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Solar radiation is the Earth’s main energy source — but it never falls evenly. That unevenness sets the whole planet’s winds and currents in motion.
- Solar radiation reaches the Earth as electromagnetic (EM) waves that travel through vacuum at the speed of light, \( 3 \times 10^{8} \) m s\(^{-1}\).
- The EM spectrum ranges from high-frequency, short-wavelength gamma rays and X-rays to low-frequency, long-wavelength infrared and radio waves.
- About 99% of the Sun’s energy reaching us is ultraviolet (UV), visible and infrared (IR); gamma rays and X-rays are mostly filtered by the upper atmosphere.
- Short-wavelength UV radiation is mostly absorbed by the ozone layer , protecting life and warming the upper atmosphere.
- Visible light reaches the surface and powers photosynthesis; it also partly warms the land and water.
- Infrared radiation warms the surface, which re-radiates heat; some of this outgoing heat is trapped by greenhouse gases (CO₂, CH₄, water vapour), keeping the Earth warm.
The UV rays lie in the wavelength range of 100 nm to 400 nm [1 nanometre (nm) = \( 10^{-9} \) m] and carry much higher energy than visible light. Prolonged exposure can damage the eyes and skin and increase cancer risk, so use UV-protective glasses and sunscreen. UV rays are also useful for killing germs in water purifiers and for powering fluorescent lights.
- Insolation is the amount of the Sun’s radiation that reaches the Earth’s surface; it warms the surface and the atmosphere.
- The solar constant is the average solar energy received per unit time per unit area perpendicular to the Sun’s rays at the top of the atmosphere — about \( 1.4 \) kW m\(^{-2}\) (about 1400 J s\(^{-1}\) m\(^{-2}\)).
- After absorption and scattering, the maximum insolation reaching the surface is lower — about \( 1 \) kW m\(^{-2}\) under clear skies.
Anna Mani , India’s pioneering atmospheric scientist, mapped solar insolation across India in the 1950s. With S. Rangarajan she published Solar Radiation Over India (1982), creating the country’s first insolation atlas . Her measurements revealed India’s vast solar potential, now being realised through large-scale solar power.
How much solar energy will be received by a \( 1\ \text{m}^2 \) area in one hour, if the insolation on the surface of the Earth were \( 1 \) kW m\(^{-2}\)?
Answer: \( E = \text{Intensity} \times \text{area} \times \text{time} \)
\( E = 1 \times 1000\ \text{J s}^{-1}\text{m}^{-2} \times 1\ \text{m}^2 \times 3600\ \text{s} \)
\( E = 3600000\ \text{J} = 3.6 \times 10^{6}\ \text{J} \)
This is roughly the energy needed to melt 5 kg of ice and heat the water to 100°C — and equals about one unit of electricity used in a household.
An interesting estimation: how much of the Earth’s surface must be covered with solar panels to supply all the electric power India uses today? Find the numbers online, assume some insolation and a conversion fraction, and you will likely find that even a fraction of the Thar desert , if covered with solar panels, could meet India’s electricity needs.
