Transcript No Slide Title

• Refraction
• Refraction of Water Waves
• Light Rays in Glass
• Atmospheric Refraction
• Apparent Depth
Refraction is responsible for why objects look bent in
water and why the setting sun looks flattened
Refraction
Reflection involves a change in direction of waves when they bounce off a barrier; refraction of waves involves a change in the
direction of waves as they pass from one medium to another. Refraction, or bending of the path of the waves, is accompanied by a
change in speed and wavelength of the waves.
Refraction Analogy
A way to visualize refraction of waves is to follow the motion of a marching
band line as it moves at an angle from one type of marching surface to
another.
Let’s say that the line moves from cement onto a muddy football field. The
first musicians to enter the field will be slowed down by the mud but the
musicians on the end of the line carry on at full speed. The result will be that
the line turns towards a line drawn perpendicular to the boundary between the
two surfaces (normal line). Also note that the marching lines will get closer
together but the number of lines passing a point per unit time will stay the
same.
Refraction of Water Waves
We have seen that the speed of a wave is dependent upon the properties of the medium through which the
waves pass. So if the medium (and its properties) are changed, the speed of the waves are changed. The most
significant property of water which would effect the speed of waves traveling through it is the depth of the
water. Water waves travel fastest when the medium is the deepest. Thus, if water waves are passing from
deep water into shallow water, they will slow down. And as mentioned, this decrease in speed will also be
accompanied by a decrease in wavelength. So as water waves are transmitted from deep water into shallow
water, the speed decreases, the wavelength decreases, and the direction changes. The frequency of the waves
will however stay the same. If the waves’ travel from shallow water to deep water, the waves bend in the
opposite direction as they speed up and the wavelength increases.
normal line
Light Rays in Glass
Light can be perceived as a series of wave fronts just light like the band analogy is composed of a
series of marching lines. Often it is more useful to draw the direction of the moving wave fronts as a
line drawn perpendicular to the wave fronts called a ray. When light moves from a less optically dense
medium (lower n) to a more optically dense medium (higher n), the light rays bend towards the normal
line. The reverse is true when moving from a more optically dense medium to a less optically dense
medium. See opposite.
Example
Light is incident on a glass prism as shown
opposite. Draw the path of the light ray a) in the
prism and b) after it leaves.
a)
First draw the normal line where
the incident ray hits the glass
prism. Glass is more optically
dense than air so the light will slow
down and bend towards the
normal line and then continue in a
straight line in the material as
there is no change in the medium.
b)
When the ray hits the boundary
between glass and air it will speed
up again as it moves into air and
therefore bend away from the
normal.
Atmospheric Refraction
This involves changes in wave properties including direction as a wave moves through the atmosphere.
Sound in Air
Sound waves are refracted when parts of the wave fronts travel at
different speeds. This happens in uneven winds or when sound is
traveling through air in uneven temperature.
On a warm day the air near the ground may be appreciably warmer
than the air above. Since the sound travels faster in warmer air, the
speed of sound near the ground is increased. The refraction effect thus
bends the sound gradually away from the ground, making it appear that
sound does not carry well.
On a cold day (or night) the reverse is true and the higher speed of the
wave fronts above cause a bending of the sound toward earth. When
this happens, sound can be heard over longer distances. Think of the
times that you have heard the whistle of the train at night as it rolls into
Dow but haven’t heard it other nights.
Light
The setting sun often looks oval shaped. Why is this?
The sun actually has sunk below the horizon and yet is still visible. This
is because light is refracted by the Earth’s atmosphere. Since the density
of the atmosphere changes gradually, the refracted rays bend gradually
to produce a curved path. The same thing occurs at sunrise, so our
daytimes are about 5 minutes longer because of atmospheric refraction
of light.
When the sun (or moon) is the near the horizon, the rays from the lower
edge are bent more than the rays from the upper edge. (see diagram).
The eye traces these rays back in a straight line so it looks like the sun
(or moon) is squashed in a vertical direction.
Mirage
Another application of atmospheric refraction of light is a mirage. This is very much like the atmospheric refraction of sound in that
hot air near the ground will speed up part of the light wave front nearest to it and thus produce a gradual bending of the light ray
upwards.
An example is shown opposite. Light from the top of the
tree is refracted away from the ground towards the
observer’s eye. The eye and brain together play a trick
on you as they perceive the light to have come in a
straight line which it did not. The eye thus forms an
inverted image of the tree and the brain interprets this as
a tree standing on the edge of water.
A motorist experiences a similar situation when driving
along a hot road. It appears as if there are shimmering
pools of water on the road but light from the sky is
really being refracted through a layer of hot air.
Convection currents add to this effect by giving the light
a shimmering appearance.
Apparent Depth
Have you ever looked down into a fountain or swimming pool and have seen an object sitting on the bottom but when
you went to fish it out found out that it was much deeper than it seemed.
This is because the light from the object is refracted as it crosses
the boundary from water into air. According to our refraction
rules the light waves will speed up and thus bend away from
the normal. The lens in the eye will focus these rays and the
brain will interpret the rays as have come in a straight line and
thus have traveled from an object that was a lot closer than in
reality.
Just like a mirage, this is an illusion, but is very much real in that
the light rays exist, and so can be photographed.
Have you ever felt that a thick frosted glass mug seems to
contain less liquid refreshment than it looks? Well, you are right!
Your brain perceives the light to have come from the edges of
the glass but careful inspection will show that it didn’t. Great
marketing skills do the rest!
Example
Where must the spear fisherman aim in order to hit the
fish?
He must aim his spear below where he sees the image
of the fish as the light reflected from the fish bends away
from the normal as it moves into air. If the fisherman
throws along his line of sight the spear will shoot above
the fish’s head.