sample essays of IELTS

46 Vision

Human vision like that of other primates has evolved in an arboreal
environment. In the dense complex world of a tropical forest, it is more
important to see well that to develop an acute sense of smell. In the
course of evolution members of the primate line have acquired large eyes
while the snout has shrunk to give the eye an unimpeded view. Of mammals
only humans and some primates enjoy color vision. The red flag is black
to the bull. Horses live in a monochrome world .light visible to human
eyes however occupies only a very narrow band in the whole
electromagnetic spectrum. Ultraviolet rays are invisible to humans
though ants and honeybees are sensitive to them. Humans though ants and
honeybees are sensitive to them. Humans have no direct perception of
infrared rays unlike the rattlesnake which has receptors tuned into
wavelengths longer than 0.7 micron. The world would look eerily
different if human eyes were sensitive to infrared radiation. Then
instead of the darkness of night, we would be able to move easily in a
strange shadowless world where objects glowed with varying degrees of
intensity. But human eyes excel in other ways. They are in fact
remarkably discerning in color gradation. The color sensitivity of
normal human vision is rarely surpassed even by sophisticated technical
devices.