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The speed of light

In the above example a hexagonal mirror is shown. Note that Michelson originally used an octagonal mirror.

He found he could only see a steady image of the light when the speed of rotation of the mirror exactly matched the speed of light, or was a multiple of it.

If the first frequency of rotation of the mirror at which a steady image can be seen is f, then a steady image of the lamp will also be seen at frequencies 2f, 3f etc.

For a hexagonal mirror, f corresponds to the mirror completing one sixth of a rotation.

Calculations:

If the distance between the rotating hexagonal mirror and the concave mirror = 35km and the rotational frequency of mirror = 710 rev s-1

[please do not use standard notation in your answers - it will be scored as incorrect]

(a) Calculate distance the ray of light travels = metres
(b) The time taken for the mirror to rotate 360o =
seconds
(c) The time taken for the light to travel
seconds
(d) The speed of light


If an octagonal mirror is situated 50km from the concave mirror repeat the above for a rotational frequency of 375 rev s-1

(a) Calculate distance the ray of light travels = metres
(b) The time taken for the mirror to rotate 360o =
seconds
(c) The time taken for the light to travel
seconds  
(d) The speed of light
ms-1


What is the slowest frequency of rotation required for an octagonal mirror situated 13km away in order to see a steady image?
rev s-1

Look at your answers so far: Why is the distance between the mirrors made as great as possible?

Press the button to check your answers:

NOTE that since the 1980s the speed of light in a vacuum has been defined to be 299792458 ms-1 .  This comes from the definition of the metre.