The eye is a marvelous organ, composed of several parts, which collects visible light from the electromagnetic spectrum. We do not see things, or objects, rather, we see the light that is reflected off of them. The process of vision goes something like this:
First, incoming light reflected off an object must pass through the cornea, the clear protective coating that surrounds the eye. The light is refracted (bent) slightly as it does this.
Second, the light passes through a timy opening called a pupil. This is the dark spot in the middle of the colored part of the eye, which is called the iris. The iris controls the size of the pupil, making it small in bright light and large in dim light.
The light then passes through a lens, which refracts the light more and focuses what is called a real image on the back of the eye. The thickness of the lens can be changed to help focus the image being produced.
The black surface of the back of the eye is called the retina. The retina has lots of photoreceptor cells called rods and cones that pick up the image being focused on them. This image is passed along to the optic nerve, which comes out the back of the eye and goes to the brain. The brain interprets the nervous stimuli sent, which is called vision, or seeing things.