• Question: Does all colour of light travel at the same speed?

    Asked by anon-267491 on 9 Nov 2020.
    • Photo: Sam Geen

      Sam Geen answered on 9 Nov 2020:

      This is a great question! For light – yes, all colours travel at the same speed in empty space. But for some water waves, longer wavelengths travel faster, for some slower, and for some it doesn’t matter at all: https://spark.iop.org/speed-water-waves (this link has some equations)

      The speed of light comes from “Maxwell’s equations” after a Scottish scientist James Clerk Maxwell. When you solve the equations, it comes out as the same value regardless of colour.

      However! Light does travel at different speeds in different materials, like glass. When light enters or exists glass, the angle it travels at changes, and this angle depends on the colour. This is why prisms separate out the light to make a rainbow effect.

    • Photo: Daisy Shearer

      Daisy Shearer answered on 9 Nov 2020:

      In a vacuum, yes! But in some materials, different colours travel at different speeds. For example, in glass (if your teacher has a prism, you can see this in action– it’s because of something called ‘refraction’) and also in water which is why we see rainbows.

    • Photo: Jesse Dykes

      Jesse Dykes answered on 11 Nov 2020:

      As the others have said, in a vacuum like outer space all colours travel at the same speed. But in any other medium (air, glass, water) there will be some difference in speed. This is called dispersion, if you want to look up more about it.

      Dispersion can be inconvenient, because as you might have seen, all colours or frequencies of light are on a spectrum. So the light from a light bulb is made up of a range of frequencies that altogether look white, and even laser light is made up of a small range of frequencies. Dispersion means that some of your light will arrive sooner than the rest of it, and this can cause a lot of problems.

Comments

  • Photo: Colin

    Colin commented on :

    In a vacuum Indeed it does – all electromagnetic radiation travels at 299792458 metres per second – however in materials this can differ – as explained above. But for a vacuum visible light, x-rays, ultraviolet, infrared, microwaves and radio waves will travel at the speed above.