Mon Prof Augmenté
Timing a Light Beam to a Mirror
A telescope fires a short pulse of light at a mirror far away, and the light bounces straight back. Because light always travels at the same known speed, the time the round trip takes tells you exactly how far away the mirror is.
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Age 12: open this week
Browse Age 12
In the picture
- The domed observatory on the hilltop, its shutter open
- The thin green beam climbing from the dome into the night sky
- The square panel of corner-cube prisms on the grey dusty ground
- The stopwatch in the foreground, its hand part way around the dial
Examples
- Shouting at a cliff and counting the seconds until the echo comes back
- A builder’s laser measure that reads a room’s length in an instant
Key takeaways
- Light travels about 300,000 kilometres every second
- The beam covers the gap twice, so halve the round-trip time
Carry on in the app
Three questions on this lesson. It has left the free week, so its quiz is for subscribers.
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