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CHAPTER 25
OPTICAL INSTRUMENTS
THE CAMERA
THE EYE
MAGNIFIER
MICROSCOPE
TELESCOPE
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CAMERA
Main Parts of Camera:
Enclosed light tight chamber
Light detector – film or photo cells
Lens combination – to focus the image on
the film or photo cells.
Shutter – To allow the light to reach the film
or photo cells.
Aperture – Along with the speed of the
shutter to control the amount of light that
reaches the film or photo cells.
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The Diaphragm controls the amount of light to
reach the film or photo cells for a specific setting
of shutter speed.
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F-number or f-stop is defined:
������� = �� ������ℎ
���������� �����
������� = �
�
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Example 25.1
Telephoto lens for 35 mm camera
Focal length 200 mm
Fstops = 5.6 to 45
What is the range of aperture diameters?
What is the corresponding range of intensities?
������� = �
�
Therefore
� =�
�������=���
!.#= 36��
To
� =�
�������= 200mm
45= 4.4��
Intensities = square of D
+36��,�
+4.4���,= 65
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THE PROJECTOR
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Example 25.2
Slide 24 x 36 mm
Object on screen 1.2 x 1.8 m
Image distance s = 5 m
What focal length?
Need � =-.
-=
/.��
�01/�23�= 50
And -.
-=−
5.
5= 50
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And 67 = 5�
5�
6= 50
Thus
6 = 0.1�
Then
1
�= 1
6+1
67=
1
0.1�+1
5�= 9.8��
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THE EYE
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COLOR VISION
Retina has two types of cells – Rods and Cones
Rods – sensitive to all light but signals to brain only
black or white.
Cones – there are three types of cones, sensitive
to red, blue or green.
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Defects in cones cause color deficiencies or in
extreme cases color blindness.
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Other defects
First - Normal vision
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Nearsighted Eye
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Farsighted eye
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To Correct need glasses
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More complicated defect – astigmatism
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Near Point and Far Point
Range that a normal eye can have objects in focus
is from the “near point” to the “far point”.
This is due to the ageing of the muscle that
controls the lens of the eye.
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Other eye problems you can worry about for the
next 50 years.
Cataracts
Glaucoma
Macular Degeneration
Having cataracts has a good side. It is easy to fix
(now) and at the same time the “fix” can take care
of the problem of the near point receding.
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THE MAGNIFIER
If you want to look at something small – your book
uses an example of an inchworm.
To get your best look you can put the object (the
inchworm) at your near point.
To get a better look you need help.
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Place a magnifier lens between the inchworm and
your eye and you can move the object closer than
your near point.
The image on your retina will be larger.
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Angular Magnification
Angular Magnification is the ratio of two angles –
the angle subtended at the eye with the magnifier
lens and the angle subtended at the eye without
the lens.
< ==′
=
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In the figure with the object distance 6 = 25��
The magnification will be found with trig.
tan = = B
25��≈ =��� D� �6
tan =′ = B
�≈ =′��� D� �6
Therefore magnification is
< ==′
==
B�E
B25��E
=25��
�
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THE MICROSCOPE
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The object gives an image in a position that will be
the object position for the second lens. Thus
maxnification.
Shown here:
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Magnification will be the product of magnification
of the first lens and the magnification of the
second lens.
The magnification of the objective lens is
�/ = −6/7
6/
As the book says the microscope is usually used
with the object very close to the focal point thus
6/ ≈ �/ and �/ ≈ −5F.
�F
Magnification of the eyepiece is
<� =25��
��
And < =�/<� =+�!G�,5F
.
�F�H
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TELESCOPES
Refracting
You can see the refracting telescope is much like
the microscope.
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Reflecting
Eyepiece in center of telescope.
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Eyepiece at end of telescope.
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Eyepiece on side of telescope.