If a magnifying glass will magnify the sun’s rays, then wouldn’t wearing prescription glasses actually be dangerous for our eyes?


If so, does prescription glasses that come with photochromatic or polarised lenses help to combat this?

In: Biology

All lenses focus light, including the lens in your eye. Prescription glasses just move that focal point to where it should be, they don’t collect any more light than your eyes do. That being said – Do you look directly in the sun when you wear glasses (or when you aren’t)? That is dangerous, with or without glasses because sunlight is strong enough to damage your retina permanently.

Sun rays are dangerous when they are focused into a point. Lenses of your glasses can do it, but that point is well behind your eyes. At the distance you are using them, it is only intensifying the sun rays by a little.

Unless you are myopic, that is, in which case they actually spread the sunlight into lesser intensity.

Take the glasses off and shine light through them onto the ground. You can see the effect as a region of intensified or shaded light.

Your eye already has a natural lens built-in. Its job is to focus the light that comes in to your eye to a single point, or nearly a single point, called the focal point. To be able to see clearly, the focal point should land exactly on your retina (the back of your eyeball). This relies on your lens being a very specific shape, and the distance between your lens and retina to be a very specific length.

Some people over time will develop issues with their eyes. The most common issue is myopia (near-sightedness), which is usually when the eyeball gets squeezed out a little bit into an oval shape. This causes the distance between your lens and retina to increase just a bit. Since your eye lens creates the focal point at a very specific distance away, the lengthening of the eye causes the focal point to happen too early, before reaching the retina, resulting in blurred vision.

The opposite can also happen, called hyperopia (far-sightednesd), which causes the distance between the lens and retina to shrink. This causes the focal point to be behind the retina–in other words, all the light hits the retina too soon before it’s been fully focused. This also results in blurred vision.

Sometimes the lens in your eye itself can be misshapen due to damage or genetic factors. A misshapen lens can cause a whole host of distortion problems, as you can probably imagine. These kinds of problems are typically tossed under the umbrella term of “astigmatism”.

What corrective lenses (eyeglasses, contacts) do is pre-focus the light in just the right way so that the error in your natural eye lens is canceled out. If you have myopia, for example, meaning the light in your eye gets focused too early, your corrective lenses will spread light outwards a little bit before it comes into your eye. That way, when your eye tries to bend the light back in, it bends at a more shallow angle, pushing the focal point back just enough to land where it’s supposed to.

Is this dangerous? No more dangerous than just seeing normally. Your eye is naturally focusing all that light into a point on its own. Anything that corrective lenses do in terms of focusing light is just the lenses compensating for error in this process.

Putting a magnifying glass in sunlight and focusing it to burn things makes lenses seem dangerous, sure. But that’s exactly why you should never look at the sun in the first place–*your eye will do that naturally*. It’s why looking at the sun hurts. You’re literally burning your retinas. The correct set of corrective lenses will technically make this worse, as their job is to correct the focal point in your eyes, but they don’t make you any worse off than a person with perfect vision and no corrective lenses. If anything, having terrible vision should make you ever so slightly more resistant to looking at the sun, since you can’t quite focus the light properly, and wearing corrective lenses just negate that resistance by fixing the problem. But that’s all just tiny theoretical differences that hardly matter, the takeaway should still be *do not look directly at the sun no matter what your vision is like or what corrective lenses you wear*.