Joe is hiking through the woods when he decides to stop and take in the view. he is particularly interested in three objects: a squirrel sitting on a rock next to him, a tree a few meters away, and a distant mountain. as joe is taking in the view, he thinks back to what he learned in his physics class about how the human eye works. light enters the eye at the curved front surface of the cornea, passes through the lens, and then strikes the retina and fovea on the back of the eye. the cornea and lens together form a compond lens system. the large difference between the index of refraction of air and that of the aqueous humor behind the cornea is responsible for most of the bending of the light rays that enter the eye, but it is the lens that allows our eyes to focus. the ciliary muscles surrounding the lens can be expanded and contracted to change the curvature of the lens, which in turn changes the effective focal length of the cornea-lens system. this in turn changes the location of the image of any object in one's field of view. images formed on the fovea appear in focus. images formed between the lens and the fovea appear blurry, as do images formed behind the fovea. therefore, to focus on some object, you adjust your ciliary muscles until the image of that object is located on the fovea.