Light is produced by many objects in our surroundings, and this light ten reflects off of the most other objects. It is usually this reflected light that launches the processes of vision. Light hits the front surface of the eyeball, passes through the cornea and the lens, and then hits the retina, the light-sensitive tissue that lines the back of the eyeball. The cornea and lens focus the incoming light, so that a sharp image is cast onto the retina. The shape of the lens can be adjusted by a band of muscle that surrounds it.
On the retina, there are two types of photoreceptors, cells that respond directly to the incoming light. The rods are sensitive to much lower levels of light and so play an essential role whenever we're moving around in semidarkness. But the rods are also color-blind: they distinguish among different intensities of light, but they provide no means of discriminating one hue from another.  Cones are much less sensitive than rods and so need more inoming light to operate at oll. There are three different types of cones, each having its own pattern of sensitivies to different wavelenghts. Cones also have another crucial function: They allow us to discern fine detail. The ability to see detail is reffered to as acuity.