Question

 How does the eye adjust itself to deal with light of varying intensity? 

Answer 1

 The iris regulates the amount of light entering the human eye by automatically adjusting the size of the pupil according to the intensity of light that the eye receives. If the intensity of light is high, the iris contracts the pupil so as to reduce the amount of light entering the eye. If the intensity of light is low, the pupil expands to allow more light to enter the eye.