In 1870, the Crookes radiometer was the must-have toy of the year. It was marketed as the Light Mill, which will make much 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, created 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 sunlight makes the weather vane move around inexplicably.
He sought to explain it and devised 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, 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.
The Crookes radiometer (light mill) is a fascinating device that demonstrates the principles of gas molecules’ interaction with light. Here’s how it works:
Construction: A Crookes radiometer consists of a glass bulb containing a partial vacuum or low-pressure gas, typically with a set of vanes inside. The vanes are usually white or light and have one side coated with a light-absorbing material, such as black or blue.
Thermodynamic Principle: The radiometer operates based on the principle of thermodynamics and the transfer of momentum. When light shines on the vanes, the radiation pressure of the photons interacting with the vanes creates a force.
Absorption and Reflection: The dark or light-absorbing side of the vanes absorbs more photons from the incident light, while the reflective side reflects the photons. This difference in absorption and reflection properties creates a temperature difference between the two sides of the vanes.
Gas Molecule Interactions: The temperature difference between the two sides of the vanes creates a pressure gradient within the radiometer. The gas molecules inside the bulb interact with the van’s heated and cooler sides differently.
Molecular Motion: The gas molecules near the hot side of the vanes gain energy and increase their kinetic motion. As a result, they collide with the gas molecules on the opposite side of the vane, transferring some of their energy and momentum.
Rotational Motion: Due to the uneven distribution of gas molecule impacts on the vanes, a net force is exerted on the vanes. This force causes the vanes to rotate toward the side with the lighter-absorbing or more reflective surface.
Continuous Rotation: As the vanes rotate, the lighter-absorbing or more reflective side faces away from the light source, reducing the radiation pressure on that side. At the same time, the other side faces the light source, increasing the radiation pressure. This imbalance of forces sustains the rotation of the vanes.
Crookes radiometer in sunlight creates heat in the form of light. So each of the four vanes has silvery, reflective, and darker black sides. The 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 to the warmer side. Sometimes, they displace 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 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 toward 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. The weaker the light source, the slower the radiometer will spin.