406 Week 12: Lenses and Mirrors
Figure 169: A simplified anatomical diagram of the human eye.- Optic Nerve:which pipes all of the information transduced from the light image
cast on the retina to the brain. The retina (especially the macula) is very sensitive
to light and easily damaged. To control the amount of light entering the eye, the... - Iris:The iris is a ring of pigmented tissue that can open or contract to let more or
less light into the... - Pupil:The pupil is the aperture for light into the eye. When it is dark, the iris
opens and lets all the light possible into the retina (which is very sensitive and
capable of seeing with remarkably little light). When it is very bright, the iris
closes down to a pinpoint. This actually increases visual acuity – see thepinhole
camera– independent of the action of the... - Lens:The lens of the eye is normally in a state of tension maintained by suspensory
ligaments calledzonulesthat keep it flattened out, with a maximally long focal
length. A ring ofciliary musclessurrounding the lens can be contracted, which
removes a part of this tension, predictably bulging the lens and thereby reducing
its focal length. This process is calledaccommodation.
It is important to understand that accommodation can onlyreducethe focal length of
the lens, not increase it, as well as the fact that the cornea is responsible for most of the
focal length of the combined system – the actual lens is more of a “correction” to the
overall focal length already achieved by the cornea alone. We now need to understand
the three common conditions that describe the eye.normal eye farsighted eye nearsighted eyecorrected corrected
farsighted eye nearsighted eyeFigure 170: The focal length of the relaxed (combined) lensing acting of the eye for a normal eye, a
farsighted eye (hyperopia), and a nearsighted eye (myopia).
The focal length of arelaxedlens of an eye withnormalvision is on the retina, so distant