Close your eyes for a moment. Everything goes dark — not because the world has vanished, but because you have shut out the one thing your eyes need to work: light. Light is what lets us see. With no light at all, even the brightest red apple and your own hand would be completely invisible.
Some things make their own light. We call these light sources — the Sun, a lamp, a candle flame, a torch, a glowing screen. Everything else, like a chair or a page or the Moon, makes no light of its own. We only see those things because light from a source bounces off them and into our eyes.
Here is a lamp. Switch it On and watch what happens: it throws light out in straight lines in every direction. These straight paths of light are called rays. Light always travels in dead-straight lines until it hits something — it never bends around corners on its own. Switch the lamp Off and the rays vanish, leaving darkness.
Sunlight looks like it arrives the instant the Sun peeks over the hill, but it has actually raced all the way from the Sun — a journey that takes light about eight minutes!
First find out exactly