• Question: is it true that if you could see light traveling away from ou it would look red?

    Asked by danielburniston to Jack on 12 Mar 2012.
    • Photo: Jack Snape

      Jack Snape answered on 12 Mar 2012:

      Yes! Strangely enough.. it is.

      You can think of light as a wave, with peaks and dips. The distance between 2 peaks is called the wavelength. Different colours of light have different wavelengths – blue light has a shorter wavelength than red light.

      When light is moving towards you the peaks get squashed up so it gets bluer. When it’s moving away from you the peaks get stretched out so it looks redder.

      This is how we know that the universe is expanding, because far away galaxies emit redder light. It’s very useful for working out how fast things are going!

Comments

  • Photo: laurenceharwood

    laurenceharwood commented on :

    Yes and no.

    We use the term “red shift” to give a visual illustration to the fact that light travelling away from any point has its wavelength stretched. I’ve used an illustration previously of a tennis ball attached to a string that you throw away. The coils of the spring stretch. It is very similar with light going away from you.

    However, the wavelength of ALL light (to be correct – electromagnetic radiation) not just that tiny bit of the electromagnetic spectrum that we can see does this – so “red shift” simply means a shift to longer wavelength – whether or not we can see the light or not.

    So even though we can’t see them, ultra-violet light (shorter wavelength than visible) which makes us get sun-burnt and infra-red light (which we feel as warmth) will also go to longer wavelength even though we can’t see them. All the other wavelengths do as well.

    So Jack is totally correct when talking about visible light – but all wavelengths do this. A real life example of this effect is when a police car or ambulance with its siren going approaches and goes past yo, the tone changes. This is because the sound waves are behaving just like light waves and are being squashed when the police car/ambulance approaches you and stretched when it goes away. This is also a “red shift” even if we are dealing with sound now. The correct term is the “Doppler effect”.