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Chapter 29 – Reflection &
Refraction
Chapter preview
Sections
1. Reflection
2. The Law of Reflection
3. Mirrors
4. Diffuse Reflection
5. Reflection of Sound
6. Refraction
7. Refraction of Sound
8. Refraction of Light
9. Atmospheric Refraction
10.Dispersion in a Prism
11.The Rainbow
12.Total Internal Reflection
Reflection & Refraction
Section 29.1—Reflection
Reflection
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Reflection – some or all of a wave
bounces back into the first medium when
hitting a boundary of a second medium
When all the wave energy is reflected
back instead of being transmitted, it is
total reflection
If some energy is transmitted and some
is reflected, the wave is partially reflected
Reflection
Reflection & Refraction
Section 29.2—The Law of
Reflection
The Law of Reflection
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The direction of incidence and reflection is best
described by straight-line rays
Incident rays and reflected rays make equal angles
with a line perpendicular to the surface, called the
normal
Angle of Incidence – angle made by the incident
ray and the normal
Angle of Reflection – angle made by the reflected
ray and the normal
Law of Reflection – the angle of incidence and
the angle of reflection are equal
The Law of Reflection
Reflection & Refraction
Section 29.3—Mirrors
Mirrors
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Virtual Image – the
point located behind a
mirror where an object
appears to originate
Your eye cannot tell the
difference between an
object and its virtual
image
The image is as far
behind a mirror as the
object is in front of the
mirror
Mirrors
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For reflections in a plane
mirror, object size equals
image size and object
distance equals image
distance.
The law of reflection holds for
curved mirrors.
a. The image formed by a
Convex mirror is smaller than
the object.
b. When an object is close to a
concave mirror, the image can
be larger than the object.
Reflection & Refraction
Section 29.4—Diffuse
Reflection
Diffuse Reflection
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Diffuse Reflection – light incident on a rough surface is
reflected in many directions
A surface’s roughness is dependent upon the wavelength of the
wave incident upon that surface; the longer the wavelength, the
smoother the surface will appear
To a piece of paper, light is reflecting diffusely
The Law of Reflection is Always Observed
(regardless of the orientation of the surface)
Reflection & Refraction
Section 29.5—Reflection of
Sound
Reflection of Sound
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An echo is reflected sound
Sound reflects from all surfaces of a room
Acoustics is the study of the way sound
reflects off of objects in a room
Reverberations – Multiple reflections of
sound within a room
The walls of concert halls are designed to
make the reflection of sound diffuse
Reflection of Sound
Reflection & Refraction
Section 29.6—Refraction
Refraction
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Refraction – the change in direction of a wave
as it crosses the boundary between two media in
which the wave travels at different speeds
Wave Fronts – lines that represent the position
of different crests
At each point along a wave front, the wave is
moving perpendicular to the wave front
The direction of motion is best represented by a
ray
Refraction
Incident Ray
Less Rigid
Medium
_________________
Refracted Ray
More Rigid
Medium
Refracted ray
bends toward
the normal
Refraction
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When one medium ends and another begins, that is called a
boundary.
When a wave encounters a boundary that is more dense, part
of it is reflected and part of it is transmitted.
The frequency of the wave is not altered when crossing a
barrier, but the speed and wavelength are.
The change in speed and wavelength can cause the wave to
bend, if it hits the boundary at an angle other than 90°.
