Physics Study Notes
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Lesson 21 Reflection and Refraction
Introduction
Usually when waves are reflected or refracted they fall on a transparent medium. When light shines on water,
some of it is reflected and some are refracted.
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Reflection
a.
Reflection is when a wave reaches a boundary between two media and some or that entire wave bounces
back into the first medium. Let’s consider an example to understand this well. When someone fastens a
spring to a very rigid wall and sends a pulse along the spring’s length, all the wave energy is reflected back
along the spring rather than being transmitted into the wall. Waves that travel along the spring are almost
completely reflected at the wall. If that wall is replaced with a less rigid wall, then the energy would still be
reflected, but partially.
b.
To light waves, metal surfaces are rigid, when shone upon them. Light energy doesn’t propagate into the
metal, but instead returned in a reflected wave. Reflected wave from the metal has almost the full intensity
of the incoming wave, apart from small energy losses due to the friction of the vibrating electrons in the
surface. This is why silver and aluminum shines as they reflect almost all frequencies of visible light.
c.
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 at glass perpendicularly, about 4% of its energy is reflected. Except
for slight losses, the rest is transmitted.
Law of Reflection
a.
In 1-D, reflected waves simply travel back in one direction from which they originated. Direction of
incident and reflected waves is best known by straight-line rays.
b.
Incident rays and reflected rays make
equal angles with a perpendicular line
to the surface, called normal. The angle
made by the incident ray and the
normal is called angle of incidence,
which is equal to the angle made by the
reflected ray and the normal, called
angle of reflection. Such relationship is
called law of reflection.
c.
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
a.
Rays of light are reflected from the mirror surface in all directions. Number of rays is infinite and every one
obeys the law of reflection. Virtual images are created through reflection or refraction that can be seen by
an observer but cannot be projected on a screen because light from the object doesn’t actually come to a
focus.
b.
Humans cannot ordinarily tell the difference between an object and its virtual image. Something seen in the
mirror is exactly the same far distant from the mirror itself. That’s why an identical object would appear on
the other side of the mirror. This is the case for flat mirrors.
Diffuse Reflection
a.
Light reflects in many directions when it is incident upon rough surface. This situation best describes
diffuse reflection. 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.
b.
If differences in elevations of a surface are small, less than one-eighth the wavelength of the light that falls
on it, that surface is considered polished. Thus, a surface can be polished for long wavelengths, but not for
short wavelengths. Whether a surface is a diffuse reflector depends on the length of the waves it reflects.
c.
Normal pages are diffuse, as they have irregular shape when seen in microscope. This tells us that light is
reflected at them, it can go in as many possible directions as it can, when bumped onto paper. However, in
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Physics Study Notes
Lesson 21 Reflection and Refraction
mirror, it follows a straight line
path. Paper’s various shapes
allow us to read letters from it
from almost any angles. This is
why; most of the objects we see
around us are by diffuse
reflection.
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Sound Reflection
a.
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.
b.
Since sound reflected in all directions, designers of walls, ceilings, floor, and furniture need to understand
the reflective properties of surfaces, by studying acoustics.
c.
Reverberation occurs with the persistence of sound, as in the case for echo, due to multiple reflections. In
big hall rooms, like auditorium or concert hall, sound level is lower, since the reflective surfaces are more
absorbent. That’s why, in making of these designs, a balance of reverberations and absorption must be held
into account.
Refraction
a.
Refraction is when waves bend but only one part of each wave is made to travel faster or slower than
another part. Waves travel faster in deep water than in shallow water.
b.
Wave fronts are drawn when drawing a diagram of wave. These lines represent positions of different crests.
At each point along a wave front, wave is moving perpendicular to the wave front. Rays can be used to
represent the direction of motion of the wave, perpendicular to the wave fronts.
Sound Refraction
a.
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. That’s why, on warm days, air near the ground
is much warmer than air above. Since sound travels faster in warmer air, speed of sound near ground is
increased. Refraction is gradual, not abrupt.
b.
But during colder days, when layer of air near the ground is colder than air above, speed of sound near the
ground is decreased. Higher speed of wave fronts above cause a bending of sound toward the ground.
When this happens, sound can be heard over long-distances.
Light Refraction
a.
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Wave fronts would be curved if the source of light were close, just like wave fronts of water waves near a
stone thrown into the water are curved. When light rays enter a medium in which their speed decreases, like
when passing from air to water, rays bend toward the normal. But when light rays enter a medium in which
their speed increases, like when passing from water into air, rays bend away from the normal. Light paths
are reversible for both reflection and refraction.
Atmospheric Refraction
a.
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. 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.
b.
Refraction of light can be very much like refraction of sound. Undeflected wave fronts would travel at one
speed and in a different direction. Their greater speed near the ground causes light ray to bend upward.
c.
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.
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Physics Study Notes
Lesson 21 Reflection and Refraction
10 Dispersion in a Prism
a.
Light of frequencies closer to the natural frequency of electron oscillators in a medium travels more slowly
in the medium. Since there are more interactions with the medium in the process of absorption and
reemission, this occurs. Visible light of higher frequencies travels slower than light of lower frequencies, as
the natural or resonant frequency of most transparent materials in the UV part of the spectrum.
b.
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, separation of
different colors of light is quite apparent. Thus, dispersion occurs when the separation of light into colors
are arranged according to their frequency, by interaction with a prism or diffracting grating, for example.
11 Rainbow
a.
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.
b.
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.
c.
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.
12 Total Internal Reflection
a.
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. This is 100% reflection.
b.
Silver or aluminum mirrors reflect only 90-95% of incident light and are marred by dust, dirt. Prisms are
more efficient. 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. A small critical angle, pronounced refraction due to the unusually low speed of light in diamond,
produces wide dispersion and a wide array of colors.
c.
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. These are also
called light pipes, as they can be used in places very inaccessible. These can be used in communications,
where thin glass fibers are replaced with thick, bulky copper cables to carry thousands of simultaneous
telephone messages between major switching centers. These are replacing electric circuits and microwaves
in communication technology.
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