TwiddleLab / Physics / Light and Sight
Light Travels in Straight Lines
What you will learn
- Light travels in straight lines. We draw it as rays.
- A pinhole lets one ray through from each point, so the picture is upside down, and its size depends on the two distances.
- A shadow is where the rays are blocked: closer to the lamp, the shadow grows.
How the lesson goes
Why is the picture in a pinhole camera upside down? First: is it?
A candle shines through a tiny hole onto a screen in a dark box.
Move the candle and the screen. Follow the two rays.
One ray from the flame tip, one from the base. Each goes straight through the hole.
Make the hole bigger. Can you have bright and sharp at once?
Find a hole size where the picture is bright enough and still sharp.
From one point of the flame, light passes through every part of the hole. Through a 4 mm hole that light spreads into a spot several mm wide on the screen. Many overlapping spots make a blur. This trade-off is why real cameras use a lens instead of a pinhole.
Light rays.
In one material, light travels in straight lines. We draw its path as a ray: a line with a direction.
A pinhole image is upside down because rays from the top and bottom cross at the hole. Its height = object height × (hole to screen ÷ object to hole).
A shadow is the region the rays cannot reach because something blocks them.
Simplified: a ray is a model. Light is really a wave, and through very tiny holes it spreads out a little (diffraction). For everyday sizes, rays work very well.
Game: Shadow puppets. Make each shadow fill its outline.
Three puppets. Fewer moves, more stars.
Where does the ray from the flame tip land on the screen?
A ray never turns in air. From the flame tip it slopes down to the hole, so after the hole it is still sloping down. With the screen farther than the candle, it lands even farther from the middle: the picture is bigger and upside down.
Make your shadow theatre poster.
Your three shadows, at the sizes you made them.
Next lesson: Mirrors: Angle In, Angle Out · Part of the course: Light and Sight
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