1The two things behind every shadow
Light goes straight — and a shadow is just blocked light
You only need two ideas. Watch each one:
Light travels in straight lines
Light shoots out and keeps going straight until something stops it. Think of it as zillions of tiny straight beams flying out from where the light is made.
A shadow is where light is blocked
Put something in the way and the beams can't get through. Behind it, where no beams reach, you get a dark patch on the wall. That dark patch is the shadow.
2Two kinds of light
The pinpoint lamp vs the big glowing panel
The light source is one part of the puzzle. Compare light coming from one spot with light coming from a broad panel:
The pinpoint lamp
Light leaves from one tiny spot, so every beam hits the pencil's edge from the same direction.
The big glowing panel
Light leaves from a whole spread of spots, so beams reach the edge from many different directions at once.
3Your turn — make a shadow
Drag the pencil and watch its shadow follow
Here's one tiny pinpoint lamp on the left, a wall on the right, and a pencil in between. Drag the pencil up, down, near the wall, or far from it — and watch the straight beams paint its shadow onto the wall.
Grab the teal pencil and move it around. Notice: with this one tiny lamp, the shadow's edge stays crisp wherever you put it.
4Now change the gap to the wall
Same broad lamp. Move the pencil toward the wall.
You moved a pencil under a point lamp in the practice. Now the lamp is a fixed broad panel. Slide the pencil from the middle of the beam toward the wall. Predict how the gray edge will change.
Guess before you move the pencil
Keep this broad panel lamp the same size. As the pencil gets closer to the wall, will its gray shadow edge become narrower or wider?