PHY097 · Environmental Physics · Classes 8–12
Albedo measurement
Objective: To measure the albedo (reflectivity) of different surfaces with a light meter.
🧰 Apparatus
- Light meter or phone light-sensor app
- Surfaces: snow/white sheet, sand, grass, asphalt, water
- Stand to hold the meter at a fixed height
📐 Formulas and SI units
α = reflected / incident (0 to 1)
absorbed = (1 − α) × incident
💡 Theory
Albedo is the fraction of incoming sunlight that a surface reflects: α = reflected/incident. Fresh snow reflects 80–90%, grass about 25%, asphalt and ocean only 5–8%. Surfaces with low albedo absorb more energy and get hotter, which matters for city heat and melting ice.
🪜 Procedure
- Hold the meter facing up to measure the incoming light.
- Turn it to face the surface at the same height and measure the reflected light.
- Calculate α.
- Repeat for each surface.
- Rank the surfaces and relate to how hot they get.
📋 Observations Simulated values
✅ Result and conclusion
300 W/m² reflected out of 1000 W/m² gives α = 0.30, typical of grass or a mixed landscape.
🔬 In a real lab
Light meters respond to visible light only, while albedo depends on the full solar spectrum; readings also change with the sun angle.
⚠️ Safety
Never look at the Sun; work in shade when possible on hot days.
❓ Quick check
1. Which has the highest albedo?
2. Albedo has…
3. White roofs keep buildings cooler because they have…
🏋️ Practice
EasyIncident 800 W/m², reflected 640. α?
Answer: 0.8
MediumAbsorbed for α = 0.06 and 1000 W/m²?
Answer: 940 W/m²
ChallengeWhy are cities hotter than villages?
Answer: Dark roads and roofs absorb more (urban heat island)