What makes solar eclipse so dangerous to look at?

Have you ever wondered why you’re advised against looking at the eclipse with bare eyes? After all, it’s the same sun being blocked by a planetary object. The fact that the sun doesn’t magically power ups during the eclipse but looking at it during then can make you go blind has to do with your eyes not the sun itself.

The fundamental science behind this is the anatomy of human pupil which varies its size to adapt to the luminance or simply the intensity of light entering your eyes. In dark, an adult’s pupil can stretch/dilate up to 8 mm in diameter to allow more light to enter your eyes while it can shrink down to 2 mm in bright surroundings. But what is bright enough to make pupil shrink to the lowest? Holladay L. in 1926 in his journal “The fundamentals of glare and visibility” established that it takes only less than a luminance of 1000 cd/m2 for a human pupil to shrink to its lowest. Beyond that it’s other supporting elements that protects your retina like squeezing your eyes or shutting them altogether.

At noon, the luminance of sun can go up to 1.6×109 cd/m2 which is about 2 millions times more than the light required for your pupil to shrink to lowest. Know that the pupil controls the amount of light that enters your eye chamber and if your forcefully or accidentally insert more light, your retina can burn permanently.

While your bare eyes cannot see the sun under normal circumstances anyways; the eclipse makes it much worse. During eclipse the luminance drops and pupil enlarges and you looking at the eclipse with those enlarged pupils completes the recipe of disaster. The moment that stray ray shines from the edge of eclipse it can damage your eye permanently before your pupil gets a change to respond to it. At this point it important to mention that it’s perfectly safe to see the eclipse with bare eyes while the moon is covering the sun completely and there’re no rays coming from edge but would you take your chances? I suggest not because the ray will hit your before even you know it.

Look out for Diamond Ring (or not!)

The Baily’s beads effect or commonly known as diamond ring eclipse is the first peek of sun from behind a total eclipse which appears like a diamond ring. The only difference is that this one can burn your eyes as the pupils are fully dilated to accept any and all light and boom! Suddenly the most violent and powerful gush of light enters your retina and damaging it permanently. Image this situation as you are sleeping in dark room and the first thing you look after opening the eyes is the sun.