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

  1. Place a small thermometer on a sheet of material (e.g., black paper, white paper, aluminium foil).

  2. Hold an infrared lamp (or a warm hand) 10 cm above the material.

  3. Record the temperature every 10 s for 2 minutes.

  4. 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

  1. Heat a hot plate to 100 °C.

  2. Place a sheet of material on the plate and cover it with a transparent glass.

  3. Measure the temperature of the material with a contact thermometer.

  4. 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

MaterialAbsorption 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.