How Light Travels

Here is a question so obvious that almost nobody asks it: why can you see this page?

The page isn't glowing. Cover the window and switch off the lamp and it vanishes completely, even though the page is still right there in front of you. So seeing clearly isn't something your eyes do on their own. Something has to arrive.

That something is light, and once you know the single most important thing about how it moves, a surprising amount falls out of it — sharp shadows, mirrors, eclipses, and why you can see round a corner but not through a wall.

That one thing is this: light travels in straight lines. Nothing in this lesson is more useful than that sentence.

By the end you'll be able to:

• explain why we can see things that don't give out any light of their own
• use straight-line travel to explain why shadows form and why they're sharp
• sort materials into transparent, translucent and opaque
• describe what a plane mirror does to your reflection

Some things make light. Most things just pass it on.

A very short list of things around you actually make light: the Sun, a flame, a bulb, a phone screen, a firefly. These are light sources.

Everything else — this page, your hand, a tree, the Moon — makes no light at all. You see them only because light from a source lands on them and bounces off into your eyes.

That's the whole of seeing:

source → object → your eye. Light leaves a source, bounces off the thing, and arrives at you.

Switch off every source and the middle step has nothing to work with, so you see nothing. The objects didn't go anywhere; the light did.

The Moon is the example worth remembering, because it looks so much like a source. It isn't. The Moon is a cold grey rock that makes no light whatsoever — every bit of moonlight you have ever seen is sunlight that travelled to the Moon, bounced, and came back to you.

Now the rule that does the real work in this lesson:

Light travels in straight lines.

Not curves. Not round corners. Straight, until something stops it or bounces it. And once you accept that one sentence, shadows stop being mysterious — they're just the place the straight lines couldn't reach.