Radiometer - Impress the In-Laws...Leave it by a Window*

Experience a classic Crookes radiometer for yourself!  The metal inside the sealed glass will spin when exposed to strong light.  Leave it by a window on a sunny day...the stronger the light, the faster it spins!

Why does it turn you ask?  Well, in case anyone asks, a common myth (one originally held even by the scientist who invented it in 1873, Sir William Crookes) is that the momentum of the absorbed light on the black faces makes the radiometer operate.  But this can't be right because the white faces move forward (not away from) towards the light source.  Photons of light do exert radiation pressure on the faces, but other effects dwarf these forces. The final explanation it seems depends on having just the right degree of vacuum and is more related to the flow of heat than the direct effect of photons.

For the more adventurous, try out the Precision model.  They are handmade in Germany and are incredibly sensitive, spinning at up to 3,000 rpm in bright sunlight.  With a bit of patience, it will even respond to just the infrared radiation from the heat of your hand!

Recommended for Age 12 & Up (with adult supervision)

*  Especially if you can be nonchalant about it...