The dark shape is where the light does not land.
The sun is shining. A dark shape sits on the floor. What is making it?
Eve can say this.
The sun is shining. A dark shape sits on the floor. What is making it? Move the lamp. Then hold your hand in the light.
Words Eve says
The sun is shining. A dark shape sits on the floor. What is making it? Move the lamp. Then hold your hand in the light.
Your hand, the sun
Use sunlight or a lamp. Never look straight at the sun.
Stand in sunlight or by a bright lamp. Hold out your hand. Look at the dark shape on the ground.
Move your hand closer to the ground. Then farther. Watch the shape change.
Do not look at the sun. Look at the floor.
What if there were two lamps?
Look closer
Notice
The dark shape sits on the far side of the ball, away from the lamp.
Move the lamp and the dark shape moves. The ball did not go anywhere.
The dark is not a thing poured on the floor. It is missing light.
What do you think will happen?
If you move the lamp closer to the ball, what happens to the dark shape?
Change where the lamp sits.
Keep looking
What must be true?
Light leaves the lamp in straight lines.
The ball is in the way, so some of those lines never reach the floor.
The dark shape is the patch of floor that did not get the light. Move the lamp, and a different patch is missed.
The missing light has a name.
We call that patch a shadow. A harder word for the fully dark middle is umbra. You do not need the word to see the rule: light travels in straight lines, and a shadow is the light that did not arrive. The lines we draw from the lamp past the ball are a map. They are not strings.
The sun is shining. A dark shape sits on the floor. What is making it? Light travels in straight lines. The ball is in the way, so the floor behind it does not get the light. That missing light is the shadow. Move the lamp. The dark shape moves. Hold your hand in the sun and look.
Words Eve says
The sun is shining. A dark shape sits on the floor. What is making it? Light travels in straight lines. The ball is in the way, so the floor behind it does not get the light. That missing light is the shadow. Move the lamp. The dark shape moves. Hold your hand in the sun and look.
The light that did not land
People have watched shadows for as long as there has been noon.
A thousand years ago, Ibn al-Haytham studied light in a dark room with a small hole. The picture on the wall was upside down because light still travelled in straight lines.
The rule you can test with a hand is the same rule: if light goes straight, something in the way leaves a dark shape.
- NASA: Why is the sky blue, and how light travels · Sunlight as something that travels, and can be blocked.
- Exploratorium: Shadows · Hands-on light and shadow with ordinary lamps.