Chapter 27
Optical Instruments (Lecture II)
Dr. Jie Zou
PHY 1161
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Outline
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Corrective optics
Nearsightedness and farsightedness
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The cause
The correction
Refractive power
Dr. Jie Zou
PHY 1161
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Corrective Optics
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Use a two-lens system to correct
abnormal vision:
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The lens of the eye
An external lens, for example the glasses
Nearsightedness
Farsightedness
Dr. Jie Zou
PHY 1161
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Nearsightedness
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Normal vision:
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Can focus on objects at infinity when the ciliary
muscles of the eye relax.
The far point is essentially infinity.
Nearsightedness:
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A totally relaxed eye can focus only out to a finite
distance from the eye.
The far point is not infinity, but a finite distance
from the eye.
Objects near the eye can be focused, while
objects beyond the far point are fuzzy.
Dr. Jie Zou
PHY 1161
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Eye Shape and Nearsightedness
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An eye that is elongated can
cause nearsightedness.
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The eye converges the light
coming into it in too short a
distance.
The focal length of the eye is
less than the distance from
the lens to the retina.
An object at infinity forms an
image in front of the retina.
Dr. Jie Zou
PHY 1161
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Correcting Nearsightedness
A diverging lens (concave) in front of the eye removes
some of the excess convergence.
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(1) The diverging (concave) lens with a proper focal
length forms a virtual image of a distant object at the
far point of the nearsighted eye.
(2) The relaxed eye can now focus on this image of the
object.
Dr. Jie Zou
PHY 1161
6
Example 27-2 Extended Vision
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A nearsighted person
has a far point that is
323 cm from her eye. If
the lens in a pair of
glasses is 2.00 cm in
front of this person’s
eye, what focal length
must it have to allow her
to focus on distant
objects.
Answer: f = -321 cm
Dr. Jie Zou
PHY 1161
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Farsightedness
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Normal vision:
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Near point distance ≈ 25 cm.
Farsightedness:
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Having a near point that is much farther from the
eye than the typical 25 cm.
Can see clearly beyond the near point of the eye,
but cannot focus on closer objects.
Dr. Jie Zou
PHY 1161
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Eye Shape and Farsightedness
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Farsightedness can be
caused by an eyeball that is
shorter than normal, or
because the eye’s lens
becomes sufficiently stiff
with age.
An object inside the near
point comes to a focus
behind the retina.
Dr. Jie Zou
PHY 1161
9
Correcting Farsightedness
Using a converging lens in front of the eye to add to its
insufficient convergence.
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(1) For a near object inside the near point of the eye,
the convex lens forms an image that is beyond the near
point.
(2) The eye can now focus on the image of the object.
Dr. Jie Zou
PHY 1161
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Example 27-3
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A farsighted person wears
glasses that enable him to
read a book held at a
distance of 25.0 c from his
eyes, even though his nearpoint distance is 57.0 cm. If
his glasses are at a distance
of 2.00 cm from his eyes,
find the focal length
required of his lenses to
place the image of the book
at the near point.
Answer: f = 39.5 cm
Dr. Jie Zou
PHY 1161
11
Refractive Power
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Refractive power = 1/f
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The shorter the focal length, the
more strongly a lens refracts
light.
SI unit of refractive power:
diopter = m-1.
Example:
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A lens with a refractive power of 10.0
diopters has a focal length of 10.0 cm
(a converging lens); A lens with a
refractive power of –10.0 diopters has
a focal length of –10.0 cm (diverging
lens).
Dr. Jie Zou
PHY 1161
12
Homework #15
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Chapter 27, P. 969-970, Problems:
#9, 10, 22, 30, 31 (Physics, Walker, 4th
edition).
Dr. Jie Zou
PHY 1161
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