responding to a different range of light wavelengths. The rod and cone
cells contain specific protein molecules, called rhodopsin and cone
opsins, that absorb light and initiate the process of transformation of
the light energy into a neural signal. Rhodopsin and the various cone
opsins absorb light in slightly different regions of the visible light
spectrum (Fig. 14.2). There are short-wavelength (S), medium-wave-
length (M), and longer-wavelength (L) absorbing cones. S, M, and L
cones are also, respectively, sometimes referred to as blue, green, and
red cones.
S-Cone Rod M-Cone L-Cone
420 498 530 560_. 100
BS
o 75
o
S Q 50
a ®
ec 25t t t t t
400 450 500 550 600 650
Wavelength (nm)Figure 14.2. Response of rod and cone photoreceptor cells in the human retina
to wavelengths in the spectrum of visible light. The S-cone has a maximal
response at around 420 nm (violet-blue light), the M-cone at around 530 nm
(green light), and the L-cone at around 560 nm (green-yellow light). Rod pho-
toreceptors have a response maximum at around 498 nm (blue-green light).
Color vision depends on comparing the amount of activation of
the three different cone photoreceptor types by various wavelengths