Understanding Psychology (10th Ed)

Key Concepts
What basic processes underlie 
the sense of vision? 
How do we see colors?
M O D U L E 1 1 
 
Vision: Shedding Light on the Eye
Gamma
rays
X-rays
Ultraviolet
rays
Infrared 
rays
Radar
FM
TV
Shortwave
AM
AC
electricity
Wavelength in meters
10
–14
10
–12
10
–10
10
–4
10
–2
10
1
10
2
10
8
10
4
10
6
10
–6
10
–8
400
500
600
700
Violet Blue
Green
Yellow
Red
Wavelength in nanometers (billionths of a meter)
Visible light
FIGURE 1 
The visible spectrum—the range of wavelengths to which people are sensitive—is 
only a small part of the kinds of wavelengths present in our environment. Is it a benefi t or 
disadvantage to our everyday lives that we aren’t more sensitive to a broader range of visual 
stimuli? Why?
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Module 11 
Vision: Shedding Light on the Eye 
105
Illuminating the Structure 
of the Eye
The ray of light being refl ected off the tree in Figure 2 fi rst travels through the cornea , 
a transparent, protective window. The cornea, because of its curvature, bends (or 
refracts ) light as it passes through, playing a primary role in focusing the light more 
sharply. After moving through the cornea, the light traverses the pupil. The pupil is 
a dark hole in the center of the iris , the colored part of the eye, which in humans 
ranges from a light blue to a dark brown. The size of the pupil opening depends on 
the amount of light in the environment. The dimmer the surroundings are, the more 
the pupil opens to allow more light to enter. 
A camera’s lens focuses the
inverted image on the film in
the same way the eye’s lens
focuses images on the retina.
Optic nerve
Fovea
Retina
Nonsensor cells
of retina
Iris
Cornea
Pupil
Lens
Blind spot
FIGURE 2 
Although human vision is far more complicated than the most sophisticated 
camera, in some ways basic visual processes are analogous to those used in photography. 
Like the automatic lighting system on a traditional, nondigital camera, the human eye dilates 
to let in more light and contracts to block out light.
Like the automatic lighting system on 
a camera, the pupil in the human eye 
expands to let in more light (left) and 
contracts to block out light (right). Can 
humans adjust their ears to let in more 
or less sound in a similar manner?
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