Stand in the sun and a dark shape follows you: your shadow. Why is it there?
Key idea
Light travels in straight lines. When something blocks it, the place behind it gets no light: a shadow. The closer the object is to the light, the bigger its shadow.
Things that block light make shadows: we call them opaque, like wood or your hand. Transparent things, like clear glass, let light through. Translucent things, like wax paper, let some through.
Why it works
Rays from the lamp spread out in straight lines. Close to the lamp, the block stops a wide fan of rays, so a big patch of wall goes dark.
Worked example: Puppet show
You want a giant shadow puppet on the wall. Where do you hold your hand?
- Shadows grow as the object gets closer to the light.
Answer: Close to the flashlight, far from the wall.
Common mistake
Thinking a shadow is a kind of reflection or is made of something. It is only the place the light cannot reach.
Try it at home
- In a dark room, shine a flashlight on a wall. Make hand shadows.
- Move your hand toward the light and away. Measure the shadow's width with a ruler each time.
- Outdoors, never look straight at the Sun.
Going further. The Optics Bench shows light rays.