LASER-3

day we know that the speed of light c is 2.9979 3 108
m/s.
How do we know that? You can’t exactly time a light beam
with a stop watch. Our understanding of light and its speed
parallels the development of the technology that was used to
measure this speed.
In the early 1600s, Galileo attempted to measure the
speed of light by using his pulse-beat and comparing the time
between opening his lantern while on a hilltop and seeing
the light from his assistant’s lantern from a distant hilltop.
His attempts were not successful, but he correctly concluded
that light is simply too fast to be measured by the slow human
reaction.
Around 1675, the Danish astronomer Ole Roemer made
the first accurate measurement of the value of c (FIGURE B4-2).
Roemer had made many careful observations of Jupiter’s
moon Io and knew that Io’s orbital period is about 1.76 days.
As a result, he expected that he could predict Io’s eclipses
accurately. He was astonished to find that Io seemed to be
behind its predicted position when the Earth was farther away
from Jupiter (point A) and ahead when the Earth was closer
(point B). Roemer correctly attributed this effect to the time
required for light to travel from Jupiter to Earth. Light takes
about 16.5 minutes to travel across the diameter of the Earth’s
orbit (2 AU); Roemer’s actual measurement was