The retina is the light-sensing part of the eye that holds the rods and cones, which contain enzymes that absorb light and allow us to see. When light strikes the retinal molecule, it changes its shape. This activates a cascade of chemical reactions that informs the brain that light has entered the eye. When the levels of light sensitive molecules are low, due to Vitamin A deficiency, there will not be enough retinal to detect the light at night. During the day there is enough light to produce vision, despite low levels of retinal. So it's only night vision that can be improved by eating carrots.