Lesson 17: The Speed of Light!
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The speed of light is 3.0x108 m/s. There have been several experiments that confirmed this value. One of the
earlier successful experiments was the famous Michelson Experiment in 1924. !
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The Michelson Experiment!
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Mount Saint Antonio
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1) Light is shone on the surface of the rotating nsided mirror rotating at a frequency f!
2) A moment later light bounces back. If the frequency
is not right, the flat face of the mirror will not be lined
up right when the light comes back
3) If the frequency is just right, the light reflects off the
flat side and is detected
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Rotating!
n-sided
mirror
curved
mirror
Mount Wilson
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The minimum rotational frequency needed for detection happens when the mirror rotates one of its n sides
before the light returns.!
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This is 1/n of the mirror’s total rotation:!
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And since T=1/f!
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The distance that the light must travel to get back is twice the distance between the mountains!
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This means that the speed of light is!
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Example: The speed of light is measured to be 2.87x108 m/s using a 10 sided mirror rotating at 300 hz. What is
the distance between the mountains?!
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Practice Problems:!
!1. Students using a 12-sided set of rotating mirrors in an experiment similar to Michelson’s determine the
speed of light to be 2.88 x 108 m/s. The mirrors are located 30.0 km from the fixed mirror. What is the
frequency of rotation if 1/12th of a rotation occurs before the light returns? [400 Hz]
!2. An 8-sided set of mirrors, similar to Michelson’s, rotating with a frequency of 500 Hz, is located 36.0 km
away from a fixed mirror. If the returning light is observed in the system, at what speed is the light
travelling? [2.88 x 108 m/s]
!3. An 8-sided set of rotating mirrors rotates at 545 Hz in an experiment similar to that of Michelson. How far
away should the fixed mirror be placed in order to correctly measure the speed of light? [3.44 x 10 m]
!4. A communications satellite is in orbit around Earth at an altitude of 2.00 x 10 km. If the satellite is
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directly above a ground based station, how long does it take a signal to travel between the satellite and the
station? 6.67 x 10-2 s
!5. Using a similar approach to Michelson, a student sets up a 64-sided set of rotating mirrors, 8.00 km away
from a fixed mirror. What minimum frequency of rotation would be required to successfully measure the
speed of light? 293 Hz
!6. The speed of light was measured to be 2.97 x 10 m/s using a 16-sided set of rotating mirrors and a fixed
mirror separated by 5.00 x 10 m. At what frequency was the mirror rotating? [1.86 x 10 Hz]
!7. A 64-sided set of rotating mirrors is turning at 340 Hz. If a fixed mirror is located 6.55 km away from the
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rotating mirror and the light is reflected correctly, what value for the speed of light would be obtained?
[2.85 x 108 m/s]
!8. The electromagnetic waves of frequency and wavelength described below belong to which portion of the
electromagnetic spectrum? If it falls in the visible range, approximately which colour would it be?
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1. λ = 410 nm ____________________________
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2. λ = 283 nm ____________________________
3. λ = 740 nm ____________________________
4. λ = 1100 nm ___________________________
5. f = 15 GHz ____________________________
6. f = 580 kHz ____________________________
7. λ = 1.00 x 10-9 m ________________________
8. λ = 4.0 x 1018 Hz ________________________
9. λ = 100 nm _____________________________
10. f = 410 THz ___________________________
11. f = 740 THz ____________________________