ラジオメーターの不思議／The Mysteries of the Radiometer

DENJIRO: Today, let’s perform an experiment using this apparatus. This is called a “radiometer.” Inside the glass container, there is a spindle with four vanes on it. I’m going to shine light on this. GIRL: It’s spinning! DENJIRO: The vanes inside started to spin just from the light. Let me explain why. If you look at the vanes, one side is white and the other side is black. The black side absorbs light, so the temperature rises, but the white side reflects light, so the temperature does not rise. Then, the vanes spin toward the direction of the higher temperature. This device was invented by British physicist William Crookes. Now, let’s try another experiment. I’ve removed the glass container to expose the vanes. What happens when I shine light on it now? Unlike before, it doesn’t spin. That’s because the air inside the glass container is very thin, just 1/100,000 of the atmosphere. A radiometer converts light energy into heat energy, then into kinetic energy for moving the vanes. However, the reason why it spins is still not fully known. Now, what will happen if I cool down the radiometer? Let me spray it with a cold spray. GIRL: It’s spinning in the opposite direction now! DENJIRO: Why does it spin in the opposite direction? When I cooled the glass container with a cold spray, due to radiative cooling, the black side becomes cooler than the white side, which is the opposite of what happened when I shined light on it. Therefore, it moves in the opposite direction. I hope energy will bring you all happiness. Our magic word is "Happy Energy!"