Radiometer

Relevant Resource questions :

Concepts demonstrated :

• radiation
• reflection of Infra Red (IR) radiation

• large parabolic reflectors
• heat source eg industrial soldering iron
• thermocouple
• CCD camera

• Place two parabolic reflectors at opposite ends of the front bench of a lecture theatre. Ideally so that the lecturer may walk between them.
• Position the soldering iron at the focus of one of the reflectors.
• Place the thermocouple at the focus of the other parabolic reflector.

Noni

Physics involved :

The energy from the soldering iron is radiated away and is collected by the reflector and focussed so that a planar beam of radiation may travel across the room to the other reflector.

The energy is then focussed back down onto the thermocouple.

How to use the Demonstration :

• Discuss the radiation of energy from a hot source.
• Demonstrate that the radiation travels across the room. Stand between the mirrors to block the beam and show that the temperature on the thermocouple drops.
• Ask a POE question - if a match head is placed at the focus of the second mirror. Most students will not expect the match to ignite.
• Extend the match head idea by asking students what makes a match head ignite - friction, flame, focussed light/heat?

• Car head lamps.
• Solar collectors.
• Collectors for satelite dishes, telescopes (radio and optical).
• Cold air from a refrigerator (for the other case listed in "Things to do" below.

• Repeat the experiment using a "cold source". If liquid nitrogen placed in a test tube is used instead of the soldering iron the temperature of the thermocouple should drop. Therefore cold is being radiated (sic).
  NB this is not the correct physics but it does highlight a strong student misconception.

• Place a paper, match and balloon at the other focus and wait for the effect -> gnition, and the balloon explodes.
• A sound version of this might be good for Open Days.
• When performed by the author using liquid nitrogen no effect was observed. The reflectors may need to be closer for this experiment to work.