In 1905, Albert Michelson accurately measured the speed of light using a set of rotating mirrors.  In his now-famous experiment, light was directed at an  eight-sided , rotating mirror.  The light was reflected from one side of the rotating mirror toward a distant curved mirror that sent the light back toward the rotating mirror.  If the rotating mirror completed one-eighth of a rotation (or a multiple of 1/8) by the time the light returned, the light would be reflected to an observer (as seen in the illustration).  Knowing the period of rotation that causes the light to be observed gives the time it takes the light to travel to and from the distant curved mirror.  Therefore, the speed of light was equal to the distance to and from the curved mirror divided by 1/8 of the period of rotation.
Physics  (Pearson Education Canada, 2007) p650, fig 13.24. Reproduced with permission.

 


Read
Read "Michelson's Experiment" and "Example 13.1" on pages 650 and 651 of your textbook.


Try This
Determining the Speed of Light Using a Microwave Oven. Read "Quicklab" on page 651 of your textbook.  Perform the procedure that follows.


Problem

How can a microwave oven be used to determine the speed of light?

 

Materials

  • microwave oven
  • microwave-safe dish, such as a casserole dish
  • marshmallows (large)


Procedure

  • Place a layer of marshmallows in a microwave-safe dish or casserole dish, ensuring the marshmallows cover the bottom of the dish with no empty spaces between the marshmallows.

  • Remove the turntable from the microwave oven.

  • Place the dish in the centre of the microwave, and cook on high power until you see the marshmallows begin to melt in various locations.

  • Remove the dish from the oven and let it cool.

  • Measure the distance between adjacent melted spots from centre to centre. This distance is one-half the wavelength of the microwave.

  • Calculate the average distance of separation for all the spots.

  • Calculate the wavelength by multiplying the average separation distance by two.

  • Record the frequency of the microwave oven, which is listed on the door or back of the microwave or in its documentation.

Observations

Record the distance of separation between adjacent melted spots (measured centre to centre) in a table like this one. If you do not have access to a microwave, use the values in the table.

 

Measurement Number

Distance of Separation (cm)

1

approx. 6.0

2

approx. 6.0

3

approx. 6.0

4

approx. 6.0

 

microwave frequency = 2.50 × 10 9 Hz

 

 

Conclusion

What is the speed of light based on your calculations?

 

 

Do light and all other classifications of EMR always travel at 3.00 × 10 8  m/s?  This common value gives the speed of light when it travels in a vacuum or in air.  But what happens to the speed of light when it enters water or a glass?  Does the speed slow down, speed up, or stay the same?  Can light be stopped?  Furthermore, if the medium, such as water, in which light travels is moving, can light travel faster than 3.00 × 10 8  m/s?