In 1870, the Crookes radiometer was the must-have toy of the year. It was marketed as 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 rotates the grinder. The higher the energy of the light, the more the grinder turns.
How does Crookes radiometer work? (Structure And Process)
A radiometer responds to radiant energy and rays like beams of light or heat. The lights heat molecules, 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 vein 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.
It shouldn’t be subject to air currents since it’s sealed off in a glass bulb. Its inventor, Sir William Crooke, came up with this whole thing for that 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 makes the weather vane move around inexplicably.
He sought to explain it and came up with a proposal called the pressure of light. What Crookes thought was going on was the sunlight hitting the weather vane was pushing it around. 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. 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 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 silvery, reflective, and darker black sides. That darker black sides tend to absorb light and enhance heat. It means that one side of a vane is hotter than the other. Cool things down by sending colder air on the silvery side to the darker side to cool it off. 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. Sometimes, they displace some warmer molecules, which go to the other side. By definition, warmer air molecules have more energy. They’re more excited, 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, 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. The radiometer will move from left to right. It explains why the radiometer does not work in a total vacuum.
The air moves in a direction and by Newton’s third law in the radiometer. 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.