Mon Prof Augmenté
Timing a Light Beam to a Mirror
Age 12 · Physics
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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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
Quiz: Timing a Light Beam to a Mirror (Age 12)
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You time how long a light pulse takes to travel to a distant mirror and come back. To turn that time into a distance, what else must you know?
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A second mirror is set up twice as far away as the first one. How long does a light pulse take to go out to it and come back?
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The clock measures the time for the pulse to travel out to the mirror and back again. How do you get the distance to the mirror from it?
The answers, each with its explanation, are in the app. This lesson has left the free week, so its quiz is part of the subscription.
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