Page 61 - Physics 10_Float

GEOMETRICAL OPTICS

long. Light rays from a distant object are focused in front of
the retina and a blurred image is produced (Fig.12.38-a).
Interesting information
Distant object Relaxed lens
Some animals like fish has the
ability to move their eye lenses
Far point of
(a) forward or backward and
nearsighted eye
Image formed in hence, are able to see clearly
objects around them.
front of retina
Virtual image formed Diverging lens
Distant object by diverging lens

Far point of
(b)
nearsighted eye
Image formed
Fig. 12.38: Correction of near sightedness on retina
The nearsighted eye can be corrected with glass or contact
lenses that use diverging lenses. Light rays from the distant
objects are now diverged by this lens before entering the eye.
To the observer, these light rays appear to come from far
point and are therefore focused on the retina, thus forming a
sharp image (Fig.12.38-b).
Farsightedness (hypermetropia)

The disability of the eye to form distinct images of nearby
objects on its retina is known as farsightedness.
When a farsighted eye tries to focus on a book held closer
than the near point, it shortens its focal length as much as it
can. However, even at its shortest, the focal length is longer
than it should be. Therefore, the light rays from the book For your information
would form a blurred image behind the retina (Fig.12.39-a).
A thin film can be placed on the
Near point of Tensed lens Image formed lenses of eyeglasses to keep
farsighted eye behind retina t h e m f r o m r e f l e c t i n g
wavelengths of light that are
(a) Object
highly visible to the human
eye. This prevents the glare of
Virtual image formed reflected light.
by converging lens Converging lens

(b)
Near point of Object Image formed
farsighted eye on retina

Fig. 12.39: Correction of farsightedness

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