Page 240 - Physics_Form_2

Physics for Secondary Schools

Note that lenses used for vision correction Note that the myopic eye produces too
are usually described in terms of lens much convergence of light rays from a
power, P, which is given by the reciprocal distant object, while a hyperopic eye
focal length expressed in metres. Hence, produces insufficient convergence of
rays of light from a near object.
1
P =
FOR ONLINE READING ONLY
f Presbyopia
Presbyopia is the gradual loss of the eye’s
The unit of lens power is the dioptre, D. ability to focus on nearby objects because
Thus, a lens with f = 0.50 m has a power of old age. The term “presbyopia” comes
of 2.0 D, f = 0.250 m corresponds to 4.0 from a Greek word that means “old eye.”
D, and so on. The numbers used for the This defect is normally observed after
prescription of the glass lenses are usually the age of 40 years. Presbyopia is caused
powers expressed in diopters. by loss of eye lens flexibility, which is
necessary to change its focal length in

Long-sightedness (hyperopia) order to focus on objects. This results
In the hyperopic (long-sightedness or far- in insufficient accommodation, leading
sightedness) eye, the eyeball is too short to image formation behind the retina, as
or the cornea is not curved enough. This shown in Figure 6.24.
causes the rays of light from a near object
to be focused behind the retina. A person Contracted Image formed
muscles
behind retina
suffering from long sightedness can see
distant objects clearly, but cannot see
nearby objects clearly. The near point of
the eye may be more than a metre away,
making ordinary reading difficult. Figure
6.23 illustrates the hyperopic eyes.

Cornea
Focal
Insufficient
accommodation

Figure 6.24: Presbyopic eye

Correction of hyperopia and presbyopia
In the case of either a presbyopic or a
Lens hyperopic eye, the near point is farther
from the eye than normal. Therefore, to
Figure 6.23: Hyperopia see clearly an object at a normal reading
distance, which is about 25 cm, a convex

234
Student’s Book Form Two

Physics Form 2 Final.indd 234 25/10/2025 10:28