During the latter half of the 19th century, Sir William Crookes was conducting research on Thallium. While trying to ascertain the atomic weight of Thallium, he noticed that the vacuum scales he was using proved to be sensitive to light and heat. Looking into this phenomenon he created the Solar Radiometer which drew significant attention from the scientific community including names such as A. Einstein and J C Maxwell.

Today, Solar Radiometers come in all shapes and sizes, but what they all have in common is a transparent bulb which houses a set of vanes mounted on a spindle in a near vacuum. On the spindle, lies a set of lightweight metal vanes with each wing having one side painted black and the other left bare. When left exposed to a light source (or even the heat of your hand!), the vanes will begin to rotate with an apparent lack of motive power. This rotation will always be in the same direction with the dark sides being repulsed from the light source, and the light sides being attracted. When exposed to cold, the vanes will rotate in the opposite direction.

A Solar Radiometer Is A Form Of Heat Engine

At its core, a Solar Radiometer is a form of heat engine, a system that converts heat energy into mechanical or kinetic energy. This is achieved when the radiometer is exposed to light and the dark side of the vane becomes hotter than the light side through a process known as Black Body Absorption. The individual atoms on the blackened side will subsequently have a higher velocity than those of the silver side.

Although there is a near vacuum in the bulb, it is deliberately imperfect to allow for just enough air molecules to flow freely. Under normal circumstances, the individual collisions of the gas molecules against the vanes will cancel each other out, rendering the mechanism inert. But when the radiometer is subjected to a light source and a difference in temperature is present, gas molecules hitting the warmer sides will absorb a small portion of the heat and bounce off at a higher speed. Due to Newton’s 1^st and 3^rd Laws of Motion, the same amount of energy that repels the gas molecule is also exerted on the vanes. Although these molecules are tiny and exert a very small force, the near vacuum allows for the vanes to rotate more freely.

When the source light becomes brighter, more heat will be gathered on the dark side. This provides a greater kinetic energy on the atomic level to repel air molecules with more force to causing the vanes to spin faster. 

There are numerous experiments you can do with a Solar Radiometer. We have compiled a few below;

1. What source of light provides the best results?

Materials: torch, lamp with an incandescent bulb, mirror

By comparing different light sources, what makes the radiometer spin the fastest?

2. What effect does a mirror have on the radiometer?

By utilising a mirror to direct even more light towards the radiometer, will the vanes rotate faster or slower? Why do you think that is? What changes are caused when the mirror is held at different angles and differences?

3. What effect does changing the ambient temperature have on the radiometer?

Materials: Hairdryer

Use a hairdryer to direct a stream of air at different temperatures towards the radiometer. Will the vanes rotate normal light? What happens after a few seconds?