Science for you to try - Chemical photography: seeing in a different light

Infrared in a shoe box

The research team from Imperial College London use infrared light to produce ‘chemical photographs’ of objects. But infrared is outside of the visible spectrum, so how was it discovered?

Try this experiment and follow in the footsteps of Sir William Herschel, a British astronomer and Fellow of the Royal Society to find out.

Procedure

You will need:

• Glass prism
• 3 Alcohol thermometers
• Permanent black marker or black paint
• Scissors or a prism stand
• Cardboard box
• Blank sheet of white paper

Safety: Wear safety glasses.

The experiment should be done outdoors on a sunny day. The setup is shown below.

1. Place the sheet of white paper flat in the bottom of the cardboard box.

2. Take three thermometers and blacken the bulbs by painting them with black paint, or colouring them with a black marker pen.

3. Attach the glass prism near the edge of the box facing the sun. If you don’t have a prism stand you can place the prism in a rectangular cut in the top edge of the box. This should hold the prism snugly, but you should still be able to rotate the prism.

4. Rotate the prism until you get the widest possible spectrum on the white sheet of paper. You may have to tilt the box.

5. After securing the prism in the cut, place the thermometers in the shade and record the air temperature. Then place the thermometers in the box so that one of the bulbs is in the blue region, another is in the yellow region, and the third is just beyond the red region.

5. Record the temperatures in each of the three regions of the spectrum: blue, yellow, and just beyond the red every minute for a total of five minutes. 

6. Calculate the difference between the final temperatures measured in the spectrum and the shade for all three thermometers.

What's next?

Herschel's experiment was important not only because it led to the discovery of infrared light, but also because it was the first time that it was shown that there were forms of light that we cannot see with the naked eye.

1. Why do you think you had to blacken the thermometer bulbs?

2. Which thermometer recorded the highest increase in temperature? Why do you think this is?

3. There are many other types of electromagnetic radiation that we cannot see, can you name some of these? Hint, one of them lies beyond the other end of the visible spectrum.

Teacher's instructions

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