Hot Light: Exploring the Infrared

Seeing Red

Remote controls, night-vision goggles, CD players and supermarket checkout scanners all use infrared light in their operation. Although students are generally very familiar with the colours of the rainbow - the components of visible light - they are less familiar with the concept of "invisible" light and have few opportunities to explore this part of the electromagnetic spectrum.

In this activity, students will explore infrared radiation by replicating a simple experiment designed and carried out by Frederick William Herschel in the 18th century.

Frederick William Herschel

Frederick William Herschel was born in Hanover, Germany in 1738. He earned his living as a musician but became fascinated with astronomy and, as a result of his observations, discovered the planet Uranus. Herschel was also interested in heat and its relationship to light. He noted that different colours of light seemed to be of different temperatures, so he created the experiment that students will perform in this activity to test his hypothesis. After noticing the temperature pattern for the different colours of visible light, he decided to try measuring the temperature beyond the red portion of the spectrum. His experiment proved that there was radiation beyond the visible spectrum. Further experimentation proved that this radiation could be reflected, refracted, absorbed and transmitted in a manner similar to visible light. What Herschel had discovered was a form of light (or radiation) beyond red light, which is now known as infrared radiation.

Investigation

Learning Objectives:

Students will:

• Carry out procedures to explore a given problem and to ensure a fair test, controlling major variables
• Use instruments effectively and accurately for collecting data
• Compare the properties of visible light to the properties of infrared radiation
• Communicate results using data tables and graphs
• Draw a conclusion based on experimental data and explain how the evidence supports the conclusion

Teaching Strategies:

• Experiment

Materials Needed

1. Glass prism (plastic prisms do not work as well) - 1

2. Alcohol thermometers - 3

3. Sheet of cardstock – 1

4. Sheet of blank white paper – 1

5. Cardboard box, shallow - 1

6. Clear adhesive tape - 1

7. Scissors - 1

8. Flat black paint and brush - 1

9. Watch with a second hand or stopwatch - 1

Procedure

A good spectrum is rarely achieved with an artificial light source, so this experiment is best performed outside on a bright sunny day.

Step 1

Paint the bulb of each thermometer using flat black paint (try to use the same amount of paint for each thermometer) and let them dry. This will ensure that the thermometers absorb as much heat as possible.

Step 2

Attach the thermometers to a piece of cardstock with clear adhesive tape so that the temperature scales line up.

Step 3

Place the thermometers in the shade to measure the ambient air temperature while setting up the rest of the experiment.

Step 4

Line the box with a sheet of white paper.

Step 5

Cut a notch, slightly narrower and deeper than the prism, from the top of the narrow edge of the cardboard box. This will hold the prism in place. Place the prism in the notch and rotate it gently to produce the widest possible spectrum on the sheet of paper.

Step 6

Recreate the chart below in a science notebook or journal.

	Thermometer 1	Thermometer 2	Thermometer 3
Temperature in the shade
	Thermometer 1 (blue region)	Thermometer 2 (yellow region)	Thermometer 3 (beyond red)
Predicted temperature after 10 minutes
Temperature after 1 min.
Temperature after 2 mins.
Temperature after 3 mins.
Temperature after 4 mins.
Temperature after 5 mins.
Temperature after 6 mins.
Temperature after 7 mins.
Temperature after 8 mins.
Temperature after 9 mins.
Temperature after 10 mins.

Step 7

Record the temperature in the shade for each thermometer on your experimental results chart.

Step 8

Place the thermometers inside the box so that one bulb is in the blue region, one in the yellow and one just beyond the red region. Predict the temperature you expect to observe for each colour and record it on the chart. Justify your prediction.

Step 9

Wait five minutes and then take the temperature reading on each thermometer. Do not remove the thermometers from the spectrum or block the light during the reading. Write down each reading on your experimental results chart.

Step 10

• Graph the results.
• Create a Venn diagram to compare the properties of visible light and the properties of infrared radiation based on what you learned in this experiment, what you already know and any further investigations you could do.

Observations and Conclusions

1. Look at the temperature readings you recorded on the temperature chart. Is there a pattern to the temperatures?

2. What do you conclude from this experiment about the region of the spectrum beyond red?

3. How did your predictions compare to your observations? Were you surprised by what you observed?

Discussion

1. What does the prefix infra mean? Why do you think it is called infrared?

2. Why is the Herschel experiment historically significant?

3. How else could you prove the existence of infrared radiation?

4. What hypothesis could you make about the temperature of ultraviolet light based on your graph?

5. What are the strengths and weaknesses of this experiment?

6. Was this a fair-test? Why or why not?

Extended Activities

1. Have students research a device which employs infrared light such as a remote control, night-vision goggles, CD players or supermarket checkout scanners, to find out how it works, who invented it and how the device is used in everyday life.

2. Have the students create their own experiment which proves that infrared radiation can be reflected (e.g., attempt to turn on a TV (or any device that used a remote control) by reflecting the infrared waves off a series of mirrors, etc.).

3. Have students find out what other forms of invisible electromagnetic radiation exist.

4. Have students research Frederick William Herschel and his many discoveries.

5. See an infrared view of the Mona Lisa taken by the NRC at Mona Lisa – A Scientific Exploration

6. For more information about the electromagnetic spectrum and its application to telescopes, check out Tools for Astronomers.