The back of your eye is packed with rods and cones (mostly rods, about 120 million of them vs 6m cones) and the photon is absorbed by them. Inside rods you have stacks of proteins called optic disks, that have rhodopsin in them, which has a small molecule called 11-cis retinal. When light comes through the pupil and hits the rod, some of it will hit the rhodopsin and cause a change in retinal to 11-trans retinal. This change in shape will cause a cascade that closes sodium channels on the rod and essentially turns the cell "off". The turning off of the rod will turn on a bipolar cell which then turns on a retinal ganglion cell that finally sends a signal via the optic nerve to the brain