Radiometer
Introduction
A radiometer is a device that measures the intensity of radiant energy, typically that of infrared radiation. It is a scientific instrument used in fields such as meteorology, astronomy, and climatology. The term radiometer is derived from the Latin words 'radius' meaning 'ray' and 'metron' meaning 'measure'.
History
The radiometer, also known as a light mill, was invented in 1873 by the British scientist William Crookes as the by-product of some chemical research. In the course of his studies on the radiations from the rare earth element thulium, Crookes discovered that the mill turned faster in sunlight than in artificial light.
Design and Function
A typical radiometer consists of a glass bulb containing a partial vacuum. Inside the bulb, on a low friction spindle, is a rotor with several (usually four) vertical lightweight metal vanes spaced equally around the axis. The vanes are polished or white on one side and black on the other. When exposed to a source of light, the vanes turn with no apparent motive power, the dark sides retreating from the radiation source and the light sides advancing.
Working Principle
The operation of a radiometer is not as simple as it seems at first glance, and has been the subject of much scientific debate. The common explanation, that the heat on the black side of the vane causes the gas to expand and push the vane around, has been found to be incorrect. Instead, the force that drives the vanes comes from the residual gas in the bulb. When the gas molecules are heated by the black side of the vane, they move faster and bounce off the vane with more force than those hitting the white side. This difference in force pushes the vane around.
Applications
Radiometers have a wide range of applications. They are used in meteorology to measure the intensity of solar radiation. In astronomy, they are used to measure the intensity of radiation from celestial bodies, which can provide information about their temperature, composition, and motion. In climatology, they are used to measure the amount of solar radiation reaching the Earth's surface, which is a key factor in climate modeling.