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Chapter 24. Optical Instruments
Eyeglasses, microscopes and
telescopes aid our senses by
using lenses and mirrors to
form images we wouldn’t be
able to see, or see as well,
with our eyes alone.
Chapter Goal: To
understand some common
optical instruments and their
limitations.
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Lenses in Combination
The analysis of multi-lens systems requires only one new
rule: The image of the first lens acts as the object for the
second lens.
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The Camera
•  A camera lens forms a real
inverted image on a light-sensitive
detector in a light-tight box.
•  A adjustable focus combination
lens acts as a single lens with an
effective focal length
•  The effective focal length is
changed by varying the spacing
between a converging lens and
diverging lens.
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EXAMPLE 24.2 Focusing a camera
QUESTION:
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EXAMPLE 24.2 Focusing a camera
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Human eye
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Focusing and Accommodation
•  The ciliary muscles to change the curvature of the lens
surface.
•  Tensing the ciliary muscles causes accommodation, which
decreases the lens’s radius of curvature and thus decreases
its focal length.
•  The farthest distance at which a relaxed eye can focus is
called the eye’s far point (FP). The far point of a normal
eye is infinity.
•  The closest distance at which an eye can focus, using
maximum accommodation, is the eye’s near point (NP),
normally 25 cm.
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EXAMPLE 24.4 Correcting hyperopia
QUESTION:
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EXAMPLE 24.4 Correcting hyperopia
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EXAMPLE 24.4 Correcting hyperopia
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EXAMPLE 24.5 Correcting myopia
QUESTION:
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EXAMPLE 24.5 Correcting myopia
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The Microscope
•  A specimen to be observed is placed on the stage of a
microscope, directly beneath the objective, a converging
lens with a relatively short focal length.
•  The objective creates a magnified real image that is
further enlarged by the eyepiece.
•  The lateral magnification of the objective is
• Together, the objective and eyepiece produce a total
angular magnification
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The Telescope
•  A simple telescope contains a large-diameter objective
lens which collects parallel rays from a distant object
and forms a real, inverted image at distance s' = fobj.
•  The focal length of a telescope objective is very nearly the
length of the telescope tube.
•  The eyepiece functions as a simple magnifier.
•  The viewer observes an inverted image.
•  The angular magnification of a telescope is
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The Telescope
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Diffraction effects
A lens both
focuses and
diffracts light.
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The Resolution of Optical Instruments
The minimum spot size to which a lens can focus light of
wavelength λ is
where D is the diameter of the circular aperture of the lens,
and f is the focal length. In order to resolve two points, their
angular separation must be greater than θmin, where
is called the angular resolution of the lens.
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Estimate of the angular resolution of the eye.
This corresponds to resolving two dimes (1 cm) at a
distance of 1/(3e-4) = 3e3 cm = 30 m.
The density of rods and cones has evolved to match the
diffraction limit.
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A moment of appreciation
The microscope opened the microscopic universe making
possible the study of biology at the cellular level.
The telescope opened the celestial universe enabling the
discovery of our place in the solar system and in the
universe.
Let us not forget too the art of the emulsion photographic
film permitting the observation of small dim signals.
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The second lens
in this optical
instrument
A. Causes the light rays to focus closer than
they would with the first lens acting alone.
B. Inverts the image but does not change where
the light rays focus.
C. Causes the light rays to focus farther away
than they would with the first lens acting
alone.
D. Prevents the light rays from reaching a
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The second lens
in this optical
instrument
A. Causes the light rays to focus closer than
they would with the first lens acting alone.
B. Inverts the image but does not change where
the light rays focus.
C. Causes the light rays to focus farther away
than they would with the first lens acting
alone.
D. Prevents the light rays from reaching a
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A photographer has adjusted his
camera for a correct exposure with a
short-focal-length lens. He then decides
to zoom in by increasing the focal
length. To maintain a correct exposure
without changing the shutter speed, the
diameter of the lens aperture should
A.  stay the same
B.  be increased
C.  be decreased
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A photographer has adjusted his
camera for a correct exposure with a
short-focal-length lens. He then decides
to zoom in by increasing the focal
length. To maintain a correct exposure
without changing the shutter speed, the
diameter of the lens aperture should
A.  stay the same
B.  be increased
C.  be decreased
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You need to improvise a magnifying
glass to read some very tiny print.
Should you borrow the eyeglasses from
your hyperopic friend or from your
myopic friend?
A.  The myopic friend
B.  The hyperopic friend
C.  Either will do
D.  Neither will work
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You need to improvise a magnifying
glass to read some very tiny print.
Should you borrow the eyeglasses from
your hyperopic friend or from your
myopic friend?
A.  The myopic friend
B.  The hyperopic friend
C.  Either will do
D.  Neither will work
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A biologist rotates the turret of a
microscope to replace a 20× objective
with a 10× objective. To keep the same
overall magnification, the focal length of
the eyepiece must be
A.  Halved.
B.  Doubled.
C.  Kept the same.
D.  The magnification cannot be kept the
same if the objective is changed.
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A biologist rotates the turret of a
microscope to replace a 20× objective
with a 10× objective. To keep the same
overall magnification, the focal length of
the eyepiece must be
A.  Halved.
B.  Doubled.
C.  Kept the same.
D.  The magnification cannot be kept the
same if the objective is changed.
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Four diffraction-limited lenses focus plane
waves of light with the same wavelength l.
Rank order, from largest to smallest, the spot
sizes w1 to w4.
A. 
B. 
C. 
D. 
E. 
w2 = w3 > w4 > w1
w1 = w2 > w3 > w4
w4 > w3 > w1 = w2
w1 > w4 > w2 = w3
w2 > w1 = w3 > w4
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Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley.
Four diffraction-limited lenses focus plane
waves of light with the same wavelength l.
Rank order, from largest to smallest, the spot
sizes w1 to w4.
A. 
B. 
C. 
D. 
E. 
w2 = w3 > w4 > w1
w1 = w2 > w3 > w4
w4 > w3 > w1 = w2
w1 > w4 > w2 = w3
w2 > w1 = w3 > w4
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