The outer segments of rod and cone cells are the photosensitive part. In rod cells this part contain rhodopsin or visual purple. Rhodopsin is a compound of protein (opsin) and pigment (retinene). The animals which have very high number of rod cells can see in dark because rod cells are highly sensitive to light. Retina of owls contains only rods. The animals which have high number of cone cells can see in light. The retina of fowls contain only cones.