• Question: How many mirrors can a laser bounce of before disappearing?

    Asked by anon-219534 to Ali, Bella, Declan, Emma, Laurence, Nicolas on 7 Jun 2019.
    • Photo: Laurence Datrier

      Laurence Datrier answered on 7 Jun 2019: last edited 10 Jun 2019 9:04 pm

      Depends on the laser and the mirror! Mirrors have different reflectivities, meaning the amount of light you lose to absorption (or sometimes transmission) will be different depending on the material coating your mirror. The reflectivity is also wavelength dependent, so different “colours” of light will be reflected differently. You lose ~10 % of reflected light to your average bathroom mirror.

      Here at LIGO we use special coatings that are very reflective for the laser that we use – the mirrors at the ends of the interferometer’s arms are 99.9999% reflective, and the laser is reflected ~280 times with minimal loss.

    • Photo: Declan Jonckers

      Declan Jonckers answered on 10 Jun 2019:

      The mirrors used to make spacecraft insulation reflect about 97% of the light you can see. This means that if you bounced a visible laser off of 75 of these mirrors, you’d still have about 1/10th the amount of light you started with.

      By the sounds of it, the mirrors used for LIGO are quite a lot shinier!

    • Photo: Ali Hussain

      Ali Hussain answered on 10 Jun 2019:

      Good question. If you are trying to make the laser beam disappear, just get the wrong type of mirrors. Then let’s say you lose 50% of intensity with each bounce. In theory, will the laser ever disappear?

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