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Transcript
Lesson 24
Reflection and Refraction
Eleanor Roosevelt High School
Chin-Sung Lin
Reflection & Refraction
• Reflection – When waves strike the surface of a new
medium, waves remain in one medium instead of
entering a new medium
Reflection & Refraction
• Refraction – When waves strike the surface of a new
medium at an angle, their direction changes as they
entering a new medium
Reflection & Refraction
• Usually, waves are partly reflected and partly
refracted when they fall on a new transparent
medium
Reflection
Reflection
• Reflection – Reflection is when a wave reaches a
boundary between two medium and some or all of
the wave bounces back into the first medium
– When the boundary is very rigid, all the wave energy is
totally reflected back
Reflection
• Reflection – Reflection is when a wave reaches a
boundary between two medium and some or all of
the wave bounces back into the first medium
– If the boundary is a less rigid medium, then the energy
would be only partially reflected
Reflection
• To light waves, metal surfaces are rigid when shone
upon them. Light energy returned in a reflected wave
• When light shines perpendicularly on the surface of
still water, about 2% of its energy is reflected and the
rest is transmitted
• When light strikes glass perpendicularly, about 4% of
its energy is reflected. Except for slight losses, the
rest is transmitted
Law of Reflection
• In one dimension, reflected waves simply travel back
in one direction from which they originated
• In two dimension, incident rays and reflected rays
make equal angles with a perpendicular line to the
surface, called normal
Law of Reflection
• The angle of incidence, which is equal to the angle of
reflection. Such relationship is called the law of
reflection
θi = θr
• The incident ray, normal and the reflected ray all of
them lay on the same plane. This law applies to both
partially reflected and totally reflected waves
Mirrors
• Rays of light are reflected
from the mirror surface in all
directions. The number of
rays is infinite and every one
obeys the law of reflection
Mirrors
• Virtual images are created through reflection that
can be seen by an observer but cannot be projected
on a screen because light doesn’t actually start there
Diffuse Reflection
• Diffuse Reflection – Light reflects in many directions
when it is incident upon rough surface
• Even though the reflection of each of these single rays
obeys the law of reflection, these many different angles
that incident light rays encounter cause reflection in
many directions
Diffuse Reflection
• If differences in elevations of a surface are less than
one-eighth the wavelength of the light that falls on it,
that surface is considered polished
• Whether a surface is a diffuse reflector depends on
the length of the waves it reflects
Diffuse Reflection
• Normal pages are diffuse, as they have irregular
shape when seen in microscope. This tells us that
light is reflected on them, it can reflect back in as
many possible directions as it can, when bumped
onto paper. However, in mirror, it follows a straightline path. Paper’s various shapes allow us to read
letters from it
Reflection of Sound
• Echo is an example of reflected sound. Fraction of sound
energy reflected from a surface is more when the surface
is rigid and smooth, but less when the surface is soft and
irregular. Non-reflected sound energy is absorbed or
transmitted
Reflection of Sound
• Reverberation occurs with the persistence of sound,
as in the case for echo, due to multiple reflections
Reflection of Sound
• In big hall rooms, like auditorium or concert hall,
sound level is lower, since the reflective surfaces are
more absorbent
Reflection of Sound
• A balance of reverberation and absorption must be
held into account while doing acoustic design
Reflection Example
• Light is incident on a flat surface, making an angle of 10o
with that surface. (a) What are the angle of incidence
and the angle of reflection? (b) Sketch the path of the
reflected beam on the diagram. (c) If the mirror rotate
counter-clockwise 20o without changing the light source,
what are the new angle of incidence and the new angle
of reflection?
Normal
Refraction
Refraction
• Wavefronts are drawn when drawing a diagram of
wave. These lines represent positions of different
crests
Refraction
• At each point along a wavefront, wave is moving
perpendicular to the wavefront. Rays can be used to
represent the direction of motion of the wave,
perpendicular to the wavefronts
Refraction
• Refraction is the bending of waves when only one
part of each wave is made to travel faster or slower
than another part
Refraction
• Waves travel faster in deep water than in shallow water
Refraction of Sound
• Sound waves are refracted when parts of a wave
front travel at different speeds. This usually takes
place when sound is traveling through air of uneven
temperature
Refraction of Sound
Refraction of Sound
Refraction of Light
• When light rays enter a medium in which their speed
decreases, like when passing from air to water, rays
bend toward the normal
Refraction of Light
• When light rays enter a medium in which their speed
increases, like when passing from water into air, rays
bend away from the normal
Refraction of Light
Refraction of Light
• Light paths are reversible for both reflection
and refraction
Law of refraction (Snell’s Law)
Refraction Indices
Refraction of Light Example
• What’s the speed of light in diamond?
Refraction of Light Example
• What’s the speed of light in diamond?
