Investigating Infrared Radiation

Infrared (IR) radiation is a type of electromagnetic radiation that transfers thermal energy. All objects emit IR; hotter objects emit more each second. Radiation does not need a medium — you can feel heat from a fire across a room, and the Sun warms Earth through space.

Experiment: distinguishing good and bad emitters

Apparatus: Leslie cube filled with hot water (sides: dull black, dull white, shiny metal) and an IR detector (thermopile/IR sensor) or thermometer placed at equal distance.

Method: Point the detector at each surface in turn. Keep the distance and water temperature the same.

Observation: Reading is highest for dull black, lower for dull white, lowest for shiny metal.

Conclusion: Dull black is the best emitter. Shiny surfaces are poor emitters (good reflectors).

Experiment: distinguishing good and bad absorbers

Apparatus: Two identical cans, one painted dull black and one shiny; equal volumes of water at the same starting temperature; lamp; two thermometers.

Method: Place cans the same distance from the lamp. Switch on for a fixed time. Measure the temperature rise.

Observation: Water in the black can warms faster.

Conclusion: Dull black is the best absorber. Shiny is a poor absorber.

Alternative: Two identical metal plates (black and shiny) with drawing pins stuck on by wax. Warm equally from a distance. Pins drop first from the black plate.

What affects the emission rate?

• Surface temperature: higher temperature greatly increases emission rate.
• Surface area: larger area emits more energy per second.
• Surface finish: dull black surfaces emit more; shiny silver surfaces emit less (they reflect IR).
• Energy balance: constant temperature when energy in equals energy out.

$$\text{power absorbed} = \text{power emitted} \Rightarrow \text{temperature is constant}$$

Real-world connections

• Solar heaters and some radiators use dark, dull surfaces to absorb/emit IR well.
• Shiny space blankets and kettles reduce heat loss by reflecting IR.
• Animals with large ears (e.g., fennec fox) lose heat faster due to larger surface area.

Safety and common mistakes

• Hot water and lamps can burn. Use heatproof mats and avoid touching hot surfaces.
• Keep distances equal and starting temperatures the same for a fair test.
• Radiation works through a vacuum; do not confuse it with conduction or convection.
• All objects emit IR, not just glowing ones. Glowing means very hot and also emitting visible light.