The explanation talks about the wave-particle duality, but I think that’s a distraction. This is a dipole force phenomenon; the beam’s intensity is greatest at the center, and where the focal point occurs, as shown in the drawing at the end of the post. This gives rise to a gradient in the electric field. If you put a dielectric particle in this region, it will feel a force in the direction of the field maximum, or toward the highest intensity of light.

I agree with a couple of commenters there — I also like the demonstration where you spin the wheel and sit on a rotatable stool, and then reorient the wheel.

Laser level as the source, and lenses using gelatin as the medium, to demonstrate Snell’s law.

A Malawian youth, whose family could not afford his school tuition, learns some physics and builds a windmill to generate electricity for his village.

William scoured trash bins and junkyards for materials he could use to build his windmill. With only a couple of wrenches at his disposal, and unable to afford even nuts and bolts, he collected things that most people would consider garbage-slime-clogged plastic pipes, a broken bicycle, a discarded tractor fan-and assembled them into a wind-powered dynamo. For a soldering iron, he used a stiff piece of wire heated in a fire. A bent bicycle spoke served as a size adapter for his wrenches.

William now has a blog

How does the amount of energy per gram of TNT compare with the energy per gram of a chocolate chip cookie?

I’ve discussed before why I think the answer should be, “About the same,” if you’re doing a first-order approximation, and depending on what options you give for an answer.

We’ve also visited the energy content of a candy bar. So along that vein (or clogged artery) we have

Recrystallized Rocketry

which tests sugar as an ingredient in rocket propellant, in the form of pixy sticks, creme from an oreo cookie, and a Snickers bar.

Creme filling from one Double Stuff Oreo weighed out at 5.7 grams. Mixed with 11.4 grams finely powdered KNO3 (FireFox) with mortar and pestle.

Result: Burns very sluggishly, requires some assistance from torch flame

Adjustment: Added 0.2 grams red iron oxide (Fe2O3) to catalyze the burn. Works much better.

Evil Mad Scientist Laboratories does a pretty exhaustive analysis on the circuits, looking at battery life and potential danger of these simple circuits. Some thoughts on throwies

This data shows a couple of interesting things. First is that the power-law model seems to hold fairly well. Second, the power function that pops out is not very different from that of the data from only the first half hour– integrating both out to 24 hours gives two answers– 150 mAh and 186 mAh –that differ by only 25%. The estimate based on the long data record (150 mAh) is the more accurate one, but this does suggest that we should be able to use the data from the first half hour alone to get a fairly good “factor of two” estimate of the performance over 24 hours.

Learn how to blow bubbles with your old plastic CDs in this edition of ‘It’s Effin Science.’

How to build a cereal box spectrometer.

What do you see when you are in a completely dark room with no lights?

All you need is a big trash bag and an industrial strength vacuum cleaner, and a willing victim (er, “faithful subject of science.”) The victim (aka “subject) gets inside the bag, and once you suck all the air out of the bag with the vacuum cleaner, they’ll feel an intense pressure. SAFETY FIRST! Read this PDF writeup of the activity (from the Exploratorium’s Eric Muller) for all the ins-and-outs and safety factors in doing this with your kids. (Words to the wise — don’t put your head inside the bag!) It’s stunning — try it if you can.