How do you see this page? Light from a lamp or a window hits the page, bounces off, and goes into your eyes.
Key idea
We see things when light bounces off them into our eyes. A smooth mirror reflects light neatly: it leaves at the same angle it came in. Light also bends (refracts) when it passes from air into water or glass.
Why it works
Light slows down a little in water and glass. Like a cart rolling from pavement into sand at a slant, it turns as it slows. Lenses in glasses and magnifying glasses use this bending.
Worked example: Periscope
A periscope has two mirrors. How does it let a submarine crew see above the water?
- Light from above hits the top mirror and bounces down the tube.
- The bottom mirror bounces it sideways into the viewer's eye.
Answer: The two mirrors pass the light down and into the eye.
Common mistake
Thinking light comes out of our eyes. In a dark room with no light at all, you see nothing: there is no light to bounce off things.
Try it at home
- Put a pencil in a clear glass half full of water. Look from the side. Does it look broken?
- Put a coin in an empty cup and move your head back until the rim just hides it. Hold still while someone slowly pours in water. The coin appears!
Going further. The Optics Bench, and Refraction in Physical Science.