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

2.3.3 Radiation

Learning Objective

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

Key Concepts

• All bodies above absolute zero emit electromagnetic radiation.
• Infra‑red radiation lies in the wavelength range \$700\ \text{nm}\$ to \$1\ \text{mm}\$.
• A “good emitter” absorbs and re‑emits IR radiation efficiently; a “bad emitter” does so poorly.
• Kirchhoff’s law of thermal radiation: for a given wavelength and temperature, emissivity \$e\$ equals absorptivity \$a\$.

Qualitative Indicators of Emissivity

Materials that appear dark (e.g., black matte surfaces) are usually good emitters, whereas shiny or reflective surfaces (e.g., polished metal) are poor emitters.

Experimental Methods

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

1. Place a small sealed container (e.g., a cardboard box) with a single opening on one side.
2. Insert a sensitive thermometer or a thermistor probe at the centre of the box, away from the walls.
3. Cover the opening with a thin sheet of the test material (good or bad emitter) and secure it.
4. Expose the assembly to a uniform heat source (e.g., a hot plate set to a constant temperature).
5. Record the temperature rise inside the box after a fixed time (e.g., 5 min).

2. Radiometer (Crookes Radiometer) Method

1. Obtain a Crookes radiometer with four vanes mounted on a low‑friction spindle.
2. Cover each vane with a different test material (black matte, white glossy, polished aluminium, etc.).
3. Place the radiometer in a darkened enclosure to minimise visible light.
4. Introduce a controlled IR source (e.g., a heated blackbody plate) at a fixed distance.
5. Observe and note the direction and speed of rotation – faster rotation indicates higher IR absorption/emission.

3. IR Thermometer (Non‑Contact) Method

1. Heat identical samples of the test materials to the same temperature using a water bath.
2. Remove the samples and quickly position an IR thermometer at a fixed distance (e.g., 10 cm) from each surface.
3. Record the temperature reading displayed by the IR thermometer for each material.
4. A higher reading corresponds to a higher emissivity (good emitter).

Experimental Setup Summary

Sample Procedure (Thermometer‑in‑a‑Box)

1. Calibrate the thermometer in ambient air and record \$T_{\text{ambient}}\$.
2. Heat the hot plate to \$T_{\text{plate}} = 80^{\circ}\text{C}\$ and allow it to stabilise.
3. Place the empty box on the plate, open side facing upward.
4. Cover the opening with a black matte sheet (good emitter) and start a timer.
5. After 5 min, record the internal temperature \$T_{\text{good}}\$.
6. Repeat steps 3–5 using a polished aluminium sheet (bad emitter) and record \$T_{\text{bad}}\$.
7. Calculate the temperature difference \$\Delta T = T{\text{good}} - T{\text{bad}}\$.

Data Analysis

Using the Stefan‑Boltzmann law for a surface of area \$A\$:

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

where \$e\$ is emissivity, \$\sigma = 5.67 \times 10^{-8}\ \text{W m}^{-2}\text{K}^{-4}\$, and \$T\$ is absolute temperature.

The measured temperature rise is proportional to \$e\$, allowing a qualitative ranking of the test materials.

Safety Considerations

• Handle hot plates and heated samples with heat‑resistant gloves.
• Do not look directly at bright IR sources; use appropriate shielding.
• Ensure the radiometer is placed on a stable surface to prevent tipping.
• Allow all heated equipment to cool before storage.

Conclusion

The three described experiments provide reliable, classroom‑friendly ways to distinguish good IR emitters from bad ones.

Consistently, materials with high absorptivity (dark, matte surfaces) show larger temperature rises, faster radiometer rotation, and higher IR‑thermometer readings, confirming their status as good emitters. Conversely, reflective or shiny surfaces behave as poor emitters.