New trends in physics teaching, v.4; The ... - unesdoc - Unesco

New Trends in Physics Teaching IV
The retina itself, comparable to the film in a camera, is composed of rod-shaped and coneshaped
cells, each about 0.08 mm in diameter. The active light sensitive element for the rodsis
the chemical compound rhodopsin. The molecular structure is shown in figure 26. When light
strikes the rhodopsin pigment, the retinene group is caused to swing round in the molecule,
changing its potential energy, and setting off an electrochemical impulse in the retina's nerve
cells. The rhodopsin bleaches under illuminaticn and is restored in the dark. The light, having
transferred its energy, ceases to exist. The electrical impulse from the retinene appears to be of
too low a voltage to trigger the neurons, so it appears there is some unknown intermediate
amplifying system. The structure of the retina is shown in figure 27. Each sensory cell is attached
to one or more optic nerve fibres or axons via the synapses, bipolar cells, and amacrine cells. As
will be seen, there is not a one to one relationship between a rod or cone, and an axon. In addition,
experiment shows that the nature of signals transmitted along the axons is different from the
three colour output of the cones described below. The nearest analogue is the way in colour
television the three colours are transmitted via a black and white and chroma signal. In the case
of the eye, the output appears to be black-white, red-green and yellow-blue.
optic
nerve
fibres
Ganglion
cells
Inner
syn a p t ic
layer
Amacrine cells
Bipolar cells
Horizontal cells
Outer synaptic
layer
Receptor
nuclei
Receptors
pigmented layer
(epithelium cells)
.c 0 ne
-
rod
INNER
LAYER
MIDDLE
LAYER-
OUTER
LAYER
Figure 27. The structure of the retina.
Optical properties of the cone cells
The ends of the cone cells through which the light first passes have three times the diameter of
the opposite ends. Since light from the pupil enters roughly parallel to the length, it is totalinternally
reflected from side to side, as shown in figure 28, and concentrated on the smaller area
which, it seems, is the part of the cell containing the photosensitive pigment, less of which is
required because of this focusing action.
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