from AISH dot com: On a practical note: If you encounter a solar eclipse, even though the sun appears covered, don’t look. The sun’s corona is still as powerful as ever. People have gone blind after looking at an eclipse for as few as four seconds. There is no pain when the retina is being burned, and the resulting visual symptoms do not occur until at least several hours after the injury has occurred – by which time it is far too late.
Maimonides writes (Deyot 4) that it is a Torah obligation to guard one’s physical health. Therefore, regardless of how tempting it is to look at the sun during the eclipse, don’t do it. Sunglasses are not effective; you must use specifically approved treated plastic or glass filters, or indirectly view the sun’s projection through a pin-hole.