Understanding Psychology (10th Ed)

106 Chapter 
4 
Sensation and Perception
Why shouldn’t the pupil be open completely all the time, allowing the greatest 
amount of light into the eye? The answer relates to the basic physics of light. A small 
pupil greatly increases the range of distances at which objects are in focus. With a 
wide-open pupil, the range is relatively small, and details are harder to discern. The 
eye takes advantage of bright light by decreasing the size of the pupil and thereby 
becoming more discriminating. In dim light the pupil expands to enable us to view 
the situation better—but at the expense of visual detail. (Perhaps one reason candle-
light dinners are thought of as romantic is that the dim light prevents one from 
seeing a partner’s physical fl aws.) 
Once light passes through the pupil, it enters the lens , which is directly behind 
the pupil. The lens acts to bend the rays of light so that they are properly focused 
on the rear of the eye. The lens focuses light by changing its own thickness, a process 
called accommodation : It becomes fl atter when viewing distant objects and rounder 
when looking at closer objects.

REACHING THE RETINA

Having traveled through the pupil and lens, the image of the tree fi nally reaches its 

ultimate destination in the eye—the
retina
.
It is within the retina that the electromag-
netic energy of light is converted to electrical impulses for transmission to the brain. 
Note that, because of the physical properties of light, the image has reversed itself 
in traveling through the lens, and it reaches the retina upside down (relative to its 
original position). Although it might seem that this reversal would cause diffi culties 
in understanding and moving about the world, this is not the case. The brain inter-
prets the image in terms of its original position.
The retina consists of a thin layer of nerve cells at the back of the eyeball (see 
Figure 3). There are two kinds of light-sensitive receptor cells in the retina. The names 
they have been given describe their shapes: rods and cones.