How fast does light travel? It travels very fast indeed. Early scientists found it
ery hard to measure the speed of light. The picture shows how Galileo tried to
measure the speed of light in the seventeenth century.
'' This method works
fine for sound but
not for light!'' he says

• His friend stood on a distant hill.
• Galileo flashed his lamp and started his clock at the same moment.
• When his friend saw the light, he flashed his lamp back towards Galileo,
• When Galileo saw the flash, he noted the time taken on his clock.
Unfortunately, this method didn't work. This was because the light travelled
J uuickly from one hill to the other. It took only a tiny fraction of a second.
T o much longer for his friend to see the light and react to it, flashing his
own lamp.
This method would work quite well if Galileo wanted to measure the
ed of sound. If his friend was 3000 m away, how long would the sound
to travel between the two hills? The speed of sound is about 300 m/s.