Light travels in straight lines
Why does your shadow stretch out long and thin in the evening?
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Explain
Light goes in straight lines.
Things that block light make shadows.
Light travels in straight lines, so it can't bend around objects by itself.
When something blocks the light, it leaves a dark area behind it. This is a shadow.
Move the object closer to the lamp, and the shadow gets bigger.
Shadows form because light travels in straight lines called rays. Opaque objects block them.
Similar triangles give the shadow size: S = H × L / d.
Worked example: a 0.5 m object is 2 m from a lamp, and the wall is 6 m away. S = 0.5 × 6 / 2 = 1.5 m.
- Light leaves a source
- It travels straight
- Objects block it
Want the full story?
A real lamp isn't a single point, so shadows have a soft edge called a penumbra. Light does bend very slightly around edges, which is called diffraction. A lunar eclipse is the Earth's shadow falling on the Moon.
The Formula
shadow height = object height × lamp-to-wall distance ÷ lamp-to-object distance
- the height of the shadow, measured in metres (m)
- the height of the object, measured in metres (m)
- the distance from lamp to wall, measured in metres (m)
- the distance from lamp to object, measured in metres (m)
Try the numbers
Shadow of a 0.5 m object, with the wall 6 m away: 1 m
Try it at home
You need a torch, a toy, a wall.
You need: a torch, a toy, a wall.
Make hand shadows on the wall.
- Darken a room, and shine a torch at a wall.
- Hold a toy between them.
- Move it closer to the torch, then closer to the wall.
What's happening: nearer the torch, the toy blocks more of the light, so the shadow is bigger.
Physics is life
Your body
Your shadow follows you.
Daily life
Sundials tell the time with shadows.
Nature
Trees give us shade.
Space
Eclipses are huge shadows.
Quick check
Question 1 of 3
Words to know
Light: Energy that travels as waves you can see.