Vision, the sense of seeing, is the sense that humans use most. The part of the brain for vision is larger than the areas of all the other senses combined.
Human eyes are like cameras that send to the brain signals about patterns of light. The brain organizes these signals to produce images or vision.
The seeing sense starts with light. Just about everything in the universe either gives off light - like the sun or a light bulb - or reflects light - like the moon. If you are interested in finding out more about reflected light, check out this website.
Light travels outward from the source or surface in all directions, and human eyes are made to let in the light. The cornea is the clear bump on the front outside of the eye. The cornea gathers the spreading light to form an image (picture) of whatever is being looked at on the back inside of the eyeball. The lends fine-tunes the image to bring it into exact focus on the retine.
The inside back of the eyeball is covered by 128 million sense cells, called rods (120 million) and cones (8 million), which change the light into signals that the brain understands.
Some nocturnal creatures such as mice, rats and bats have mostly or only rods. They have great difficulty telling colors apart and are almost blind in full sunlight, but are perfectly suited to seeing at night. Other creatures, such as pigeons, chickens, turtles and most reptiles have only cones. They have no trouble distinguishing colors and see perfectly well during the day, but are totally blind at night.