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Refraction
Section 2
What do you think?
• How will the light bend as
it enters and leaves the
three glass blocks?
• Draw the rays as they
change direction. Make sure
your drawing includes
normal lines at each
interface.
• Would you describe the
combination of blocks as
converging or diverging with
respect to the incoming
light?
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Refraction
Section 2
What do you think?
• How will the light bend as
it enters and leaves the
three glass blocks?
• Draw the rays as they
change direction. Make sure
your drawing includes
normal lines at each
interface.
• Would you describe the
combination of blocks as
converging or diverging with
respect to the incoming
light?
© Houghton Mifflin Harcourt Publishing Company
Refraction
Section 2
Lenses
• A lens is a transparent object that
converges or diverges light by
refraction.
– A converging lens is thicker at the middle.
– A diverging lens is thinner at the middle.
• Light actually bends at each surface.
However, for thin lenses, we can show
light bending only once at the center of
the lens.
• Focal length (f) is the distance from the
focal point (F) to the center of the lens.
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Refraction
Section 2
Converging and Diverging Lenses
Click below to watch the Visual Concept.
Visual Concept
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Refraction
Section 2
Ray Diagrams for Lenses
• Complete the
ray drawing
to locate the
image using
the rules
above.
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Refraction
Section 2
Ray Tracing for a Converging Lens
Click below to watch the Visual Concept.
Visual Concept
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Refraction
Section 2
Images Created by Converging Lenses
• Configurations 1 and 2:
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Refraction
Section 2
Images Created by Converging Lenses
• Configurations 3 and 4:
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Refraction
Section 2
Images Created by Converging Lenses
• Configurations 5 and 6:
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Refraction
Section 2
Diverging Lens Diagram
• Complete the
ray diagram
for the lens
shown to the
left using the
three rules
from Table 2.
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Refraction
Section 2
Ray Tracing for a Diverging Lens
Click below to watch the Visual Concept.
Visual Concept
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Refraction
Thin-Lens Equations
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Section 2
Refraction
Section 2
Sign Conventions
• p is positive if the object is in front of the lens.
• q is positive if the image is behind the lens (real
and inverted).
• q is negative if the image is in front of the lens
(virtual and upright).
• f is positive for converging lenses and negative
for diverging lenses.
• h and h’ are positive if upright and negative if
inverted.
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Refraction
Section 2
Classroom Practice Problems
• When an object is placed 3.00 cm in front of a
converging lens, a real image is formed 6.00 cm
in back of the lens. Find the focal distance of the
lens.
– Answer: 2.00 cm
• Where would you place an object in order to
produce a virtual image 15.0 cm in front of a
converging lens with a focal length of 10.0 cm?
How about a diverging lens with the same focal
length?
– Answers: 6.00 cm, -30.0 cm
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Refraction
Section 2
• A 15cm object is placed 60cm from a convex lens with a focal
length of 15 cm.
1.
2.
3.
4.
Solve for the image distance and the magnification.
Is the object upright or inverted?
Is it real or imaginary?
Draw the Ray diagram using an appropriate scale.
1. 15cm object, 30 cm from a lens with a focal of length of 15cm.
2. 15cm object, 16 cm from a lens with a focal of length of 15cm.
3. 15cm object, 15 cm from a lens with a focal of length of 15cm.
4. 15cm object, 10 cm from a lens with a focal of length of 15cm.
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Refraction
Section 2
The Eye and Corrective Lenses
• Light is refracted at both the cornea (outer
surface) and the lens.
– When functioning properly, the converging lens can
adjust so that the image is focused on the retina.
• Muscles adjust the thickness of the lens.
• Many people are nearsighted (myopia) and can’t
see distant objects clearly.
• Older people are often farsighted (hyperopia)
and can’t see nearby objects.
– The lens becomes inflexible with age and can’t be
made thicker to focus on nearby objects.
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Refraction
Section 2
Nearsightedness
• The image forms in front
of the retina, possibly
because the retina is too
long.
• What type of lens is
needed in front of the eye
to correct the problem,
converging or diverging?
Explain your reasoning.
– Answer: a diverging lens
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Refraction
Section 2
Farsightedness
• The image forms behind
the retina, possibly
because the lens is
inflexible.
• What type of lens is
needed in front of the eye
to correct the problem,
converging or diverging?
Explain your reasoning.
– Answer: a converging lens
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Refraction
Section 2
Combinations of Lenses
• Microscopes and refracting telescopes use two lenses.
– The objective lens forms a real image that is located inside the
focal point of the eyepiece.
– The eyepiece magnifies the first image, creating a large virtual
image.
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Refraction
Section 2
Compound Light Microscope
Click below to watch the Visual Concept.
Visual Concept
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Refraction
Section 2
Refracting Telescope
Click below to watch the Visual Concept.
Visual Concept
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Refraction
Section 2
Now what do you think?
• How will the light bend as it enters and
leaves the three glass blocks?
• Draw the rays.
• How is this similar to a lens?
• Which type of lens?
• How would the rays exit the
three blocks if there were six equally
spaced rays instead of three?
• How would those same six rays exit a
converging lens?
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Refraction
TEKS
Section 3
The student is expected to:
7D investigate behaviors of waves, including
reflection, refraction, diffraction, interference,
resonance, and the Doppler effect
7E describe and predict image formation
as a consequence of reflection from a
plane mirror and refraction through a thin
convex lens
7F describe the role of wave characteristics
and behaviors in medical and industrial
applications
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Refraction
Section 3
What do you think?
• Suppose a beam of light entering a tank of water
strikes at a 60.00° angle with the normal. What
angle does it make with the normal after
entering the water? Sketch it.
