2.3.3 Radiation: Distinguishing Good and Bad Infrared Emitters

This section describes experiments designed to differentiate between materials that efficiently emit infrared radiation (good emitters) and those that do not (bad emitters). The concept of emissivity is central to understanding this distinction.

Emissivity

Emissivity (ε) is a measure of how effectively a body radiates thermal energy compared to a perfect blackbody at the same temperature. A blackbody is a theoretical object that absorbs all incident radiation. The emissivity value ranges from 0 to 1.

• ε = 0: Perfect reflector (no radiation emitted).
• ε = 1: Perfect blackbody (maximum radiation emitted).
• Materials with ε close to 1 are good emitters.
• Materials with ε close to 0 are poor emitters.

Experiment 1: Using a Thermal Detector

This experiment uses a thermal detector (e.g., a thermal imaging camera or a thermocouple) to measure the temperature of different materials under controlled conditions. The temperature measured will be related to the amount of infrared radiation emitted by the material.

Apparatus

• Thermal detector (e.g., thermal camera, thermocouple)
• Objects of different materials (e.g., black paper, white paper, metal, wood)
• Heat source (e.g., a lamp or a heated block)
• Thermometer (for ambient temperature)
• Stand to hold the objects

Procedure

1. Set up the objects at equal distances from the heat source.
2. Allow the objects to reach thermal equilibrium.
3. Use the thermal detector to measure the temperature of each object.
4. Record the temperatures.

Analysis

The object with the highest temperature will be the best emitter of infrared radiation (highest emissivity). The object with the lowest temperature will be the poorest emitter (lowest emissivity).

Experiment 2: Comparing Heat Loss Rates

This experiment compares the rate at which heat is lost from different materials when they are exposed to the same temperature environment. The rate of heat loss is directly related to the amount of infrared radiation emitted.

Apparatus

• Objects of different materials (e.g., black paper, white paper, metal, wood)
• Insulated containers (e.g., boxes or enclosures)
• Thermometer
• Heat source (e.g., a lamp or a heated block)
• Stopwatch

Procedure

1. Place each object inside its insulated container.
2. Ensure the containers are at the same temperature.
3. Expose the objects to the heat source for a fixed time.
4. Measure the temperature of each object at regular time intervals.
5. Record the temperature changes over time.

Analysis

The object that loses heat the fastest will be the best emitter of infrared radiation. This is because it is radiating more heat away.

Table Summarizing Emissivity (Approximate Values)

Material	Approximate Emissivity (ε)
Black Paint	0.9 - 0.98
Dark Wood	0.8 - 0.9
Silvered Paper	0.1 - 0.2
Aluminum	0.05
Glass	0.04
Human Skin	0.9

These experiments demonstrate how the rate of infrared radiation emitted by a material is related to its emissivity. Good emitters have high emissivity and lose heat more rapidly than poor emitters.