__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 =

(b) The time taken for the mirror to rotate 360^{o} =

(c) The time taken for the light to travel

(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 =

(b) The time taken for the mirror to rotate 360^{o} =

(c) The time taken for the light to travel

(d) The speed of light ^{-1}

What is the slowest frequency of rotation required for an **octagonal** mirror situated 13km away in order to see a steady image?
^{-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.