Transcript Reflection and Refraction

Reflection and
Refraction
Chapter 29
Reflection
• 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
The Law of Reflection
• 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
Mirrors
• 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
Diffuse Reflection
• 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
Diffuse Reflection
Reflection of Sound
• 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
Refraction
• 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
Refraction of Sound
• 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
Refraction of Light
• 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)
Refraction of Light
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
• 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
Atmospheric Refraction
Dispersion in a Prism
• 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
The Rainbow
• 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
Total Internal Reflection
• 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
Assignment (Due ¾)
• Read Chapter 29 (pg. 442-459)
• Do Chapter 29 Assessment #28-41