Learning goals

• To understand how does the eye accommodate.

Summary

The parts of the eye and the accommodation principle.

Click on the eye to name the parts.
Click and drag the object along the optical axis.

Learn more

In a normal eye, light rays penetrating the eye are first bent by the cornea. Then the lens makes them converge on the retina. This normal condition is known as emmetropia.  The iris controls the amount of light entering the eye by altering the size of the pupil.  Nerve cells in the retina transform the light into patterns of nerve firings which are carried by the optic nerves to the brain, where the nerve firings are analyzed, and the object producing or reflecting the light is recognized.

• Distance vision: Light rays from distant objects arrive at the eye nearly parallel to one another. In this case, the bending of such rays by the cornea suffices to focus an image on the retina , so the lens is not used. At rest, the lens takes on a flat, elongated form. There is no accommodation.

• Near vision: Objects closer than about 6 meters do not send parallel rays to the eye, and this affects the refraction of those rays in the eye. To compensate for such effects, the lens bulges, thus changing its focal length. (It cannot alter the distance to the retina.) Its more curved surface refracts incoming light more.  These changes in the shape of the lens are what is meant by the term “accommodation”.