Physics 2054C

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Physics 2054C

• Chapter 25 Optical Instruments

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Camera

• Camera
• f-stop
  focal length/aperture diameter
• Focus - depends on lens-film distance
• Shutter speed - length of exposure ? amount of
  light on film

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Camera
35 mm refers to the film size, not the focal
length which is usually about 50 mm
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The Eye

• Eye
• Normal Eye
• Eye Adjustments
• Near Point
• Far Point
• Relaxed Eye
• Diopters the power of a lens
• Correcting Lenses
• Near Sighted
• Far Sighted

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The Eye As A Lens.
A relaxed eye should focus an object at infinity
onto the back of your eye (retina).
Far point the furthest distance at which an
object can be seen.
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The Eye As A Lens.
The muscles in your eye must thicken the eyes
lens to focus nearby objects onto the back of
your eye (retina).
Near point the closest distance on which the
eye can clearly focus.
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The Eye As A Lens.
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The Eye As A Lens.
Correct by helping the eye focus the rays.
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The Eye As A Lens.
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The Eye As A Lens.
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Examples
Reading glasses of what power (D 1/F) are
needed for a person whose near point is 127 cm,
so that she can read a computer screen at 52 cm?
Assume a lens-eye distance of 2 cm.
This person is farsighted they will need a
converging lens (F gt0).
1 1 1 ? F (I O) / (I O) F
O I F (I O) / (I O)
50(-125)/(50-125) F 35.7 cm ? P 1/F
1/(.357) 2.8
O 50 cm (dO 2) I -125 cm -(di 2)
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Magnifying Glass
What are we are trying to accomplish?
Angular magnification (M) ?1/?2 In this case
?1 gt ?2 so M gt 1. Note the size of the images
on the retina.
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Magnifying Glass
We add a converging lens.
Angular magnification (M) ?1/?2 In this case
?1 gt ?2 so M gt 1.
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Example
Using a 15 cm focal length lens as a magnifying
glass, how far away must an object be placed to
obtain maximum magnification for a normal eye?
What is the magnification?
We want the image distance to be at the near
point (25 cm for a normal eye) so the image
distance is 25 cm.
1 1 1 ? O (I F) / (I - F)
-2515/(-25-15) 9.375 cm F O I
m -I/O -(-25)/9.375 2.67
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Telescopes

• Telescope
• Astronomical
• Gallilean
• Reflecting
• Angular Magnification

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Astronomical Telescope
Objective
Eyepiece
?out
Fo
Fe
?in
M -?out /?in ? -tan(?out ) / tan(?in )
-(H/Fe)/(H/Fo) -Fo/Fe
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Example

• A 200X astronomical telescope is adjusted for a
  relaxed eye when the two lenses are 1.4 m apart.
  What is the focal length of the eyepiece? What
  is the focal length of the objective?

M -Fo/Fe (the 200X is really 200X) Length
(L) Fo Fe ? L - Fe Fo M (L F)/F ?
MF L F ? MF F L F L/(M1) 1.4/(200
1) ? Fe 0.00695 m and Fo L - Fe 1.4
0.00695 1.393 m
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Galilean Telescope
Objective
Eyepiece
?out
Fe
?in
M -?out /?in ? -tan(?out ) / tan(?in )
-(H/Fe)/(H/Fo) -Fo/Fe