How solar energy travels
Solar energy travels as electromagnetic (EM) waves — oscillating electric and magnetic fields that require no medium. This is why sunlight can cross the vacuum of space. EM waves travel at the speed of light (3 × 10⁸ m/s) and span a wide range of wavelengths.
🔵 UV Radiation (100–400 nm)
Short wavelength, high energy. Can damage skin cells and DNA. Most is absorbed by the ozone layer in the stratosphere. Prolonged exposure causes sunburn and increases skin cancer risk.
🌈 Visible Light (400–700 nm)
The only part of the spectrum human eyes can detect. Drives photosynthesis in plants. Makes up about 43% of total solar energy reaching Earth.
🔴 Infrared (700 nm – 1 mm)
Longer wavelength, felt as heat. About 49% of solar energy is infrared. Greenhouse gases in the atmosphere absorb and re-emit infrared, warming Earth's surface.
📡 Radio Waves
Longest wavelength, lowest energy. Used in communications. Pass through the atmosphere without significant absorption.
Key fact: EM waves are non-mechanical — they do not need a medium to travel. Sound waves, water waves, and seismic waves are mechanical and require a medium. This is why we can hear no sound from the Sun, but we receive its light.
How surfaces reflect sunlight
Albedo is the fraction of incoming solar radiation that a surface reflects. It ranges from 0 (absorbs everything) to 1 (reflects everything). High-albedo surfaces stay cool; low-albedo surfaces absorb heat and warm up.
| Surface | Albedo | Effect |
|---|---|---|
| Fresh snow | 0.80–0.90 | Reflects most sunlight — stays cold |
| Ice / glaciers | 0.50–0.70 | High reflection — keeps polar regions cold |
| Desert sand | 0.30–0.40 | Moderate reflection |
| Grassland / crops | 0.15–0.25 | Moderate absorption |
| Forest | 0.10–0.15 | Absorbs most sunlight |
| Ocean (calm) | 0.06–0.10 | Absorbs most — stores heat |
| Asphalt / roads | 0.04–0.08 | Very high absorption — heats cities |
Ice-albedo feedback: As global temperatures rise, ice melts. Less ice means lower albedo, so more sunlight is absorbed, causing further warming — a dangerous positive feedback loop.
Why cities are warmer
Cities are significantly warmer than surrounding rural areas — a phenomenon called the Urban Heat Island (UHI) effect. The primary cause is that urban materials (concrete, asphalt, brick, steel) have low albedo and high heat capacity. They absorb large amounts of solar energy during the day and release it slowly at night, keeping cities warm even after sunset.
🏙️ Urban (Low Albedo)
Concrete roads, glass buildings, and dark rooftops absorb 90–96% of sunlight. Heat is stored and released slowly. Cities can be 2–5°C warmer than nearby rural areas.
🌾 Rural (Higher Albedo)
Vegetation, soil, and water reflect more sunlight. Plants also cool the air through transpiration (releasing water vapour), further reducing temperatures.
Solution: Urban planners use green roofs, white-painted surfaces (high albedo), and urban forests to reduce the UHI effect in cities like Mumbai, Delhi, and Bengaluru.