As we know, bifocal glasses are used by patients whose eyes are mixed with different types of refractive errors, that is myopia, hyperopia and astigmastism. They are most probably wore by the elderly. But here comes a strange phenomenon, that is young people also need bifocal glasses. Why? The elderly need them because their vision degenerates as time goes by. Young people need them because the development of vision has not jet been completed. And what is worse, this vision development can even be abnormal for different facts of the environment. So young people can not only have myopia because of improper use of eyes, but also hyperopia and astigmastism. Bifocal glasses provide both distance and near vision aids, so that the visual acuity of young people can be corrected.