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B Why my mirror does not reflect converging light?

  1. Aug 4, 2016 #1
    Hello.

    Today I used a magnifying glass and observed how it converged sun light to a smaller area.
    The smaller the area and the more violet was the light diffused by the surface. Why does this happen?

    Then I used a mirror hoping the small violet point would be reflected, but it didn't happen: the light was again diffused like in any other surface and I saw the light on the mirror. Why?
     
  2. jcsd
  3. Aug 4, 2016 #2

    Drakkith

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    I'm not quite sure what this means, but I think you're talking about the fact that the spot of light is surrounded by a violet "halo". This happens because a magnifying glass has dispersion and the focal point of the violet light is different than the rest of the spectrum. Technically this is true for all colors, but the magnitude of the dispersion is much greater in the violet region, where the change in refractive index with becomes increasingly non-linear as frequency increases. So what you are seeing is a large, out of focus violet spot.

    My guess is that most of the light was indeed reflected by the mirror as you'd expect. However, mirrors are not 100% perfectly reflective. A small portion, perhaps around 10-20% will be absorbed or diffusely reflected instead. This is probably what you saw.
     
  4. Aug 4, 2016 #3

    sophiecentaur

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    Sounds like there may be something on the surface of the mirror. Try a different mirror and a different lens if you can find one.
     
  5. Aug 4, 2016 #4
    Are you sure you got the distances right? Assuming you have a hand-held magnifying glass, you will have to get the mirror and screen really close to the glass to get a focus..
     
  6. Aug 5, 2016 #5

    CWatters

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    If you put a mirror at the focus the light arriving at the mirror comes from lots of different directions so obviously it will be reflected in lots of different directions as well. You won't see a coherent beam reflected if that's what you were expecting. A lens doesn't produce a beam like a laser. It diverges again after the focus.

    You can probably see the light on the glass because it's either not clean or not 100% clear/transparent.
     
  7. Aug 5, 2016 #6

    sophiecentaur

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    If you use a lens with a longish focal length (say from a pair of reading glasses) you can focus the sun at, perhaps 50cm away from the lens (for a +2 diopter lens). Putting a mirror in the way of the converging rays can give you a focussed spot in front of the mirror - total distance from lens to spot will be the same as the focal length of the lens.
     
  8. Aug 5, 2016 #7

    Charles Link

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    It is likely with the flat mirror, you may be sending the light back through your magnifying glass. I would have to believe the light is reflecting just fine off of the flat mirror, but if you are using the sun as a source, you don't want to get a direct view of the beam=Do NOT turn the mirror so that the beam reflects into your eyes. If you want to play around with a magnifying glass and a flat mirror, you are much better off using a much dimmer source than the sun.
     
  9. Aug 6, 2016 #8

    sophiecentaur

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    You can get a good insight as to what is happening with the rays if you blow smoke into the beam. Not cigarette smoke, of course!!!! :wink:
     
  10. Aug 17, 2016 #9
    Thank you everyone.
     
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