Describe experiments to distinguish between good and bad absorbers of infrared radiation
2.3.3 Radiation – Good vs Bad Infrared Absorbers
What is an Absorber?
An absorber is a material that takes in infrared (IR) radiation and converts it into heat. Good absorbers heat up quickly, while bad absorbers stay cool. Think of a black hoodie on a sunny day vs a shiny silver hoodie – the black one feels hot, the silver stays cool. 🌞🧥
Key Properties to Test
- Temperature rise when exposed to IR source
- Time taken to reach equilibrium temperature
- Colour and surface texture influence absorption
Experiment 1 – Thermometer Test
- Place a small thermometer on a sheet of material (e.g., black paper, white paper, aluminium foil).
- Hold an infrared lamp (or a warm hand) 10 cm above the material.
- Record the temperature every 10 s for 2 minutes.
- Plot the temperature vs time and compare the slopes.
Good absorbers show a steep rise (high slope) and a higher final temperature. Bad absorbers have a gentle slope and lower final temperature. 📈
Experiment 2 – The “Hot Plate” Test
- Heat a hot plate to 100 °C.
- Place a sheet of material on the plate and cover it with a transparent glass.
- Measure the temperature of the material with a contact thermometer.
- Repeat with different materials.
The material that reaches the highest temperature is the best absorber. The glass allows IR to pass, so the material’s absorption is the main factor. 🔬
Experiment 3 – “Cold Spot” Test (Infrared Thermography)
If you have access to an infrared camera (or a smartphone IR attachment), you can see the temperature distribution instantly. Place a black and a white sheet on a warm surface and look at the IR image. The black sheet will show a brighter (hotter) spot. This visual evidence is great for exams! 📸
Table – Typical Absorption Coefficients
| Material | Absorption Coefficient (α) | Typical ΔT (°C) after 2 min |
|---|---|---|
| Black paper | ≈ 0.95 | ≈ 30 |
| White paper | ≈ 0.25 | ≈ 10 |
| Aluminium foil | ≈ 0.05 | ≈ 2 |
Exam Tip Box
Tip: When asked to identify a good or bad absorber, look for colour, surface roughness, and material type. Black, matte surfaces are usually good absorbers; shiny, metallic surfaces are bad absorbers.
Analogy – The Sun’s Blanket
Imagine the Sun as a giant blanket. A black hoodie traps the blanket’s warmth, while a silver hoodie reflects it away. In physics terms, the black hoodie has a high absorption coefficient, so it converts IR to heat efficiently. The silver hoodie has a low absorption coefficient, so it reflects IR and stays cool. 🧥☀️
Quick Summary
- Good absorbers: high α, quick temperature rise, dark or matte surfaces.
- Bad absorbers: low α, slow temperature rise, shiny or light surfaces.
- Use simple thermometer or IR camera to test.
- Remember the colour analogy for quick recall.
Revision
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