Flashlight vs. Radiometer

A 3x 123A cell incandescent flashlight spins up a radiometer to a high speed.

Category:

Science & Technology

Tags:

• flashlight
• radiometer

[killercommunity]

i still dont understand what this does.

[spcutler]

@killercommunity Don't worry, it took scientists many years before they figured out how these things work. The basic idea is that the light source (in this case a flashlight) heats up the black surfaces faster than the light surfaces, for the same reason that black cars get hotter in the summer. Because cool air is more dense than hot air, this sets up a flow around the edges of the vanes, from the light side to the dark side. This flow drives the spinning of the vanes.

[zephmode]

@spcutler I thought it was because photons are being absorb on the black side and bounced off from the white side. And the difference in momentum causes the wheel to move.

[spcutler]

@zephmode That's a common misconception. Actually, it is possible to build a radiometer that way, but you need a much better vacuum on the inside of the bulb as well as better bearings. And the rotor spins the other way! In an elastic collision (bouncing off the white side) you get twice the momentum transfer as with inelastic (absorbed into the black side). But photons carry very little momentum, so that effect is usually dominated by other forces.

[xWHITExEAGLEx]

Actually they do have a purpose, they were used in weather stations to measure light intensity, they probobly still use them although I'm not sure if electronic luxmeters have replaced them.

[tigerhop]

In a TOTAL vacuum, with NO air drag whatsoever, it is possible for the photons to make the white side move by reflecting off it's surface, thus providing twice the impulse.

[Andrewsarchus77]

Cool

[angelwashere97]

I made one of these for my school`s science fair!!!!!!

[nicxtrem21]

This is not a regular flashlight :0

[killercommunity]

@WXVortexTX27 oh ok =)

[WXVortexTX27]

@killercommunity It doesn't really do anything. It's just for display. Like a shelf filler. I have one too. They're cool to watch. Just a little science decoration.

[killercommunity]

@spcutler thanks for the replay. but what i meant was what is the purpose of the object. i don't see any purpose to it.