Task 3: The energy of light and thermal radiation

Pupils first estimate how the course of the experiment of warming up a sheet of black paper by heat radiation of an ordinary light bulb and a LED light bulb will proceed, which source of light will warm up the paper the fastest. It is advisable to have them deduce this only based on their own experience, or based on an interpretation of why the ordinary light bulbs are being replaced by other artificial sources of light. They should not deduce it based on moving their hand or another part of the body closer to the lit up sources, because at that moment everything would be quite clear to them.

For the experimental execution of the first part of the task, it is necessary to prepare two sources of light, ideally an ordinary light bulb and a LED light bulb with a similar luminous flux. It can be for example a 100W light bulb, which has the luminous flux of approximately 1300 lm, and a LED light bulb with the input of approximately 13 W, which has a comparable luminous flux. Next, any two thermometers with the range from room temperature up to at least 50 °C but preferably up to 100 °C and two sheets of black paper are needed. We place one of the black sheets of paper to the set distance from the light bulb and put a thermometer underneath it. We place the other sheet of paper at the same distance from the LED light bulb and again we put a thermometer underneath it. It is essential for both thermometers and the sheets of paper to lie on the same surface (for example on the wooden desk, a carpet or linoleum). We turn on the ordinary light bulb and the LED light bulb as well and read the temperature on both thermometers at regular intervals. We end the measurement after reaching the agreed time, number of measurements or when exceeding the thermometer range. After finishing the measurements, pupils compare the final temperature on both thermometers and make a conclusion about which of the used light sources emits more energy. They compare the experimentally obtained result with their initial estimation. It is advisable to remind the students at the end that both sources were chosen to emit a comparable amount of light.

The second goal is to verify the difference in how bodies with differently coloured surfaces absorb the thermal and light radiation. A blackbody absorbs all incident radiation and transforms it into its internal energy. The sheet of black paper does not match the properties of a blackbody, but the difference from the white paper will be noticeable. Based on their experience pupils should know that bodies with a dark surface get warmer and faster in the sun than bodies with a light surface. Light bodies reflect the majority of the incident energy back into the surroundings, while the dark bodies absorb most of it. Pupils will verify this commonly known fact by an experiment.

For the experimental execution of the first part of the task, it is necessary to prepare a light source, ideally an ordinary light bulb with a power input of approximately 100 W. In addition, any two thermometers with a range from room temperature up to at least 50 °C but preferably up to 100 °C and a sheet of black paper and a sheet of white paper are needed. We place the black sheet of paper within a specified distance from the light bulb and put a thermometer underneath it. We place the white sheet of paper at the same distance from the light bulb and again put a thermometer underneath it. It is essential for both thermometers and the sheets of paper to lie on the same surface (for example on the wooden desk, a carpet or linoleum). We turn on the light bulb and read the temperature on both thermometers at regular intervals. We end the measurement after reaching the agreed time, number of measurements or when exceeding the thermometer range. After finishing the measurements, pupils compare the final temperature on both thermometers and make the conclusion about which of the sheets of paper absorbed more energy. They compare the experimentally obtained result with their initial estimation. At the end of the experiment, it is appropriate to stress that both sheets were placed at the same distance from the light source so the comparable amount of energy was radiated on them (light and heath).

Notes for SEN pupils:

• Pupils with learning difficulties – they do not need to measure the temperature at regular intervals, it is satisfactory if they read the temperatures once after the set time and compare them.
• Gifted pupils – pupils can plot all measured temperatures on graphs of temperature dependence on time, always plotting both compared temperatures on one graph. They can observe the trend in temperature changes. (It should be roughly linear).