Ole Roemer was the first to measure the speed of light by observing eclipses of Jupiter's moon Io from Earth. He determined that light takes about 11 minutes to travel from Jupiter to Earth when Earth is farthest compared to when it is closest. This showed that the speed of light is finite. The modern accepted value for the speed of light in a vacuum is 299,792,458 meters per second. Later experiments by Fizeau and Michelson more precisely measured the speed of light and found it is slower in materials like air and water due to the index of refraction.

1. Speed of light in
vacuum and matter

2. Speed of light
• Speed of light is the distance photons travel within a given unit
of time and is often measures as meters per second.
• if you could travel at the speed of light, you could travel around
the Earth seven and a half times in a second.

3. Speed of Light in
vacuum

4. Ole Roemer
In 1676, the Danish astronomer Ole
Roemer (1644–1710) became the first
person to measure the speed of light.
• Roemer measured the speed of light by timing eclipses of Jupiter's
moon
• The orbital period of Io is now known to be 1.769 Earth days. The
satellite is eclipsed by Jupiter once every orbit, as seen from the
Earth
• From his data, Roemer estimated that when the Earth was nearest to
Jupiter, eclipses of Io would occur about eleven minutes earlier than
predicted based on the average orbital period over many years. And
6.5 months later, when the Earth was farthest from Jupiter, the
eclipses would occur about eleven minutes later than predicted.

5. • Roemer realize that the time difference must be due to the finite speed of
light.
• The speed of light in vacuum, commonly denoted c, is a universal
physical constant that is exactly equal to 299,792,458 metres per
second.
• For practical purposes where calculations are to be made four significant
figures, the speed of light in air or vacuum may be taken to be
c = 3 x 108

6. light travels at least 10x faster than sound.

7. Speed of light in vacuum
In 1849, Fizeau calculated a value for the speed of light more
precise than the previous value determined by Ole Rømer in 1676.
He used a beam of light reflected from a mirror eight kilometers
away. The beam passed through the gaps between teeth of a
rapidly rotating wheel. The speed of the wheel was increased until
the returning light passed through the next gap and could be seen.

8. Speed of
light in vacuum

9. THE SPEED OF LIGHT IN
STATIONARY MATTER

10. Speed of light in water

11. Speed of light in air
and water
Michelson used an 8-sided mirror device that rotates while
bouncing light to the reflector and also collecting return
light.

12. Wrap up

13. Index of Refraction

14. Index of Refraction
01 02
03
04
 any optical medium is defined as the ratio between the speed of light in a vacuum and the speed of light
in the medium:
Refractive Index = Speed in vacuum
Speed in medium
In algebraic symbols n= c/v
n = index of refraction
c = speed in vacuum
v = speed of medium
Material Index of
Refraction
Glass 1.520
Water 1.333
Air 1.000