The Human Eye is also the related topic to the optics.
The image formation in human eye is shown in Fig.12.36.
Human eye acts like a camera. In place of the film, the retina
records the picture. The eye has a refracting system
containing a converging lens. The lens forms an image on the
retina which is a light sensitive layer at the back of the eye. In
the camera, the distance of lens from film is adjusted for
proper focus but in the eye, the lens changes focal length.
Light enters the eye through a transparent membrane called
the cornea. The iris is the coloured portion of the eye and
controls the amount of light reaching the retina. It has an
opening at its centre called the pupil. The iris controls the size
of the pupil. In bright light, iris contracts the size of the pupil
while in dim light pupil is enlarged. The lens of the eye is
flexible and accommodates objects over a wide range of
The camera focuses the image of an object at a given distance
from it by moving the lens towards or away from the film. The
eye has different adjusting mechanism for focusing the image
of an object onto the retina. Its ciliary muscles control the
curvature and thus the focal length of the lens, and allow
objects at various distances to be seen.
If an object is far away from the eye, the deviation of light through
the lens must be less. To do this, the ciliary muscles relax and
decrease the curvature of the lens, thereby, increasing the focal
length. The rays are thus focused onto the retina producing a
sharp image of the distant object (Fig.12.37-a).
If an object is close to the eye, the ciliary muscles increase
curvature of the lens, thereby, shortening the focal length.
The divergent rays from the nearer object are thus bent more
so as to come to a focus on the retina (Fig.12.37-b).
This variation of focal length of eye lens is called
accommodation. It is large in young people while it goes on
decreasing with age. Defects in accommodation may be
corrected by using different type of lenses in eyeglasses. In
the following sections we will describe defect of vision and
When we hold a book too close, the print is blurred because
the lens cannot adjust enough to bring the book into focus.
The near point of the eye is the minimum distance of an
object from the eye at which it produces a sharp image on the
retina (Fig.12.38). This distance is also called the least
distance of distinct vision. An object closer to the eye than
the near point appears blurred. For people in their early
twenties with normal vision, the near point is located about
25cm from the eye. It increases to about 50cm at the age 40
years and to roughly 500cm at the age 60 years.
The far point of the eye is the maximum distance of a distant
object from the eye on which the fully relaxed eye can focus. A
person with normal eyesight can see objects very far away, such
as the planets and stars, and thus has a far point located at
infinity. Majority of people not have “normal eyes” in this sense!