Electromagnetic Radiation

Sunlight takes 8 minutes and 20 seconds to reach Earth. It arrives carrying 1,361 watts per square meter, enough to power a small hair dryer from every square meter of surface it hits. That energy traveled 150 million kilometers through the vacuum of space with no wire, no pipe, and no medium at all.

Electromagnetic radiation is energy carried by oscillating electric and magnetic fields moving at the speed of light (about 3 x 10⁸ m/s). What distinguishes one type from another is wavelength. Radio waves have wavelengths measured in meters. Infrared (heat radiation) ranges from about 700 nanometers to 1 millimeter. Visible light spans a narrow band from roughly 400 nm (violet) to 700 nm (red). Ultraviolet, X-rays, and gamma rays get progressively shorter. All travel at the same speed; only the wavelength and energy per photon differ.

The energy connection: shorter wavelengths carry more energy per photon. This is why ultraviolet light causes sunburn but radio waves pass harmlessly through your body. For solar energy, the key fact is that about 43% of sunlight's energy arrives as visible light and most of the rest as infrared. A solar panel's efficiency depends on which wavelengths its semiconductor material can absorb and convert.

Worked Example

Earth's surface receives an average of about 170 watts per square meter of solar radiation (accounting for atmosphere, clouds, night, and angle).

• Pick a panel area. A typical rooftop solar panel is about 1.7 m².
• Calculate incoming solar power. 170 W/m² x 1.7 m² = 289 watts hitting the panel.

If the panel converts 20% of incoming radiation to electricity, what is its output?

289 x 0.20 = about 58 watts. That is roughly enough to run a laptop. The other 80% is reflected or absorbed as heat, which is why panel efficiency is such a central metric in solar economics.

The energy carried by individual photons connects directly to how solar cells work, which builds on this foundation.

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