How Physics is involved with the Eye

            The eye is an example of a case 2 situation (see Lenses), in which the object is beyond 2F, and the image formed is real, inverted, smaller than the object, and on the opposite side of the lens.  We know that the image formed must be real, because it is projected onto the retina (which is like a screen).  Any time an image is projected, it must be real.  At the same time, any real image must be inverted.  This might make you wonder why you don’t see objects as being upside down.  Actually, our brain flips things around so that we see them as being upright. 

Click for Full Size The Eye Credits: Franklin Branley, Henry Roth

            We know that lenses are needed to focus an object onto a screen, and in this case the retina.  There are different theories dealing with how the eye focuses its objects.  One of these theories is the 3 lens system.  The three lenses would be the aqueous lens, the lens lens, and the vitreous lens.  In this situation, the cornea would have little refractive power, and just serve as a “cover glass” to the eye.  The aqueous lens lies on top of the pupil, and has a strong refractive power, because it is a very thick lens.  The lens lens lies right after the pupil.  Its index of refraction changes because there is a density change within the lens itself.  The lens in back of the lens lens is the vitreous lens, which deals a lot with the magnification of the object.