The eyes of human beings can not keep staring straight at the sun all the time. It is because that the sun light is very very strong, the crystalline lens and vitreous body of the eyeballs are just like a condenser lens (convex lenses). When the eyes are staring straight at the sun, through crystalline lens and vitreous body, the sun light will converge as a small point of light of very high power at the retina of the eyeballs, which can easily burn the retina and do great damage to the eyes. Therefore, you had better not stare straight at the sun. Wearing a pair of sunglasses could effectively protect your eyes from the ultra-violet light when you are outside on the day of great sunlight.