Diffraction is an phenomenon that refers to the bending of the light when meets an obstacle. Due to diffraction at the pupil, the image of a point source is not a point but consist of a central disk called Airy disk and homocentric annuli of decreasing intensity. The size of the Airy disk depends on the diameter of the pupil and the wavelength of the light. Larger pupils lead to a smaller Airy disk and subsequently to higher spatial resolution of the optical system, or in this case the eye.