Straight line
Light does not go round corners here.
Three cards
A teacher holds three cards, each with a hole, in front of a torch. A screen stands behind them.
When will the light reach the screen?
What this lesson covers
The idea
Light travels in a straight line.
Three cards
A teacher holds three cards, each with a hole, in front of a torch. A screen stands behind them.
When will the light reach the screen?
- Only when the three holes are in one line
- Whenever the torch is on
- When the holes are at different heights
Line up the holes
Slide the middle card. Then look at the pipes.
A straight line
The light reached the screen only when the three holes were in a straight line. The lamp could be seen through a straight pipe but not through a bent pipe.
Light travels in a straight line.
Notes
Light travels in a straight line.
Check yourself
The torch light passed through all three holes. What does that show?
Why can you see a lamp through a straight pipe but not through a bent pipe?
The middle card is moved so that its hole is out of line. What happens to the spot on the screen?
Which statement is right?
- Light bends round the cards. The light passed only because the holes lined up.
- Light needs no holes to pass through cards. Cards are opaque. The light went through the holes.
- The holes were in one line, and light travels in a straight line — correct. Yes! That is why the spot appeared.
- Light travels in a straight line and does not go round the bend — correct. Yes! It cannot turn the corner here.
- Light is blocked by a straight pipe. The light passed along the straight pipe, so the lamp could be seen.
- A bent pipe puts the lamp out. The lamp stays on. The light does not go round the bend.
- It becomes brighter. With the holes out of line, no light reaches the screen.
- It disappears — correct. Yes! The light is blocked by the middle card.
- It moves to the middle card. The light stops at the middle card. There is no spot on the screen.
- Light needs a hole to travel. Light travels through air too, not only through holes.
- Light always travels in a zig-zag. In these experiments the light travelled in a straight line.
- Light travels in a straight line — correct. Yes! That is what these experiments show.