In 1983, the international system of units (SI) adopted the following new definition of the metre: "A metre is the length of the path travelled by light in vacuum during a time interval of 1/299 792 458 of a second."

 

This means that the speed of light in vacuum is exactly 299 792 458 m/s and that the definitions related to distance and length are based on this value.  In fact, the global positioning system, or GPS, depends on it.  A GPS receiver can pinpoint its location on the surface of Earth by calculating its distance from the global positioning satellites.

 

How is the distance between a receiver and a satellite determined?  Each GPS satellite broadcasts a precise, synchronized time signal.  A receiver, such as the one in the photo, measures how long it takes the time signal to travel from the satellite to a receiver.  Using this time information and the value for the speed of light in a vacuum, the precise distance separating the receiver and the satellite is determined.  Repeating this process for a minimum of three satellite time signals provides enough information to determine the receiver's exact position using a process called trilateration.


Image by Dariusz Sankowski from Pixabay


 

Watch and Listen

Watch the following video  to see how trilateration is used to determine the precise location of a GPS receiver.

  Global Positioning Systems 

 

 

 

GPS is just one example of an optical system that is based on our understanding of the speed of light.  Obviously, the speed of light is very fast-so fast that it takes only 1.3 seconds for light to travel from Earth to the Moon.  When the first humans orbited the Moon in Apollo 8 , there was nearly a three-second delay in all communications as the EMR made its way from Earth to the command module and back again.  On Earth, it would take about 0.0668 s for light to travel from Canada to Japan.

 

Connecting two computers by sending the light through fibre optics and switching equipment on the Internet would take about 0.18 s.  Even with a stopwatch, you would have difficulty accurately measuring the time it takes for light to travel anywhere on Earth.

 

So how was the speed of light determined if it travels so fast?  Was it determined using astronomical observations?  Can it be measured on a lab bench?  Does it slow down when it enters water and other materials, such as glass?  

 

In this lesson you will answer the following essential questions:

  • How can you measure the speed of light using another planet?
  • How can you measure the speed of light in a laboratory?
  • How can you measure the speed of light using a microwave oven?