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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.