Reflection & Refraction
Section 29.7—Refraction of
Sound
Refraction of Sound
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Sound waves are refracted when parts of a
wave front travel at different speeds
This happens in uneven winds or
temperatures
Sound waves tend to bend away from warm
ground, since it travels faster in warmer air
On a cold night, the speed of sound is
slower near the ground than above, so we
can hear over larger distances
Refraction of Sound
Reflection & Refraction
Section 29.8—Refraction of
Light
Refraction of Light
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A pond or swimming pool may appear shallower than they
actually are, a pencil in a glass of water will appear bent
All of these effects are caused by changes in the speed of light
as it passes from one medium to another, or through varying
temperatures and densities of the same medium – which
changes the directions of light rays
Index of Refraction (n) = (speed of light in vacuum)/(speed of
light in material)
Snell’s Law: n sin θ = n´ sin θ´ (where n and n´ are the indices
of refraction of the media on either side of the boundary, and θ
and θ´ are the respective angles of incidence and refraction)
Index of Refraction of a few substances
Vacuum
Air
Water
Ethanol
1.00
1.0003
1.33
1.36
Crown glass
Quartz
Diamond
1.52
1.52
2.42
Refraction of Light
The submerged object's apparent depth
equals its true depth divided by the liquid's
index of refraction: d' = d(n2/n1). Note: n2 is
the index of refraction of the medium
above the surface and n1 is the index of
refraction of the medium below the
surface.
Incident Ray
Refracted Ray
Refraction of Light
Figure 29.19
There are many effects of refraction
a. The apparent depth of the glass block is less than the real depth.
b. The fish appears to be nearer than it actually is.
c. The full glass mug appears to hold more root beer than it
actually does.
Reflection & Refraction
Section 29.9—Atmospheric
Refraction
Atmospheric Refraction
On hot days there may be a layer of very hot air in contact with the
ground, the light will travel faster through this air and will bend,
creating a mirage
Atmospheric Refraction
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When you watch the sun set, you can still see the sun for several
minutes after it has sunk below the horizon, because light is
refracted by Earth’s atmosphere
Reflection & Refraction
Section 29.10—Dispersion
in a Prism
Dispersion in a Prism
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Light of frequencies closer to the natural frequency of
the electron oscillators in a medium travels more slowly
in the medium
Since different frequencies of light travel at different
speeds in transparent materials, they will refract
differently and bend at different angles
When light is bent twice at nonparallel boundaries, as
in a prism, the seperation of the different colors is
apparent
Dispersion – the separation of light into colors
arranged according to their frequency
Dispersion in a Prism
Reflection & Refraction
•Section
29.11—The Rainbow
The Rainbow
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The rainbow takes the concept of dispersion and
multiples it through the atmosphere
The sun shines on water droplets in a cloud or when
it is raining
The light is dispersed by the raindrop into its spectral
colors
The Rainbow
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Dispersion by a Raindrop:
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Each droplet acts like a prism
•Higher drops – red is bent to the eye
•Lower drops – violet is bent to the eye
The Rainbow
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Rainbows will always appear at an angle between 40ᵒ and 42ᵒ . If you are
lucky enough to see two rainbows at the same time, the second (much
dimmer) one occurs above the main one, and results from sunlight creating
a double reflection in the water droplets. The colors are also upside down.
Reflection & Refraction
•Section
29.12—Total Internal
Reflection.
Total Internal Reflection
Critical Angle:
Figure 29.32
You can observe total internal reflection in your bathtub.
a-d) Light emitted in the water at angles below the critical angle is partly
refracted and partly reflected at the surface.
e) At the critical angle, the emerging beam shims the surface.
f) Past the critical angle, there is total internal reflection.
Total Internal Reflection
Total Internal Reflection in Diamonds
The critical angle for a diamond is 24.6ᵒ, smaller
than in other common substances. This small
critical angle means that light inside is more likely
to totally internally reflect.
Total Internal Reflection
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Critical Angle – the minimum angle of
incidence for which a light ray is totally
reflected within a medium
Total Internal Reflection – the 100%
reflection of light that strikes the boundary
between two media at an angle greater than
the critical angle
Optical fibers utilize the concept of total
internal reflection to feed light from one
location to another, these cables are very
useful for communications
Total Internal Reflection
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Optical Fibers:
• At each contact w/ the glass
air interface, if the light hits
at greater than the critical
angle, it undergoes total
internal reflection and stays
in the fiber.