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Does focusing light with a lens create a light gradient?

  1. Apr 27, 2014 #1
    *disclaimer I am not a physicist

    Had a weird thought the other day - when you focus light with a lens, for example a magnifying glass, you basically increase the 'concentration of photons' at a certain point, right? But then energy is conserved... so wouldn't focusing some of the light on one spot then create a 'lack of light' or shade on another, adjacent spot? And might this not create an intensity gradient = a heat gradient = something which could be mined for energy? I mean I asked a physicist friend who said that because of diffraction focusing light doesn't create an 'energy peak' at the focal point but rather a series of rings each of weaker intensity but this still sounds like a type of gradient to my untrained ears...

    What say you oh great hivemind?

    Thanks :)
  2. jcsd
  3. Apr 27, 2014 #2


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    Yes, focusing the light will increase the light falling on a small area at the expense of reducing the light falling on a larger area. The total light still remains the same, you are just moving some of it around. This is hard to see outside with a magnifying glass, but it may be easier it you shine a small flashlight into the magnifying glass inside in the dark.

    This effect is already used to generate power in solar power stations and is known as concentrated solar power: http://en.wikipedia.org/wiki/Concentrated_solar_power
  4. Apr 27, 2014 #3
    cool, thanks :)
  5. Apr 27, 2014 #4
    Also, it's fun to think of the eye as an example of light focusing.
    Found quite a bit of info online on the general properties of the eye and a smaller amount on the total numbers of photons hitting the retina but none that explicitly described the spatial distribution of impacting photons.

    So a rather silly back of the envelope calculation -
    Retinal area is about 1000mm^2
    Fovea centralis area is about 2.5mm^2
    so fovea area is ~500-fold smaller than total retina
    + let's assume 90% of incident light is redirected to the fovea (my wild guess)
    then remaining 10% of light redistributed over 500-fold larger area gives 4 orders of magnitude difference in number of incident photons per foveal area.
    Which, at 4e-19 J per photon at 500nm or 2e-22 degrees C per photon is not much of a heat difference... ah well
  6. Apr 27, 2014 #5


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    Not true. Just look off to the side of a bright light. The light from the lightbulb will be focused off of the fovea.

    Your original example just dealt with a single source of light. In real life light is entering the eye from every angle and a real image is formed on the retina that is a mirror image of the scene in front of the eye. The fovea is simply the area of the retina that is at the center of the eye's optical axis. It is on the optical axis that most aberrations inherent to any optical system are minimized and visual acuity is maximized.
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