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Need some info about light heat amplified by lens

  1. Mar 19, 2015 #1
    It is known that a magnifing glass can start a fire, but I don't fully understand the phenomenon and I have the next questions:
    - Does the effect depend on the shape and the material of the lens you use?
    - Where can I find some details about heat produced by light or particulary on this phenomenon? (details that contain math support, of course)
  2. jcsd
  3. Mar 19, 2015 #2
    It works by focusing the energy density of light over a large area into a much smaller area.
    More energy density = higher temperature = fire.

    The material that the lens is made of needs to be a transparent as possible since we don't want to heat up the lens itself.
    The shape of the lens determines the focal distance - that is the distance from the lens at which the light will be maximally concentrated.
    Last edited: Mar 19, 2015
  4. Mar 19, 2015 #3
    But does the lens parameters (refractive index, form, thickness) affect the quantity of energy that exits through the lens? I would try to study it experimentaly, but outside is snowing.
  5. Mar 19, 2015 #4
    I edited my original reply to cover some of this, but to extend further ..
    the shape of the lens and properties of the material mainly affect the focal distance.
    A thicker more highly curved lens will result in a closer focal distance, but then more of the incoming energy will become dissipated by heating the thicker material - unless it is 100% transparent, and I don't think that such a material exists.
  6. Mar 19, 2015 #5
    Thanks, it is more clear now. I will try to study the math involved.
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