• Suppose a beam of light emerging from beneath
the water surface strikes at a 60.00° angle with
the normal. What angle does it make with the
normal after entering the air? Sketch it.
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Refraction
Section 3
Total Internal Reflection
• Total internal reflection occurs
if the angle in the denser
medium is too great.
– Light can’t emerge so it is
reflected back internally.
– Occurs if the angle is greater than
the critical angle (c).
• Used in fiber optics, right angle
prisms, and diamond cutting.
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Refraction
Section 3
Critical Angle
• c occurs when the angle in the less
dense medium is 90°.
– At the critical angle, the emerging ray
travels along the surface.
– At greater angles, the rays are totally
internally reflected.
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Refraction
Total Internal Reflection
Click below to watch the Visual Concept.
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Section 3
Refraction
Section 3
Classroom Practice Problems
• Find the critical angle for light emerging from a
diamond into air. The index of refraction for
diamond is 2.419. Repeat for cubic zirconium
with n = 2.200.
– Answers: 24.42° for diamond and 27.04° for cubic
zirconium
• Which material is more likely to trap light
entering the top surface in such a way that it
reflects many times internally before emerging?
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Refraction
Section 3
Atmospheric Refraction
• Make a sketch like that above. On your drawing, show
how light will bend when it strikes the atmosphere.
– Remember that this is a very slight change in the index of
refraction, and it occurs gradually as the atmosphere becomes
denser.
– This bending allows us to see the sun before it rises and after it
sets.
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Refraction
Section 3
Mirages
• Mirages are caused by the refraction of light as it strikes
the hot air near the earth’s surface.
– This phenomena can be observed when driving on blacktop
roads on hot summer days.
• Inverted cars can be seen approaching, with the actual cars up
above them.
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Refraction
Dispersion
• Refraction or n depends on the
wavelength.
– Longer wavelengths refract less.
• Prisms disperse the light into a
spectrum.
• Chromatic aberration is a lens
problem where different colors
focus at different points.
– Can lead to imperfect images for
cameras with less expensive
lenses.
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Section 3
Refraction
Rainbows
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Section 3
Refraction
Section 3
Dispersion of Light
Click below to watch the Visual Concept.
Visual Concept
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Refraction
Section 3
Now what do you think?
• How do fiber optic cables keep the light trapped
inside the cable as it travels great distances and
bends around corners?
• What phenomena is responsible for trapping the
light?
• Why do different people see different colors for a
water drop when observing a rainbow?
• What phenomena is responsible for the rainbow?
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Refraction
Preview
Section 1 Refraction
Section 2 Thin Lenses
Section 3 Optical Phenomena
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Section 3
Refraction
TEKS
Section 3
The student is expected to:
7D investigate behaviors of waves, including
reflection, refraction, diffraction, interference,
resonance, and the Doppler effect
© Houghton Mifflin Harcourt Publishing Company
Refraction
Section 3
What do you think?
• Suppose you are reaching for swim goggles
floating below the surface of a pool or trying to
net a fish while out in a lake. Would you reach at
the point where you see the object, or above it,
or below it?
– Describe personal experiences that helped you
answer this question.
– Make a sketch showing how you think light behaves
when leaving the goggles, passing into the air, and
then entering your eyes.
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Refraction
Section 3
Refraction
• Why does the lawnmower turn when it strikes the grass?
– The right wheel slows down before the left one.
– Light waves behave in the same way.
• Refraction is the bending (change in direction) of light
when it travels from one medium into another.
– Caused by a change in speed
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Refraction
Section 3
How does it bend?
Upper
Lower
edge
edge
• Wave fronts (dashed lines) slow down when entering
glass.
– The lower edge slows before the upper edge, so the wave turns
to the right.
– Also, the wavelength is shortened.
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Refraction
Section 3
Wave Model of Refraction
Click below to watch the Visual Concept.
Visual Concept
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Refraction
Section 3
Ray Diagrams
• Light rays reflect and refract.
• If the light slows down, it bends toward the normal line (glass < air).
– Angles are measured with the normal line.
• Light rays are reversible.
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Refraction
Section 3
Law of Refraction
• c = 3  108 m/s
• v is always less than c, so n >1 for all media.
– nair = 1.000293
• n is dimensionless.
• n is a measure of the optical density of a material.
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Refraction
Indices of Refraction
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Section 3
Refraction
Section 3
Snell’s Law
• Angles must be
measured with the
normal.
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Refraction
Section 3
Classroom Practice Problems
• Find the angle of refraction of a light ray (589 nm)
entering diamond from water at an angle of 30.00° with
the normal.
– Answer: 15.99°
• A light ray (589 nm) traveling through air strikes an
unknown substance at 60.00° and forms an angle of
41.42° with the normal inside. What material is it?
– Answer: n = 1.309, so the material is ice
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Refraction
Section 3
Refraction
• Where does the cat see the fish?
• Where does the fish see the cat?
• Objects appear to be in line with the observed rays.
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Refraction
Section 3
Now what do you think?
• Suppose you are reaching for swim goggles
floating below the surface of a pool. Would you
reach at the point where you see the object, or
above it, or below it?
– Make a sketch showing how light behaves.
• If you are under water looking at a person
standing on the side of the pool, where is the
image?
– Make a sketch showing how light behaves.
© Houghton Mifflin Harcourt Publishing Company
Refraction
TEKS
Section 3
The student is expected to:
7D investigate behaviors of waves, including
reflection, refraction, diffraction, interference,
resonance, and the Doppler effect
7E describe and predict image formation
as a consequence of reflection from a
plane mirror and refraction through a thin
convex lens
7F describe the role of wave characteristics
and behaviors in medical and industrial
applications
© Houghton Mifflin Harcourt Publishing Company