The Human Eye and
the Colourful World
You have studied in the previous chapter about refraction of light by
lenses. You also studied the nature, position and relative size of
images formed by lenses. How can these ideas help us in the study of
the human eye? The human eye uses light and enables us to see objects
around us. It has a lens in its structure. What is the function of the lens
in a human eye? How do the lenses used in spectacles correct defects of
vision? Let us consider these questions in this chapter.
We have learnt in the previous chapter about light and some of its
properties. In this chapter, we shall use these ideas to study some of the
optical phenomena in nature. We shall also discuss about rainbow
formation, splitting of white light and blue colour of the sky.
11.1 THE HUMAN EYE
The human eye is one of the most valuable and sensitive sense organs.
It enables us to see the wonderful world and the colours around us. On
closing the eyes, we can identify objects to some extent by their smell,
taste, sound they make or by touch. It is, however, impossible to identify
colours while closing the eyes. Thus, of all the sense organs, the human
eye is the most significant one as it enables us to see the beautiful,
colourful world around us.
The human eye is like a camera. Its lens
system forms an image on a light-sensitive
screen called the retina. Light enters the eye
through a thin membrane called the cornea.
It forms the transparent bulge on the front
surface of the eyeball as shown in Fig. 11.1.
The eyeball is approximately spherical in shape
with a diameter of about 2.3 cm. Most of the
refraction for the light rays entering the eye
occurs at the outer surface of the cornea. The
crystalline lens merely provides the finer
adjustment of focal length required to focus
objects at different distances on the retina. We find a structure called iris
behind the cornea. Iris is a dark muscular diaphragm that controls the
size of the pupil. The pupil regulates and controls the amount of light