Required Practical: Investigating Infrared Radiation

Purpose of the Practical

This practical investigates how infrared (IR) radiation is emitted and absorbed by different surfaces. It helps you understand that infrared radiation is a way heat energy is transferred without direct contact. You will explore how different materials absorb or reflect infrared radiation and how this affects temperature changes detected by thermometers or IR detectors.

Apparatus and Setup

• Infrared radiation source, such as a heat lamp or an electric bulb (with the filament acting as an IR emitter)
• Thermometer or infrared detector to measure temperature changes
• Surfaces with different properties, for example:
  • Black, matt surface (good absorber)
  • White or light-coloured surface
  • Shiny, metallic surface (good reflector)
• Insulating stand or holder to keep surfaces at a fixed distance from the IR source
• Stopwatch or timer to measure exposure time
• Ruler or measuring tape to keep distance consistent

Method Steps

1. Set up the IR source so it shines directly onto the surface being tested, keeping the distance constant (e.g., 10 cm).
2. Place the thermometer or IR detector close to the surface but not touching it, to measure temperature changes caused by IR radiation.
3. Record the initial temperature of the surface before switching on the IR source.
4. Turn on the IR source and allow it to shine on the surface for a fixed time (e.g., 2 minutes).
5. Record the temperature after exposure to infrared radiation.
6. Repeat the experiment for each different surface material, ensuring the same distance and exposure time.
7. Record all temperature changes systematically in a table for comparison.

Observations and Results

You will notice that the temperature of the surface rises due to absorption of infrared radiation. The amount of temperature increase depends on the surface's ability to absorb IR radiation:

• Black, matt surfaces absorb the most infrared radiation, so they show the greatest temperature rise.
• White or light-coloured surfaces absorb less infrared radiation, so the temperature rise is smaller.
• Shiny, metallic surfaces reflect most infrared radiation, resulting in little or no temperature increase.

This shows how surface colour and texture affect infrared radiation absorption and reflection.

For instance, if a black surface’s temperature rises from 20°C to 30°C after 2 minutes of IR exposure, while a shiny surface only rises from 20°C to 22°C, this indicates the black surface absorbs more IR radiation.

Conclusion and Applications

Infrared radiation transfers heat energy from the source to the surface without direct contact. The practical demonstrates that different surfaces absorb or reflect infrared radiation differently due to their colour and texture.

This explains why black, matt surfaces get hotter in sunlight, while shiny surfaces stay cooler. It also relates to everyday applications such as thermal insulation:

• Thermal clothing often uses materials that reflect infrared radiation to keep heat in.
• Buildings use shiny foil insulation to reflect IR radiation and reduce heat loss.

Example: Calculating Temperature Change Due to Infrared Radiation

Suppose a thermometer on a black surface reads 18°C before the IR lamp is switched on. After 3 minutes of exposure, the temperature rises to 28°C. The temperature change is:

$\Delta T = 28^\circ C - 18^\circ C = 10^\circ C$

This shows the black surface absorbed infrared radiation and warmed up by 10°C in 3 minutes.