Luminous and non-luminous

Keshav spends part of his summer holiday in a village in the Western Ghats of Maharashtra. What fascinates him most is hundreds of fireflies flashing their lights at night. Jatin's grandparents explain that fireflies are seasonal insects that use their light to communicate, and that their numbers are falling because of light pollution, fewer forests and too much tourism.

On the evening bus home, Keshav looks at the moonlit hills and wonders: does the Moon make its own light? He remembers from Grade 6 that every other object in our solar system shines only by reflecting the Sun's light.

The Sun gives out (emits) its own light; it is the main source of natural light on the Earth. Stars, lightning, natural fire and some animals, like fireflies, also give out their own light.

Long ago, people learnt to make fire — the very first artificial light — and later to burn different fuels for light: animal fat, oil, wax and gas. Since electricity was invented, most of our lighting is electric.

Objects that give out their own light are luminous. Objects that do not are non-luminous. A tree, a table, a book and you are all non-luminous: we see them only because light falls on them and bounces into our eyes.

So the answer to Keshav's question: the Moon is non-luminous. It makes no light of its own; the moonlight we see is really sunlight bouncing off the Moon.

LED lamps use much less electricity, are brighter and last longer than old-style bulbs, which saves money and is better for the environment. When an LED lamp finally stops working, it must be disposed of properly or recycled, not thrown in the rubbish.