In 1870, the Crookes radiometer was the must-have toy of the year. It was marketed as something called the Light Mill, which will make a lot of sense in a second. Crookes radiometer looks like a paper-thin, see-through glass light bulb. It has two color blades: black and white. The difference in pressure between the two sides of each blade causes a force that makes the grinder rotate. The higher the energy of the light, the more the grinder turns.
How does crookes radiometer work?
Radiometer responds to radiant energy that goes out and rays like beams of light or heat. The lights heat molecules, just a few molecules inside a glass bulb. When the molecules hit the black sides of the veins, they bounce off with more energy. The black sides soak up a little more heat.
Structure of radiometer
The radiometer has four veins. Each of the veins is painted black on one side and white on the other. They are mounted on a semi-spherical glass capsule that rests on the tip of a needle. So it can rotate with very low friction. The glass chamber is partially evacuated. It is not a total vacuum, and it needs a little bit of air to work. In a full vacuum or with atmospheric pressure, the radiometer will not work.
- It’s sealed with a partial vacuum inside. The shaft was coming up and arrayed around that shaft, four little vanes on a frictionless rotor.
- The vanes have a dark side and a silvery, reflective side.
Since it’s sealed off in a glass bulb, it shouldn’t be subject to air currents. In fact, its inventor, Sir William Crooke, came up with this whole thing for that very reason. He was looking to measure thallium, which is very lightweight without air currents messing with his measurements. He figured out that a Crookes radiometer in the presence of sunlight, the weather vane starts to move around inexplicably.
He sought to explain it, and he came up with a proposal called the pressure of light. What Crookes thought was going on, is the sunlight hitting the weather vane was pushing it around. Basically, photons, the tiniest packets of light, were hitting the reflective surfaces and causing the vane to spin. But it was wrong! If Crookes had been correct, the reflective side of each weather vane should have been pushed along. And the vane would have spun in the direction of the silver side. When you put it in sunlight, it spins in the opposite direction, with the darker side of each vane leading the way.
In 1879, Osborne Reynolds proposed the blue-ribbon hypothesis explanation of what happened with the Crookes radiometer. He proposed thermal creep or transpiration.
Crookes radiometer in sunlight creates heat in the form of light. So each of the four vanes has a silvery, reflective side and a darker black side. That darker black sides tend to absorb light, enhance heat. It means that one side of a vane that’s hotter than the other side. By sending colder air on the silvery side to the darker side to cool it off, cool things down. When it does that, the balance of gasses changes. It builds upon the darker side, increasing the air pressure and decreasing the air pressure on the silvery side. That’s part one of thermal creep.
The second part is that air particles move around to the warmer side as the cold air moves around to the warmer side. And sometimes, they displace some of the warmer molecules, which go to the other side. By definition, warmer air molecules have more energy. They’re more excited, and so they strike the vane with more force.
They’re striking the black side of the vane, which on the other side meets very little resistance. Because there’s lower air pressure, and the vane spins around, with the dark side leading.
- The more light intensity, the faster the mill will move, but it does not depend only on the intensity. It also depends on the frequency of light.
The radiometer is more sensitive to the light of the flame, which is of a lower frequency than the LED light. The black side absorbs more light than the white one. It gets hotter, so the air temperature is larger than the temperature on the white side. Everybody knows that a white car is cooler than a black one. So the same happens here.
Consequently, the pressure on the black side is larger than the pressure on the white side. So there is a net force that pushes from black to white. And the radiometer will move from left to right. It explains why the radiometer does not work in a total vacuum.
In the radiometer, the air moves in a direction and by Newton’s third law. The vanes will rotate in the opposite direction. The speed at which they rotate is directly proportional to the incoming flux from the light source. So the brighter the light source, the faster the radiometer will spin. In reverse, the weaker the light source, the slower the radiometer will spin.
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