How far does light travel in a year…? How far does it travel in a second…? How long does it take to travel 30cm…?

Set up some axes on your graph paper with: - distance along the x-axis in light seconds - time along the y-axis in seconds If you plotted the path of a beam of light on your graph, what angle would the line make…?

Think about/discuss: What would it mean if the line was steeper than 45°? What would it mean if the line was less than 45°? What would the line look like for an object travelling back towards us?

Consider an asteroid moving away from us at a speed of 0.5c. think: how far in light seconds will the asteroid travel each second? what will the angle of the line be? The asteroid is initially at a distance of 1 light second. Plot a line to show the position of the asteroid against time.

A short pulse of radio waves is transmitted at time t=0. Plot a dotted line to show when the pulse hits the asteroid. At what time would the pulse hit the asteroid? How far away was the asteroid at this time? The pulse is reflected by the asteroid directly back towards the transmitter. Plot another dotted line to show the path of the reflected pulse. At what time would the pulse return to the transmitting station?

A second pulse of radio waves is transmitted at time t=1. Plot a dotted line to show when the pulse hits the asteroid. At what time does the pulse hit? How far away was the asteroid at this time? The pulse is again reflected by the asteroid. Plot another dotted line to show the path of the returning pulse. At what time would the pulse return to the transmitting station?

You should now know the position of the asteroid at 2 times. Use this information to verify that the speed of the asteroid is indeed 0.5c…!