Energy from the Sun

The Sun is about 15 crore km away. Its energy crosses that empty space as electromagnetic (EM) waves, which travel at the speed of light, 3 × 10⁸ m s⁻¹. Remember Chapter 10: sound needs a medium and cannot cross a vacuum; EM waves can.

The EM spectrum is the whole family of EM waves, like classes in a school, each in its own range:

• gamma rays and X-rays: very short wavelength, very high frequency, very high energy and harmful;
• ultraviolet (UV): just beyond violet;
• visible light: the colours we see;
• infrared (IR): just below red;
• microwaves and radio waves: long wavelength, low energy.

The higher the frequency, the more energy a wave carries.

About 99 per cent of the Sun's energy that reaches the Earth is UV, visible and IR together:

• most UV is absorbed high up by the ozone layer;
• visible light reaches the ground and powers photosynthesis;
• IR warms the ground. The warm ground gives the heat off again (re-radiates it) as IR, and greenhouse gases (carbon dioxide, methane, water vapour) trap part of it, keeping the Earth warm enough for life.

Be UV-safe. UV rays (wavelength 100 to 400 nm) carry more energy than visible light. Too much can damage your eyes and skin and raise the risk of cancer. Wear UV-protective sunglasses and use sunscreen in strong sun. UV is also useful: it kills germs in water purifiers.

Two terms:

• Insolation = incoming solar radiation: the Sun's energy reaching the Earth's surface.
• Solar constant: the Sun's energy falling each second on one square metre facing the Sun, at the top of the atmosphere: about 1.4 kW m⁻² (1400 J every second on each square metre).

On the way down, some is scattered, absorbed or reflected. At the ground, on a clear day, the most you get is about 1 kW m⁻²: like ₹1400 of pocket money with ₹400 spent on the way.

How much in an hour? Energy = intensity × area × time = 1000 J s⁻¹ m⁻² × 1 m² × 3600 s = 3.6 × 10⁶ J. That's about one unit of electricity on your electricity bill, falling on every square metre in sunshine, every hour. A small part of the Thar desert, covered in solar panels, could supply all of India's electricity.

An Indian scientist mapped India's sunshine. Anna Mani measured insolation across India in the 1950s. With S. Rangarajan, she published Solar Radiation Over India (1982), India's first sunshine atlas. Her work showed how much solar energy India could use; today it is being used across the country.