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Convex Lens

  1. Feb 27, 2013 #1
    I was just wondering if a person wearing convex lenses looks at a light source...
    Will excess heat be generated in his eyes? As it is a converging lens..

  2. jcsd
  3. Feb 27, 2013 #2


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    If you are talking about corrective lenses (glasses) then the effective f number of the eye/glasses combination would be reduced by a small fraction when the correction is +Dioptres. This would increase the brightness of the image on the retina slightly and also the 'heat' falling on the retina. Is that what you mean?
    Perhaps you could explain, a bit more fully, what you mean by your question.
  4. Feb 28, 2013 #3
    Yes! That is what I meant!
    Could you please elaborate on that?
    Also, the colours appear a little darker through lenses, what does that happen?
    ( Wearing glasses for the first time, thus having weird doubts :P )
  5. Feb 28, 2013 #4
    The extra heat you feel is probably because you have something covering your face, and not from the optics themselves.

    The lenses reflect some fraction of the light, depending on the material, presence or absence antireflective coating and on the angle. That will reduce a small percentage of the light, which could darken the colors a bit. Also, the colors will separate a little, in an effect called chromatic aberration. The magnitude of this latter effect depends on the lens material, and I would steer clear of polycarbonate, which has a particularly poor Abbe number.
  6. Feb 28, 2013 #5


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    Try Varifocals next time, if you want some real doubts about the whole Universe. Woooeee!

    But the lens question. Are you familiar with the way camera lens aperture is specified? The f number is the ratio of the focal length to the aperture. Lenses with different focal lengths but with the same aperture number will expose the sensor to the same brightness of image so it's a convenient way to get the same exposure with two different lenses.
    When you put a convex corrective lens in front of your eye, you are shortening the overall focal length and your pupil defines the aperture - so the f number will have increased (but only by a very small amount and I'm surprised you could notice it). At the same time, it magnifies the image a tiny bit. You will have noticed this, no doubt. When people wear convex lenses (reading glasses, particularly) their eyes look a bit bigger - and more attractive in many cases.

    Corrective glasses will have a focal length in the region of 1m (+ or -. depending on the prescription) so there will be no significant 'focussing' (as with a burning glass) at a spacing of 1cm or less.
    I have no explanation for your subjective colour effects. Dirt and scratches will usually 'lighten' the colours due to flare (light from all over the image gets spread out and tends to dilute the colour purity). The only thing I can suggest is that you may have some coating in the lens that is 'warm' filtering the light a bit. I guess, if the image is looking much sharper than you have been used to, the sharpness may be making the colours look more vivid because the contrast at edges is greater than without.
    If you have a simple photo editing program on your computer, you could see the effect on colours when you 'soften' a colourful image, which could reproduce the effect you are getting with and without your new glasses.
  7. Feb 28, 2013 #6


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    I speak from experience here and it is usually the other way round. Normally, when you are wearing glasses, the IR absorption of the glass actually decreases your sensitivity to radiant heat. When you take them off, a radiant source 'feels' warmer.
  8. Feb 28, 2013 #7


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    What is your prescription? If you don't have that, do you know what your vision was tested as?
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