2.3.3 Radiation: Distinguishing Good and Bad Infrared Absorbers

Objective

Describe experiments to distinguish between good and bad absorbers of infrared radiation.

Understanding Infrared Radiation

Infrared (IR) radiation is a type of electromagnetic radiation with wavelengths longer than visible light. It's often associated with heat. Different materials absorb IR radiation to varying degrees. Good absorbers are dark in colour and tend to heat up quickly when exposed to IR. Poor absorbers are light in colour and heat up slowly.

Experimental Design: Comparing Infrared Absorption

A simple experiment to compare the ability of different materials to absorb infrared radiation involves using a heat source, a thermometer, and the materials themselves.

Materials Required

• Infrared lamp (heat lamp)
• Thermometer (accurate to 0.1 °C)
• Different materials (e.g., black cloth, white cloth, aluminium foil, wooden block, plastic sheet)
• Tripod or stand to hold the materials
• Ruler or measuring tape
• Stopwatch
• Thermometer

Apparatus Setup

1. Position the infrared lamp at a fixed distance (e.g., 20 cm) from the materials. Ensure the lamp is stable and cannot move during the experiment.
2. Place each material on a separate stand or support, ensuring they are at the same distance from the lamp.
3. Position the thermometer so that the bulb is in contact with the surface of each material.
4. Record the initial temperature of the thermometer.

Procedure

1. Turn on the infrared lamp and allow it to heat up for a short period (e.g., 2 minutes).
2. At regular time intervals (e.g., every 30 seconds), record the temperature reading from the thermometer for each material.
3. Continue recording the temperature for a set duration (e.g., 5-10 minutes).
4. Record the temperature readings in a table.

Data Table

Time (s)	Black Cloth (°C)	White Cloth (°C)	Aluminium Foil (°C)	Wooden Block (°C)	Plastic Sheet (°C)
0
30
60
90
120
150

Analysis

After collecting the data, plot a graph of temperature vs. time for each material. The material that shows the steepest increase in temperature over time is the good absorber of infrared radiation. The material that shows a slower increase in temperature is the poor absorber.

Conclusion

Based on the experimental results, conclude which materials are good absorbers and which are poor absorbers of infrared radiation. Explain your findings in terms of the relationship between the material's colour and its ability to absorb IR radiation. Darker colours absorb more IR radiation than lighter colours.

Safety Precautions

• Do not look directly at the infrared lamp.
• Be careful when handling the hot materials.
• Ensure the lamp is placed on a stable surface.