CHAPTER 14 REFRACTION

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CHAPTER 14REFRACTION

• Ms. Hanan

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14-2 Thin Lenses

• Objectives
• Use ray diagram to find the position of an image
  produced by a converging or diverging lens, and
  identify the image as real or virtual.
• Solve problems using the thin-lens equation.
• Calculate the magnification of lenses.
• Describe the positioning of lenses in compound
  microscopes and refracting telescopes.

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14-2 Thin Lenses

• Vocabulary
• Converging lens
• Convex lens
• Diverging lens
• Concave lens
• Ray diagram
• Focal point

• Focal length
• Centre of the lens
• Real image
• Virtual image
• Magnification

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14-2 Thin Lenses
The first telescope, designed and built by
Galileo, used lenses to focus light from faraway
objects, into Galileos eye. His telescope
consisted of a concave lens and a convex lens.
light from far away object
convex lens
concave lens
Light rays are always refracted (bent) towards
the thickest part of the lens.
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Concave (Diverging) Lenses
Concave lenses are thin in the middle and make
light rays diverge (spread out).
If the rays of light are traced back (dotted
sight lines), they all intersect at the focal
point (F) behind the lens.
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Concave (Diverging) Lenses
F
Light rays that come in parallel to the optical
axis diverge from the focal point.
The light rays behave the same way if we ignore
the thickness of the lens.
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Concave (Diverging) Lenses
F
Light rays that come in parallel to the optical
axis still diverge from the focal point.
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Concave (diverging) Lens(example)
F
The first ray comes in parallel to the optical
axis and refracts from the focal point.
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Concave (diverging) Lens(example)
F
The first ray comes in parallel to the optical
axis and refracts from the focal point. The
second ray goes straight through the center of
the lens.
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Concave (Diverging) Lens(example)
F
The first ray comes in parallel to the optical
axis and refracts from the focal point. The
second ray goes straight through the center of
the lens. The light rays dont converge, but the
sight lines do.
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Concave (Diverging) Lens(example)
F
The first ray comes in parallel to the optical
axis and refracts from the focal point. The
second ray goes straight through the center of
the lens. The light rays dont converge, but the
sight lines do. A virtual image forms where the
sight lines converge.
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Your Turn(Concave (Diverging) Lens)
F
concave lens

• Note lenses are thin enough that you just draw a
  line to represent the lens.
• Locate the image of the arrow.

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Your Turn(Concave (Diverging) Lens)
F
image
concave lens

• Note lenses are thin enough that you just draw a
  line to represent the lens.
• Locate the image of the arrow.

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Convex (Converging) Lenses
Convex lenses are thicker in the middle and focus
light rays to a focal point in front of the lens.
The focal length of the lens is the distance
between the center of the lens and the point
where the light rays are focused.
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Convex (converging) Lenses
F
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Convex (converging) Lenses
F
Light rays that come in parallel to the optical
axis converge at the focal point.
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Convex (converging) Lens(example)
F
The first ray comes in parallel to the optical
axis and refracts through the focal point.
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Convex (converging) Lens(example)
F
The first ray comes in parallel to the optical
axis and refracts through the focal point. The
second ray goes straight through the center of
the lens.
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Convex (converging) Lens(example)
F
The first ray comes in parallel to the optical
axis and refracts through the focal point. The
second ray goes straight through the center of
the lens. The light rays dont converge, but the
sight lines do.
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Convex (converging) Lens(example)
F
The first ray comes in parallel to the optical
axis and refracts through the focal point. The
second ray goes straight through the center of
the lens. The light rays dont converge, but the
sight lines do. A virtual image forms where the
sight lines converge.
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Your Turn(Convex (converging) Lens)
Principal axis
F
convex lens

• Note lenses are thin enough that you just draw a
  line to represent the lens.
• Locate the image of the arrow.

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Your Turn(Convex (converging) Lens)
Principal axis
image
F
convex lens

• Note lenses are thin enough that you just draw a
  line to represent the lens.
• Locate the image of the arrow.

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Rules for Drawing Reference Rays
Ray From object to lens From converging lens to image
Parallel ray (P ray) Parallel to principal axis Passes through focal point, F
Central ray (M ray) To the center of the lens From the center of the lens
Focal ray (F ray) Passes through focal point, F Parallel to principal axis
Ray From object to lens From diverging lens to image
Parallel ray (P ray) Parallel to principal axis Directed away from focal point, F
Central ray (M ray) To the center of the lens From the center of the lens
Focal ray (F ray) Proceeding toward back focal point, F Parallel to principal axis
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Ray Tracing for Lenses
These diagrams show the principal rays for both
types of lenses
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Lens Mirror Equation
ƒ focal length p object distance q image
distance
f is negative for diverging mirrors and lenses di
is negative when the image is behind the lens or
mirror
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Magnification Equation
M magnification h image height h object
height
If height is negative the image is upside
down if the magnification is negative the image
is inverted (upside down)
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The Thin-Lens Equation
Sign conventions for thin lenses
M
M
q
q
p
p
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Assignments

• Class-work
• Practice B, page 501, odd questions.
• Homework
• Practice B, page 501, even questions.
• Homework due next class

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Eyeglasses and Contact Lenses

• Vocabulary
• Myopia
• Short Sightedness
• Hyperopia
• Far sightedness
• Objective Lense
• Eye Piece

• Compound Microscope
• Refracting Telescope

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Eyeglasses and Contact Lenses
Leads to the occipital cortex at the posterior
(back) of the brain
Anatomy of the Human Eye
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Normal Vision
The process in which the lens changes its focal
length to focus on objects at different distances
is called accommodation
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Myopia, Hyperopia
If the incoming light from a far away object
focuses before it gets to the back of the eye,
that eyes refractive error is called myopia
(nearsightedness).  If incoming light from
something far away has not focused by the time it
reaches the back of the eye, that eyes
refractive error is hyperopia (farsightedness).
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Myopia - Nearsightedness
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Hyperopia - Farsightedness
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Combination of Thin Lenses
In lens combinations, the image formed by the
first lens becomes the object for the second lens
(this is where object distances may be negative).
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The Compound Microscope

• In the basic compound microscope, the object to
  be magnified is placed under the lower lens
  (objective lens) with a focal length of less than
  1 cm, and the magnified image is viewed through
  the upper lens (eyepiece lens) with a focal
  length of few centimeters.
• The magnification of the image can be calculated
  by multiplying the magnifying power of the
  objective lens times the magnifying power of the
  eyepiece lens.
• The microscope is composed of a mechanical system
  which supports the microscope, and an optical
  system which illuminates the object under
  investigation and passes light through a series
  of lens to form an image of the specimen.

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The Compound Microscope
The principle of the compound microscope. The
passage of light through two lenses forms the
virtual image of the object seen by the eye.
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The Refracting Telescope
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Assignments

• Homework
• Section review, page 505, questions 1, 2, 3, 4,
  5, and 6.
• Homework due next class