PHY138 Waves, Lecture 9 Todays overview

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PHY138 Waves, Lecture 9Todays overview

• Lenses
• The Thin Lens Equation
• Lenses Used in Combination

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Its the last week of the fall semester!

• The last Waves Problem Set is due at 500 PM
  today in your TAs mailbox.
• There is no Web-CT quiz until Jan.3.
• There is a practice www.masteringphysics.com
  assignment now available (not due) on Chapter 26
  material.

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Announcements

• Test 2 is at 900 AM sharp on Friday Dec. 10. You
  will need
• A non-programmable calculator
• An aid sheet with the main equations used this
  quarter, and other info you think might be
  useful. It may be double-sided, up to 8½11 in
  size. It must be hand-written no shrink
  photocopies or condensed type!
• Room Assignments are now listed on the course
  web-site.

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Quiz 1
air
water
A fish swims directly below the surface of the
water. An observer sees the fish at 1. a
greater depth than it really is. 2. its true
depth. 3. a smaller depth than it really is.
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Light going through a prism bends toward the base
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Building a Converging Lens out of Prisms
Snells Law of Refraction is obeyed at every
interface.
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Converging Lens
Focal length, f
Focal Point
NOTE Focal length is defined for initially
parallel rays.
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Diverging Lens
Negative Focal length, -f
Virtual Focal Point
Rays appear to emerge from Virtual Focal Point
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Off-axis rays through a converging lens
Focal length, f
NOTE The ray which passes through the centre of
the lens is not bent.
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Diverging rays through a Converging Lens
Focal length, f
This follows from the principle of reversibility.
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Quiz 2
f

• What will happen to the rays emerging to the
  right of the lens if the face is moved a little
  closer to the lens?
• They will remain parallel.
• They will diverge (spread out).
• They will converge (toward a focus).

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Quiz 3
f

• What will happen to the rays emerging to the
  right of the lens if the face is moved a little
  further away from the lens?
• They will remain parallel.
• They will diverge (spread out).
• They will converge (toward a focus).

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Diverging rays through a Converging Lens
Focal length, f
q
p
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Thin Lens Equation sign conventions
image
object
f
q
p
p is positive for objects to the left of lens,
negative for objects to the right of lens
(virtual objects).
q is positive for images to the right of lens,
negative for images to the left of lens (virtual
images).
f is positive for converging lenses, negative for
diverging lenses.