PHYSICS

(fig. 22.7, a). The furthest distance from the eye that an object
can be brought into focus on the retina is called the far point
(fig. 22.7, b). In order to focus parallel light on the retina, a
diverging lens (spectacles) is used which forms a virtual image at
the far point of the eye. The corneal-lens system can now focus
light on the retina (fig. 22.7, c).
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~ Fa:..point :::11t=
~ 100cm
a
"""::~::::ITt§
c
Fig. 22.7. The short-sighted eye: a - the light from a distant object is
brought to a focus in front of the retina; b ­ the far point - the
farthest distance from the eye that an object can be brought to a focus
on the retina; c - diverging spectacles can focus light on the retina
Long-sight (Hypermetropia). A long-sighted person can see
distant objects distinctly, but not those that are close. With the
latter, objects are focused behind the retina due to the eyeball
being too short (fig. 22.8, a). An object at the near point,
however, can be seen clearly (fig. 22.8, b). A converging lens
(spectacles) is used when viewing closer objects. It forms a virtual
image at the near point (fig. 22.8, c).
Astigmatism occurs as a result of a lack of symmetry in the
cornea. When a pattern such as that in fig. 22.9 is viewed, one set
of lines appears sharper than the others. The defect can be
corrected by cylindrical lenses.
Spherical Aberration is found on all lenses bound by spherical
surfaces. The marginal portions of the lens bring rays into a
shorter focus than the central region (fig. 22.10), making the
image of a point a small blurred circle.
Chromatic Aberration. All lenses made of a single material
refract rays of shorter wavelengths more strongly than those of
longer wavelengths and in doing so bring blue light to a shorter
b
§
~~
.... =::::- ..
0.25 m >---I
0.6 m ~ -----<
Fig. 22.8.The long-sighted eye: a - the light from a near object is
focused behind the eye; b ­ the near point - the distance from the
eye that an object can be clearly seen; c - converging spectacles can
focus light on the retina
~i
Fig. 22.9. Astigmatism: to a person
viewing this pattern, and suffering
from astigmatism, one set of lines
will appear sharper than the others
Red
Fig. 22.11. Chromatic aberration:
shorter wavelengths are brought
to a closer focal point than long
wavelength
c
b
Fig. 22.10. Spherical
aberration: light from the
margins is brought to a shorter
focus than the central regions
focus than red (fig. 22.11).
The result is that the image
of a point of white light is not
a white point, but a blurred
circle fringed with color.
Cataracts are caused by a
disease that commonly occurs
with old age, in which the
lens becomes partially or totally
opaque. One remedy for
cataracts is surgical removal
and replacement of the lens.
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