1.24 x 108 m/s
Refraction of Light Example
• A ray of light travels from air to corn oil. If the ray of
light in air makes an angle of 30.00 degrees to the
normal, (a) what is the angle of refraction in corn
oil?(b) What’s the speed of light in corn oil?
Refraction of Light Example
• A ray of light travels from air to corn oil. If the ray of
light in air makes an angle of 30.00 degrees to the
normal, (a) what is the angle of refraction in corn
oil?(b) What’s the speed of light in corn oil?
(a) 19.89 degrees
(b) 2.04 x 108 m/s
Refraction of Light Example
• A ray of light travels from air to water. If the ray of
light in water makes an angle of 10.00 degrees to the
normal, what is the angle of incidence in air?
Refraction of Light Example
• A ray of light travels from air to water. If the ray of
light in water makes an angle of 10.00 degrees to the
normal, what is the angle of incidence in air?
13.35 degrees
Refraction of Light Example
• The speed of light in an unknown medium is
measured to be 2.206 x 108 m/s. (a) What is the
index of refraction of the medium? (b) Does it match
any of the materials listed in your Reference Table?
Refraction of Light Example
• The speed of light in an unknown medium is
measured to be 2.206 x 108 m/s. (a) What is the
index of refraction of the medium? (b) Does it match
any of the materials listed in your Reference Table?
(a) 1.36
(b) Ethyl Alcohol
Atmospheric Refraction
• Even though the speed of light in air is only 0.03%
less than its’ speed in vacuum, but in some cases,
atmospheric refraction is quite noticeable
Atmospheric Refraction
• Mirage is such an example where a floating image
appears in the distance and is due to the refraction
of light in Earth’s atmosphere
Atmospheric Refraction
• Refraction of light can be very much like refraction of
sound. Their greater speed near the ground causes
light ray to bend upward
Atmospheric Refraction
• When sun or moon is near the horizon, rays from the
lower edge are bent more than the rays from the
upper edge. This produces a shortening of the
vertical diameter and makes the sun or moon look
elliptical instead of round
Dispersion in a Prism
• Light of frequencies closer to the natural frequency
of electron oscillators in a medium travels more
slowly in the medium
• Visible light of higher frequencies travels slower than
light of lower frequencies, as the natural frequency
of most transparent materials in the UV part of the
spectrum
Dispersion in a Prism
• Since different frequencies of light travel at different
speeds in transparent materials, they will refract
differently and bend at different angles
Dispersion in a Prism
• When light is bent twice at
nonparallel boundaries,
dispersion occurs when the
separation of light into colors
are arranged according to
their frequency, by
interaction with a prism
Rainbow
Rainbow
• Conditions for seeing a rainbow are that the sun will be
shining in one part of the sky and that water in the
opposite part of the sky
• If this can be seen from very high altitudes, the bow can
form a complete circle. If the ground was not flat, all
rainbows could be round
Primary and Secondary Rainbow
Rainbow
• Conditions for seeing a rainbow are that the sun will
be shining in one part of the sky and that water
droplets in a cloud or in falling rain be in the opposite
part of the sky
• If this can be seen from very high altitudes, the bow
can form a complete circle. If the ground was not
flat, all rainbows could be round
Rainbow
• Rays that reach the opposite part of the drop are to
be partly refracted out into the air and partly
reflected back into the water
• Parts of the rays that arrive at the lower surface of
the drop are refracted into the air
• This refraction is similar to prism, where refraction at
the surface increases dispersion already produced at
the other surface
• This twice-refracted, once-reflected light is
concentrated in a narrow range of angles. Each drop
disperses a full spectrum of colors
Rainbow
• Often a larger, secondary bow with colors reversed
can be seen arching at a greater angle around the
primary bow
• The secondary bow is formed by similar
circumstances and is a result of double reflection
within the raindrops. Since some light is refracted
out back during the extra reflection, the secondary
bow is much dimmer
Total Internal Reflection
• At critical angles, a light ray is totally reflected within a
medium
• Light beams can also experience total internal reflection
when light strikes the boundary between two media at
an angle that is greater than the critical angle
Total Internal Reflection
Total Internal Reflection - Prism
• Silver/aluminum mirrors reflect only 90-95% of incident light
• Prisms are more efficient. This is 100% reflection
Total Internal Reflection - Diamonds
• When diamonds are cut as gemstones, light that enters
at one facet is usually totally internally reflected several
times, without any loss in intensity, before exiting from
another facet in another direction
Total Internal Reflection - Diamonds
• Diamond’s total internal reflection
Total Internal Reflection – Optical Fibers
• This total internal reflection underlies the usefulness
of optical fibers, which are usually made up of glass
or plastic, which can transmit light down its length by
means of total internal reflection
The End