A. The Goldmann lens, which gives an indirect view of
the angle of the anterior chamber.
B. The Koeppe lens, which gives a direct view of the
angle of the anterior chamber.
Gonioscopy The principle of internal reflection is a problem to an
observer who has to examine the eye. Parts of the eye are
not visible because light cannot get out of the eye. For
instance, the angle structures of the anterior chamber are
not visible because of total internal reflection.
Light has to get through the cornea for the examiner to
see the angle structures. At the cornea–air interface,
however, an abrupt change from a high to a low index of
refraction occurs, which causes total internal reflection
exactly like that of the fiberoptic tube.
If a contact lens is applied to the cornea, then light can
pass through the cornea into the contact lens, which has
a refractive index higher than the refractive index of
corneal tissue. Once the light is past the cornea, it can
travel to the eyepiece and be seen either through a mirror,
as noted in the Goldmann lens, or through
simple refraction, as noted in the discussion of the Koeppe