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Lecture Outline
Chapter 22
College Physics, 7th Edition
Wilson / Buffa / Lou
© 2010 Pearson Education, Inc.
22.1 Wave Fronts and Rays
Along a wave front, all the
waves have the same
phase.
A wave front is the line or
surface defined by
adjacent portions that are
“in phase”
An easy way to think of
this, is that each “wave
front” separates
wavelengths.
© 2010 Pearson Education, Inc.
22.1 Wave Fronts and Rays
Waves propagate outward from a source.
Waves move at a certain speed and that speed
is greatest in a vacuum. C =
We use rays because it shows the direction of
energy flow.
© 2010 Pearson Education, Inc.
22.1 Wave Fronts and Rays
Why do we see objects?
Because rays from
objects, or appearing to
be from them, enter our
eyes and converge
where?
Our brains cannot tell
whether rays actually
come from objects. This
is one way magicians
can fool our eyes!
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22.2 Reflection
The basic rule of reflection is that the angle of
incidence equals the angle of reflection. Note
that the angles are measured from the normal to
the surface.
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22.2 Reflection
• Let’s look at incident pulse, reflected pulse,
angles of incidence and reflection, and the
normal.
• Finally, Law of Reflection is…
22.2 Reflection
Specular reflection
is reflection from a
smooth surface.
In this case,
incident rays are all
parallel to one
another, and
reflected rays are
parallel to one
another.
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22.2 Reflection
Diffuse reflection is reflection from a rough
surface. The reflected rays are not parallel.
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22.2 Reflection
• Driving on a dry
night vs. driving on
a dark, rainy, night.
• Specular Reflection
is a major cause of
accidents on rainy
nights.
22.2 Reflection
• Two mirrors are
perpendicular to to
each other, with a
light ray incident on
one the mirrors
shown to the right.
Sketch the path of
the reflected ray.
Find where the
reflected ray would
go.
• Let’s Draw This:
22.2 Reflection
Reflected rays are drawn using
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22.3 Refraction
Refraction occurs because the speed of light is
different in different media. As light travels
across a boundary between media, it bends.
Thus, there is a change in direction of a wave
when the medium changes.
The relationship between the angle of
incidence and the angle of refraction is:
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22.3 Refraction
Again, the angles are measured from the normal
to the surface. Direction of incident is different
than the direction of the transmitted.
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22.3 Refraction
The index of refraction is the ratio of the speed
of light in vacuum to the speed of light in the
medium.
The index of refraction is always equal to or
greater than 1.
The index of refraction is also a measure of
optical density.
Example…
22.3 Refraction
The frequency of the wave stays the same, but
the wavelength of the light is shorter where
the index of refraction is greater.
Let’s rearrange this please!
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22.3 Refraction
• Light from a helium – neon laser with a
wavelength of 632.8 nm travels from air
into water. What are the speed and
wavelength of the laser light in water?
22.3 Refraction
Snell’s law can be rewritten using the index of
refraction:
A ray bends toward the normal if the second
medium has a larger index of refraction, and
away from it if it is smaller.
© 2010 Pearson Education, Inc.
22.3 Refraction
• Light in water is incident on a piece of crown
glass at an angle of 37 degrees.
– A) Will the transmitted ray be
• Bent toward the normal?
• Bent away from the normal?
• Not be bent at all?
– B) What is the angle of refraction?
22.3 Refraction
• A simplified representation of the crystalline
lens in a human eye shows it to have a
cortex (outer layer) on n = 1.386 and a
nucleus (core) of n = 1.406.
– If a beam of monochromatic light of wavelength
590 nm is directed from air through the front of
the eye and into the crystalline lens, calculate the
frequency, speed, and wavelength of light in the
cortex and in the nucleus.
22.3 Refraction
• A beam of light
traveling in air
strikes the glass
top of a coffee
table at an angle
of incidence of 45
degrees. The glass
has an index of
1.5. What is the
angle of refraction
for the light
transmitted into
the glass?
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22.3 Refraction
Mirages are formed by light refracting
through air of different temperatures (and
therefore densities).
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22.3 Refraction
Refraction can distort underwater views, make
straight objects appear bent, and make
submerged objects appear shallower than they
really are.
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22.3 Refraction
Refraction through the atmosphere can make
the Sun appear flattened at sunset, and
increase the length of the day.
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22.4 Total Internal Reflection and
Fiber Optics
There’s a limit of how
far the refracted ray
can bend.
If the incident angle is
large enough (critical
angle), the angle of
refraction is 90
degrees, at this angle
of larger, light is no
longer refracted, but it
is internally reflected.
Total Internal Reflection
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22.4 Total Internal Reflection and
Fiber Optics
The angle for which this occurs is called
the critical angle:
Light impinging on the boundary at this or
a larger angle will be reflected; this is
called total internal reflection.
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22.3 Refraction
• What is the critical
angle for light
traveling in water
and incident on a
water-air boundary?
• Imagine a beautiful
beachy scene…
22.4 Total Internal Reflection and
Fiber Optics
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22.4 Total Internal Reflection and
Fiber Optics
Fiber optics use total internal
reflection to guide light along
the fiber. The reflection means
that there are no losses out
the sides of the fiber.
Low energy losses, not
affected by electromagnetic
disturbances, can change light
energy into electrical energy –
used in phone and computer
lines.
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22.5 Dispersion
Dispersion occurs because the index of
refraction depends slightly on wavelength. White
light is composed of all colors.
Dispersion varies with different media.
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22.5 Dispersion
The index of refraction of a
particular transparent
material is 1.4503 (640 nm)
for the red end of the
visible spectrum and
1.4698 (434 nm) for the
blue end. White light is
incident at an angle of 45
degrees.
Inside the prism, the angle of
refraction of the red light is:
larger than, smaller than, or the
same as the angle of refraction
of the blue light?
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What is the angular separation of
the visible spectrum inside the