Can staring straight at the sun blind you?

Most people don’t look into the sky much, anyway, but occasionally something like an eclipse makes the sun of more than usual interest. When that happens don’t stare directly at it because it can blind you, possibly permanently.

The human eye has a tough job. It functions as a superb optical instrument under a huge range of conditions, and it does this even though its operator has no training. It doesn’t help that the light so carefully collected and focussed by the eye is actually quite harmful. Light triggers chemical reactions in our tissues, some necessary, some unwanted, and the eye is particularly vulnerable. The cornea, at the front, and the retina, at the back, are very efficient at mopping up the mess caused by light. The lens, in the middle, comes with built in protective proteins which last until middle age but after that it starts to give in and slowly clouds, often leading to cataracts.

Sometimes, though, the defences and running repairs just aren’t enough, like if you’re staring directly at the midday sun. If you do then your eyes will respond automatically to try and save themselves — your pupils will contract to pinholes and you will squint — but the sun is just too bright. After around ninety seconds your retina will be “burnt” by the light. Maybe your vision will return to normal. Maybe.

Light from different parts of the spectrum affects the eye in different ways but it all causes damage if strong enough. The nastiest is ultraviolet light which is the main culprit behind snow blindness, cataracts and many other eye problems. At the far end of the spectrum infrared, or heat radiation, if concentrated can fry your eye just like a bug under a malevolent child’s magnifying glass. Between these two extremes lies ordinary visible light, and even that is damaging thanks to the chemical reactions it stimulates.

Staring directly at the sun will blind you — the only question is for how long, and the answer could be forever. Eclipses are interesting — but they aren’t that interesting.

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