• Question: How do you measure the speed of light?

    Asked by anon-267534 on 10 Nov 2020.
    • Photo: Liza Sazonova

      Liza Sazonova answered on 10 Nov 2020:

      Cool question! There are lots of different ways. Physics undergrads even do an experiment to measure the speed of light in labs (I did one in my second year).
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      We did a sort of simple laser experiment. We can make laser beams of a known frequency and shine it in two different mirrors that both reflect the light onto the same screen (like a piece of paper). The mirrors are arranged in a special way, so that the light travels slightly longer from one of them than the other. We can look at a difference, called the interference pattern, and use this to calculate the speed of light. There’s more details to it than that, if you’re interested look up Michelson’s interferometer! A cool thing is that this is basically how we detect gravitational waves too now with the LIGO experiment.
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      There are other ways; physicists have been quite creative in trying to measure something so fast so precisely 🙂

    • Photo: Iain Tullis

      Iain Tullis answered on 11 Nov 2020: last edited 11 Nov 2020 12:20 am

      One way to measure the speed of light is to set up a rotating “toothed wheel” (which is something like a cogwheel). You arrange for a light to travel from a small source, thru a gap in the toothed wheel and then to a mirror some distance away. You set up the mirror reflect the light back towards the source. You look thru the next gap in the toothed wheel and adjust the speed of the motor driving the wheel. When the motor speed is just right, light that starts its journey (when the gap between teeth allows light to start to travel from the source to the mirror) and it has to get back again to make it thru the gap – which has now moved in front of you eye. If you know the speed of rotation, the size of the toothed wheel, how far the wheel needs to travel for you to see it, and the distance from the mirror and back you can then calculate the speed of light. A man called Fizeau did this in about 1850 and the distance to the mirror was 8 km and the error of measurement was about 5%. Twenty five years later, with better equipment, the error was reduced to about 0.05% of the modern value of c. This is called Fizeau–Foucault apparatus. https://hsm.stackexchange.com/questions/577/speed-of-light-measurement

    • Photo: Daisy Shearer

      Daisy Shearer answered on 11 Nov 2020: last edited 11 Nov 2020 8:45 am

      There are quite a few different ways to measure the speed of light. My favourite is using a laser! We call this a ‘time of flight’ measurement. Essentially, you shine a laser onto something called a ‘beam splitter’. This divides the beam into two separate paths. If you use mirrors to direct one beam in one direction to a detector, you can send the other beam away for a bit and then reflect it back to the detector. You can then take measurements of when the laser light hits the detector and because the two split beams have travelled different distances, we can use the difference in path distance and the time difference between the beams being detected to calculate the speed of light using the equation: speed=distance/time. A slightly more sophisticated version of this experiment is to use a Michelson Interferometer and oscillate the second mirror back and forwards to cause interference between the split beams.

      Another cool experiment for measuring the speed of light is with chocolate in a microwave. If you take out the turntable and put a slab of chocolate in there for about 20 seconds, then you will see various melted areas. If you measure the distance between the melted points, you can work out the speed of light because microwaves are a type of electromagnetic radiation (aka. light). This distance is half the wavelength of the microwaves, which gives you the full wavelength when you multiply it by two. Most microwaves operate at a frequency of 2.45 gigahertz. Using the equation: speed of light = wavelength x frequency, you can then calculate light speed 😊
      Here’s a nice description of the experiment with a diagram: http://www.planet-science.com/categories/over-11s/physics-is-fun!/2012/01/measure-the-speed-of-light-using-chocolate.aspx

    • Photo: Joshua McAteer

      Joshua McAteer answered on 11 Nov 2020:

      This probably isn’t the answer you want, but since others have already given the method for measuring the speed of light in a vacuum I thought I’d mention that in optics and radiofrequency engineering the speed of light is very important.

      Different materials have different speeds of light. They are all slower than the vacuum speed. When light moves from a material that has a higher speed of light to a lower speed it is bent, sometimes by a large amount. Lenses, like in glasses, microscope, and telescopes use glass or plastic curved surfaces to direct light for a purpose – such as correcting focus or magnification.

      A machine called a ‘refractometer’ is used to measure the angle change of the light when it (typically) moves from air to the other material. This is used to work out the speed of light in different kinds of glass. Glass or plastics can have other chemicals added to them to change the speed of light, often barium or lead is added to glass. There are lots of uses for refractometers, including measuring the amount of alcohol in beer as beer is mostly water with a little ethanol (alcohol).

      Refractometers don’t normally list the actual speed of light, they typically list a number which is more useful for the specific application. For glass and plastics for optics they measure the ‘refractive index’ which is how much slower light travels in the material, and for alcohol, they are calibrated to show the % alcohol.

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