How are eyes damaged by staring at the sun?
Solar retinopathy is a result of too much ultraviolet light flooding the retina. In extreme cases this can cause blindness, but is so painful that it is rare for someone to be able to stare at the sun for that long. Typically, eye damage from staring at the sun results in blurred vision, dark or yellow spots, pain in bright light or loss of vision in the center of the eye (the fovea). Permanent damage to the retina has been shown to occur in ~100 seconds, but the exact time before damage occurs will vary with the intensity of the sun on a particular day and with how much the viewer's pupil is dilated from decongestants and other drugs they may be taking. Even when 99% of the Sun's surface (the photosphere) is obscured during the partial phases of a solar eclipse, the remaining crescent Sun is still intense enough to cause a retinal burn. Note, there are no pain receptors in the retina so your retina can be damaged even before you realize it, and by then it is too late to save your vision!