Relevant Resource questions :

Concepts demonstrated

• radiation
• air pressure
• detection of radiation

Equipment needed :

• Crookes radiometer
• light
• dry ice or liquid nitrogen
• CCD camera

• Shine a light onto the radiometer to make it rotate in the direction shown.
• When cooled (for example in a fridge) it rotates in the opposite direction.
• See also "How to use Demonstration" below.

None.

Physics involved :

The black side of the wings get hotter resulting in an increase in air pressure. This increase in air pressure pushes away the wing. Compare this to low and high pressure areas on an aeroplane wing. When cooled the opposite happens, the black side absorbs heat resulting in the pressure decreasing which causes the wings to be attracted.

How to use the Demonstration :

• Ask students to predict what will happen when a light is shone on the demonstration.
• Perform the experiment.
• Leave the light shining for a long time and ask what will happen when the light is removed. It should travel in the opposite direction.

Things to do or add :

• Create some sort of fridge to cool the radiometer such that it can still be seen by the students in a lecture theatre.

  - glass box with dry ice in it or N2 gas ??

  - CCD camera to televisions