Everyone has heard that it is dangerous to look at the sun anytime during the eclipse except during the short period of totality. I heard a good description of the reasons and want to pass it along. You may already be totally (ha ha) aware, but if not, this will give you information to pass along to others, particularly kids, to prevent any potential risks. Every eclipse results in eye damage, some permanent, to some viewers. I know we are all more likely to follow the rules or adhere to safety guidance if we fully understand the “whys”.
The eclipse provides a double whammy for possible eye damage:
A. The iris responds to total energy, the more intense the light the more it contracts, reducing the size of the pupil. On a bright day or in a bright room, the pupil is small, even if we look towards a less bright object. As the sun is blotted out by the moon during the beginning phases of the eclipse, the light levels, overall, are reduced, so the pupil will dilate even if you look towards the sun.
B. The eye’s lens focuses the energy that passes through the pupil onto the retina over an area on the back of the eye. During a cloudless day, if you look directly at the sun, the pupil size would be minimized but there would be enough energy to burn a large circle on the retina, resulting in potential damage, maybe permanent. During the eclipse, the pupil will be dilated so any energy from the sliver of sun not covered by the moon will be focused on a part of the retina at even higher energy levels, per cell (rods and cones), than the full sun would provide, but over a smaller area. So, damage can occur to those parts of the retina exposed to that energy, but the damage would be a crescent shape rather than a large disc.
So, if you, friends or family want to look directly at the sun during the eclipse, be sure to have solar glasses from a reputable vendor, usually shown as ISO or ISO certified. SUN GLASSES WILL NOT WORK. The proper glasses cut out far more than 99% of the energy. Other approaches, like telescopes or other optics, with the proper solar filtering, or indirect projection schemes are also good.
During the short period of totality, no eye protection is necessary, but it will probably be more interesting to note what is happening around you, rather than looking at the sun. Then, before looking at the sun as the moon recedes, put the glasses back on if you want to view the sun.