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

2.3.3 Radiation – Good and Bad Emitters of Infra‑red (IR) Radiation

Learning Objective

Describe, design and evaluate experiments that differentiate good and bad emitters of infrared radiation, and explain the results using emissivity, absorptivity, reflectivity, colour/texture and thermal equilibrium (Kirchhoff’s law).

Key Concepts (Syllabus Core)

• All bodies with a temperature above 0 K emit electromagnetic radiation. At ordinary temperatures the dominant part of this radiation lies in the infrared (IR) region (≈ 700 nm – 1 mm).
• Thermal radiation is an electromagnetic wave; it can travel through a vacuum and does not need a material medium.
• Kirchhoff’s law (core syllabus point): for a given wavelength λ and temperature T, $$e(\lambda,T)=a(\lambda,T)$$ where e is emissivity and a is absorptivity. Because the sum of absorptivity, reflectivity and transmissivity equals 1, a surface that absorbs strongly also emits strongly and reflects weakly.
• When the rate of energy absorbed equals the rate emitted, the object is in thermal equilibrium and its temperature stays constant. If absorption > emission the object warms; if emission > absorption it cools.
• Surface colour and texture determine the three radiative properties:
  • Dark, matte surfaces – high absorptivity → high emissivity → good emitters.
  • Bright, glossy or polished metal – low absorptivity → low emissivity → bad emitters. They also have high reflectivity, sending most incident IR back out.
• Quantitative definition:
  • Good emitter: e ≈ 1 (≈ 0.9 – 1.0).
  • Bad emitter: e ≈ 0 (≈ 0.0 – 0.1).

Qualitative Indicators of Emissivity

Experimental Methods to Distinguish Emitters

1. Thermometer‑in‑a‑Box (IR Radiation Detector) Method

1. Build a sealed container (e.g., a cardboard box) with a single opening on the top.
2. Fix a calibrated digital thermometer or thermistor probe at the centre of the box, away from the walls.
3. Cover the opening with a test sheet (black matte, white glossy, polished metal, etc.). Ensure the sheet lies flat with no air gaps.
4. Place the box on a uniform heat source (hot plate) set to a constant temperature (e.g., 80 °C). The hot plate emits IR that passes through the sheet into the box.
5. After a fixed interval (5 min) record the internal temperature.
6. Repeat for each material, keeping all other conditions identical.

2. Crookes Radiometer Method

1. Obtain a Crookes radiometer (four vanes on a low‑friction spindle).
2. Cover each vane with a different test material (same size, same orientation).
3. Place the radiometer in a darkened enclosure to suppress visible‑light effects.
4. Introduce a controlled IR source (e.g., a heated black‑body plate) at a fixed distance (~10 cm).
5. Observe the direction of rotation and measure the angular speed (rpm) after 30 s. Faster rotation ⇒ greater IR absorption ⇒ higher emissivity.

3. Non‑Contact IR Thermometer Method

1. Prepare identical flat samples of each test material (same area, same surface finish).
2. Heat all samples in a water bath to the same temperature (e.g., 60 °C) and allow them to equilibrate.
3. Remove the samples, quickly position a calibrated IR thermometer at a fixed distance (10 cm) from each surface, and record the displayed temperature.
4. A higher reading indicates a higher emissivity (good emitter).

Experimental Setup Summary

Sample Procedure (Thermometer‑in‑a‑Box)

1. Calibrate the thermometer in ambient air; note T_ambient.
2. Set the hot plate to T_plate = 80 °C and allow it to stabilise for 3 min.
3. Place the empty box on the plate, opening upward.
4. Cover the opening with the black‑matte sheet (good emitter) and start a timer.
5. After 5 min record the internal temperature T_good.
6. Repeat steps 3‑5 with a polished aluminium sheet (bad emitter) and record T_bad.
7. Calculate the temperature difference: ΔT = T_good − T_bad.
8. Perform at least three trials for each material and use the average ΔT for analysis.

Data Analysis

For a surface of area A at absolute temperature T, the radiated power is

where e is emissivity and σ = 5.67 × 10⁻⁸ W m⁻² K⁻⁴ (Stefan‑Boltzmann constant). In the box experiment the incident IR power is the same for each sheet, so the observed temperature rise is directly proportional to e. Ranking by ΔT therefore gives a qualitative ordering of emissivity:

• Largest ΔT → highest e → good emitter.
• Smallest ΔT → lowest e → bad emitter.

Safety Considerations

• Wear heat‑resistant gloves when handling hot plates, heated samples or the IR source.
• Do not look directly at bright IR emitters; use appropriate shielding or goggles.
• Secure the radiometer on a stable surface to prevent tipping.
• Allow all heated equipment to cool before storage or disposal.
• Keep electrical equipment away from water during the water‑bath portion of the IR‑thermometer method.

Conclusion

The three classroom‑friendly investigations provide clear, observable evidence of the differences between good and bad IR emitters. Dark, matte surfaces consistently produce larger temperature rises, faster radiometer rotation and higher IR‑thermometer readings, confirming their high absorptivity and emissivity (good emitters). Shiny or reflective surfaces behave oppositely, demonstrating low emissivity (bad emitters). These results illustrate Kirchhoff’s law, the link between colour/texture and emissivity, and the concept of thermal equilibrium – all core points of the Cambridge IGCSE 0625 syllabus.