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

2.3.3 Radiation

Objective

Describe experiments that can be used to distinguish between good and bad absorbers of infrared (IR) radiation.

Key Concepts

• Infrared radiation is part of the electromagnetic spectrum with wavelengths from about 0.7 µm to 1000 µm.
• All bodies emit IR radiation; the amount emitted depends on temperature and surface properties.
• A good absorber (also a good emitter) absorbs most of the incident IR radiation and re‑emits it efficiently.
• A bad absorber (also a poor emitter) reflects or transmits most of the incident IR radiation.

Typical Experimental Set‑ups

1. Black‑body vs. White‑body Plate Experiment

This experiment compares a black-painted metal plate (good absorber) with a white‑painted plate (bad absorber) when both are exposed to the same IR source.

1. Apparatus:

   • Two identical metal plates (e.g., aluminium), one painted matte black, the other matte white.
   • Infrared lamp (constant power supply).
   • Two identical thermometers or thermocouples attached to the back of each plate.
   • Insulating stand to hold plates at the same distance from the lamp.
   • Timer.

2. Procedure:

   1. Place the plates side by side, each 10 cm from the IR lamp.
   2. Turn on the lamp and start the timer.
   3. Record the temperature of each plate every 30 s for 5 min.
   4. Turn off the lamp and continue recording for another 5 min to observe cooling.

3. Observations:

   • The black plate’s temperature rises rapidly and reaches a higher steady‑state temperature.
   • The white plate rises more slowly and stabilises at a lower temperature.
   • During cooling, the black plate loses heat faster than the white plate.

4. Conclusion:

   The black plate is a good absorber (and emitter) of IR radiation, whereas the white plate is a poor absorber.

2. Infrared Transmission Through Different Materials

Identify materials that transmit IR radiation poorly (good absorbers) and those that transmit it well (bad absorbers).

1. Apparatus:

   • IR source (e.g., heated filament).
   • IR detector or thermopile connected to a voltmeter.
   • Samples: black cardboard, aluminium foil, clear glass, and plastic film.
   • Mounting frame to hold samples between source and detector.

2. Procedure:

   1. Zero the detector with no sample in place.
   2. Insert each sample one at a time and record the detector reading.
   3. Repeat three times for each sample and take the average.

3. Observations:

   • Black cardboard and aluminium foil give very low detector readings – they absorb most IR.
   • Clear glass and plastic film give relatively high readings – they allow IR to pass (poor absorbers).

4. Conclusion:

   Materials that block or absorb IR are good absorbers; those that transmit IR are bad absorbers.

Data Presentation

Example table summarising temperature rise in the black‑body vs. white‑body experiment.

Theoretical Background

The power radiated by a surface is given by the Stefan‑Boltzmann law:

\$P = \varepsilon \sigma A T^{4}\$

where \$ \varepsilon \$ is the emissivity (≈ absorptivity for opaque bodies), \$ \sigma = 5.67\times10^{-8}\,\text{W m}^{-2}\text{K}^{-4} \$ is the Stefan‑Boltzmann constant, \$ A \$ is the area, and \$ T \$ is the absolute temperature.

Good absorbers have \$ \varepsilon \approx 1 \$ (black surfaces), while bad absorbers have \$ \varepsilon \ll 1 \$ (white or reflective surfaces).

Safety Considerations

• Handle hot plates with tongs or heat‑resistant gloves.
• Do not look directly at the IR lamp; use protective goggles if the lamp is very bright.
• Ensure electrical connections are secure to avoid short circuits.

Extension Questions

1. How would the results change if the experiment were performed in a vacuum chamber?
2. Explain why a polished metal surface appears shiny but is a good absorber of IR radiation.
3. Design an experiment to quantify the emissivity of an unknown material using the Stefan‑Boltzmann law.