It is a common observation that a magnifying glass when used to converge rays of sun to a fine point in paper, the paper burns due to intense heat.

Why doesn't this happen in our eyes? Our eyes are also doing the same, they are converging the sunlight into our retina but our eyes don't get burned, or to say get heated.

Is viterous or aqueous humor absorbing heat? If so, then the tiny amount of fluid present in the eye shouldn't be enough to cool the eye for a longer period of time, but we can spend the entire day at sunlight without any eye stress or pain.

Size of eye lens small compared to magnifying glass? However a magnifying glass of the size of eye lens should also be able to produce significant heating by converging sunlight.

As far as I know, the heating effect is due to Infrared rays not because of UV rays so we can safely ignore them in this case, right?


1 Answer 1


If you look straight at the sun, you do burn your retina, in the same sense as you get burnt by contact to a hot surface: living tissue is full of water, so the burning process does not look the same as for dry paper e.g., and the harm will be done long before combustion.

To go a little further, the first layer of retina in vertebrates is made of so-called Muller cells, which conduct the light as optical fibres do to the photoreceptor cells, but it has also been argued that they can in some measure filter too strong light in order to protect photoreceptors. See http://www.pnas.org/content/104/20/8287.full

These cells are also strongly vascularised, which allows to regulate its temperature. You can read http://creation.com/mueller-cells-backwardly-wired-retina-v-dawkins

  • $\begingroup$ Converging sun rays at my hand (tissue, as you said) produces enough heat to draw back my hand spontaneously. But with our eyes, we can look at sun for a slightly longer period of time without noticing any change except extreme brightness. Even when we aren't looking directly at the sun, the sunlight is still entering our eyes. Why this indirect sunlight (For a long period of time) doesn't produce any heat (Which should, as we are exposed to a longer period of time)? $\endgroup$
    – sarthak-ag
    Feb 12, 2016 at 13:06
  • $\begingroup$ Light converges on your retina only when looking at an object at an infinite distance. The ambient light is not converged to a single focal point on the retina, so the heating is not more than on your skin. I advise strongly against performing this experiment of comparing for how long you can hold your hand in focalised sun light and look at the sun. See also my edit. $\endgroup$
    – Joce
    Feb 12, 2016 at 14:16
  • $\begingroup$ I didn't read those articles you linked to as they are too scientific for me to understand but your ambient light description seems to solve my question to the finest. Thanks. Btw, I didn't performed any such experiments for the sake of this question because I know it's harmful, I was just narrating my previous experience, we all've done these, once at least. $\endgroup$
    – sarthak-ag
    Feb 12, 2016 at 15:56

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