Light: how we see, shadows, and reflection
Light travels in straight lines — and almost everything about shadows, mirrors, and how your eye works follows from that one fact.
The single most useful fact about light is that it travels in straight lines until it hits something. We even have a word for a single straight path of light: a ray. Hold on to that and shadows, mirrors, and seeing all make sense.
How we see things
Your eye doesn't send anything out. You see an object because light from a source (the Sun, a lamp) bounces off it and travels in a straight line into your eye. That's why you can't see in a completely dark room — there's no light to bounce. Things that make their own light, like the Sun or a bulb, are luminous; everything else is seen by reflected light.
Shadows: straight lines make sharp edges
A shadow forms because light can't bend around an object to reach the space behind it. Since light travels straight, the object blocks a sharp patch of darkness shaped like its outline. Move the object closer to the light and the shadow grows — the same idea as holding your hand near a torch.
Reflection and the one rule
When light hits a smooth, shiny surface it bounces off in a predictable way. Measure both rays from the normal — an imaginary line drawn at right angles to the mirror — and:
The incoming and outgoing rays make equal angles with that normal. Every mirror, every time.
Two kinds of reflection
- Smooth surface (a mirror): all the rays bounce the same way, so you get a clear image. This is regular reflection.
- Rough surface (paper, a wall): the rays scatter in all directions, so you see the object but no image. This is diffuse reflection — and it's why most things don't act like mirrors.
Always draw the normal first. It's a dotted line at to the surface, and both angles are measured from it — never from the mirror itself. Skip it and your angles will be wrong.
Last revised 8 February 2025.