
Also, observe this set-up from various angles. From the side, as pictured on the web site, is pretty good. From directly above is bad.
But if it doesn’t work, you can have fun trying to figure out why it doesn’t work. I think that’ll be far more educational than an immediately successful experiment!
As AWB said, view it from the side. It really helps if you have a rectangular container - if you use a cylindrical glass or jar, the curved side will add distortions. Also, you need to be able to see the column of light, not just the bright illuminated spot. A laser pointer should work well, but even that isn’t very visible. You can make it more visible by adding a drop of milk to the water, and some incense (sp?) smoke to the air.
Alternatively, get a flat panel or board of some kind, and hold it vertically so that the lower half is in the water. Shine a flashlight along this board. If the beam is parallel to the board you can’t see anything, but if you tilt the light slightly towards the board, you can see a sort of oblique cross-section of the beam. This should bend at the water surface. You might have to fool around with the flashlight to make it produce a fairly focused beam. An adjustable flashlight (e.g. Mag light) may work.
Ideally, what you want is not a pencil-beam of light, but a sheet, such as you might produce by shining it through a vertical slit. Use a rectangular object (a thich piece of glass, say, or an aquarium) on top of a white screen of some sort (most likely just a piece of paper). Shine the beam down at a slight angle, so that some of it is hitting the screen on the bottom. Looking at it from above, you should see the bending nicely.
AWB’s suggestions are good. You might want to use a little corn starch instead of salt – it goes a long way. You can also spritz something into the air to see the beam above the water to help you see the beam there. Use a laser beam as your source.
Have a look at the experiment on gradient index in “The Flying Circus of Physics” – mix sugar into water and let it form a gradient. The light will bend CONTINUOUSLY in the water, not only at the interface.
Please note: Light that enters the water at normal incidence (Perpendicular to the surface)WILL NOT BEND!
Thanks all.
I’ll try to round up the needed equipment and supplies and do a repeat run.
scr4: Yes, it is very instructive to try to figure out why an experiment “didn’t work”. Just the other day, we put some salt water and some tap water in the freezer. They both froze. Now, we’re trying different concentrations of salt, different times in the freezer, etc.
Cal: Is The Flying Circus of Physics the book by Walker?
KarlGauss:
Yes, The Flying Circus of Physics is the one by Jearl D. Walker, who went on to write The Amateur Scientist column at Scientific American after Strong died. It’s the best al-round reference on oddball physics, although we could se an updated edition – it’s almost a quarter of a century old!