Experiment objective

To measure the presence and properties of invisible infrared radiation.

Read more background about this activity in Willard S. Boyle's bio...

You need

  • An ordinary electric household clothes iron
  • A piece of glass
  • A sheet of polyethylene plastic
  • Some heavy duty aluminum foil
  • A thermometer or digital thermometer
  • Something round like a basketball, balloon, or salad bowl

Make a curved bowl-shaped mirror (a spherical mirror) by shaping some aluminum foil on a basketball or an overturned shallow bowl. Try to keep it as smooth as possible. It should be about 10” in diameter.

Turn the iron on high and set it at one end of a table. Set up the thermometer about 40cm from the iron with the dish shaped aluminum mirror behind it so that the open end of the bowl faces the iron and the bulb of the thermometer is at the focus of the bowl.

In this picture a digital thermometer was used (available in most hardware or automotive stores for about $10) and the sensor was placed in the focus of the spherical mirror. The mirror was made using the basketball in the background, and a pepper mill (not visible) is behind the mirror holding it up.

The rays of invisible infrared light coming from the iron hit the curved back of the mirror then reflect and converge on a spot a few centimeters in front of the centre of the mirror. That's where the bulb of the thermometer or sensor from a digital thermometer should be.

Try the thermometer bulb or sensor in a few different places and watch the temperature reading. You should be able to find a spot that's a bit warmer. That is the focus. Once you set it up, do not touch or move the bulb of the thermometer or the sensor for the duration of the experiment.

After everything is set up, wait about 5 or 10 minutes until the temperature of the thermometer is steady. Write this temperature down as the basic warm temperature. Now place something solid in between the iron and the thermometer, like a cereal box--anything that is opaque to light. Wait about 10 minutes or until the temperature stabilizes. It should be about one or two degrees C lower.

Now place a sheet of glass between the iron and the thermometer assembly. Wait again for 10 minutes or until the temperature stops changing.

Now try a sheet of polyethylene plastic, wait ten minutes and write down the temperature.

Experimental setup: a hot iron is on the left, a plastic sheet of polyethylene in middle, with the reflecting aluminum mirror and bulb of thermometer in focus of dish like mirror on the right.

Make a table showing the temperatures for each of the conditions: nothing, blocked, glass, and polyethylene. Which material, glass or plastic, has a higher temperature?

(Hint: Polyethylene absorbs IR less than glass) Key point: heat can be transferred by invisible rays called infra red radiation or IR. Some materials may be transparent to visible light, but block IR. Glass is one of these. So a glass greenhouse traps radiant heat or IR radiation. The Earth's atmosphere also traps IR. Carbon Dioxide in the atmosphere is partly responsible for this. Do you see now why global warming is called the greenhouse effect? By the way, this is also why cars get hot in the sun, and also why frost forms on car windows on clear cold nights. The radiation of IR from the car windows to deep cold space of the night sky, cools the windows faster than the surrounding air, and so water vapour in the air condenses on the window as frost.