7. Invisible Light (Radiation)

Any object at finite temperature emits thermal radiation – invisible light that can travel through space. The radiated energy can be absorbed by an other object, resulting in an increase of its temperature. This is called radiative heat transfer.

Radiative heat transfer happens very fast as thermal radiation moves at the speed of light

Make sure that you have the following lab supplies:

1. An IR camera
2. A few pieces of paper (science book or journal)
3. A few binder clips
4. A ruler
5. A timer (using a timer, mobile phone or similar)
6. A closed jar of ice water (less than 5 °C)
7. And/or (only with teacher/adult supervision), a closed jar of hot water (more than 75 °C)

Before you do anything, write down your answer (prediction) about the following questions:

1. What will happen to the temperature of a piece of paper when it faces a jar of hot or ice water from the side (See Figure 5)
   1. Explain why you think this will happen.
2. What will happen to the temperature of the paper when it is further away from the hot or ice water jar?
   1. Explain why.
3. What will happen to the temperature pattern of the paper when it does not face the jar (See Figure 6)
   1. Explain why

1. Put a jar of hot or ice water on a table where there is no other hot or cold objects on it.
2. Attach one or two binder clips to a piece of dry paper so that it stands on the table.
3. Move the paper close to the hot or cold jar - about 25-30mm away, facing the jar.
4. Keep the paper in place for one minute. Take an IR image of the side of the paper facing away from the jar every 10 or 20 seconds and record the highest temperature of the paper in Table 3A below. To ensure consistent color contrast across IR images, make sure that the jar does not show up in the IR view when you take images.
5. Do not touch the paper or move your hands too close to it when taking images, or else the result might be affected.
6. Once the temperature of the paper stabilizes, move the paper an extra 25-30mm further away from the jar (but still facing the jar). Take an IR image and record the highest temperature of the paper in Table 3B below.
   1. Repeat this procedure for two or three more times - until the paper is about 150-200mm away from the jar.

Repeat items 1-6 with the paper not facing the jar - see Figure 6.

Record your data in Table 3C and Table 3D below.

Use the IR images that you and your partner have taken as evidence to check your prediction. Whether your prediction is correct or not, explain your observation based on the concepts of thermal radiation and light absorption (perhaps geometric optics, too, if you have learned it before). Back your explanation with your IR images.

1. What happened to the temperature pattern of a piece of paper when it faced a jar of hot

or ice water from the side. See Figure 5?

1. Explain why.
2. What happened to the temperature pattern of the paper when it was further away from the hot or ice water jar?
3. Explain why.
4. What happened to the temperature pattern of the paper when it did not face the jar. See Figure 6.
5. Explain why.

Repeat the experiment with a jar of a different temperature. If you have used a jar of hot water, try a jar of ice water this time, or vice versa.

QUESTIONS

1. Suppose you put a hot water jar at 100 °C on the left side of a piece of paper and an ice water jar at 0 °C on the right side (Figure 7). The distances from the jar to the paper are equal and the paper faces both jars. What will the thermal pattern on the paper look like?
   1. Explain your answer.
2. Describe how a house gains and loses energy through radiative heat transfer from its

windows in the day and at night. See Figure 7