Page 59 - Physics 10_Float

GEOMETRICAL OPTICS

12.15 THE HUMAN EYE
The image formation in human eye is shown in Fig.12.35. Iris Retina
Human eye acts like a camera. In place of the film, the retina
Lens
records the picture. The eye has a refracting system Object
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
Cornea
the camera, the distance of lens from film is adjusted for Light rays
proper focus but in the eye, the lens changes focal length. Image
Light enters the eye through a transparent membrane called Fig.12.35: Image formation in
human eye
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 For your information
distances.

Accommodation
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 We see because the eye forms
images on the retina at the
objects at various distances to be seen.
back of the eyeball.
Distant object

(a)
Relaxed lens
Image on retina
Close object

(b)
Tensed lens Quick Quiz
Image on retina
Fig.12.36: Human eye accommodation
How the size of the pupil of our
eye will change:
If an object is far away from the eye, the deviation of light through (a) in dim light?
the lens must be less. To do this, the ciliary muscles relax and (b) in bright light?
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.12.36-a).

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