WAVE LESSON 5: How Do We Know Light Behaves
as a Wave?
Wavelike Behaviors of Light
An age-old debate which has persisted among scientists is related to the question, "Is
light a wave or a stream of particles?"
The fact is that light exhibits behaviors which are characteristic of both waves and
particles.
Light exhibits certain behaviors which are characteristic of any wave:
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.
Diffraction involves a change in direction of waves as they pass through an
opening or around an obstacle in their path.
The interference of waves causes the medium to take on a shape which results
from the net effect of the two individual waves upon the particles of the medium.
Reflection
A wave doesn't just stop when it reaches the end of the medium. Rather, a wave will
undergo certain behaviors when it encounters the end of the medium. Specifically, there
will be some reflection off the boundary and some transmission into the new medium.
EXAMPLE: Light reflection on water. Some of the light enters the water
and some bounces back causing a reflection.
One characteristic of wave reflection is that the angle at which the wave
approaches a flat reflecting surface is equal to the angle at which the wave
leaves the surface.
Refraction
The transmitted wave undergoes refraction (or bending) if it approaches the boundary at
an angle.
A wave will bend one way when it passes from a medium in which it travels slow into a
medium in which it travels fast; and if moving from a fast medium to a slow medium, the
wavefront will bend in the opposite direction. Second, the amount of bending is
dependent upon the actual speeds of the two media on each side of the boundary.
EXAMPLE: Line of sight when looking at a fish under water. The line of sight bends as
light goes from a fast medium to a slow medium.
Diffraction
Water waves have the ability to travel around corners, around obstacles and through
openings. Sound waves do the same.
But what about light? Do light waves bend around obstacles and through openings? If
they do, then it would provide still more evidence to support the belief that light behaves
as a wave.
When light encounters an obstacle in its path, the obstacle blocks the light and tends to
cause the formation of a shadow in the region behind the obstacle. Light does not exhibit
a very noticeable ability to bend around the obstacle and fill in the region behind it with
light. Nonetheless, light does diffract around obstacles. In fact, if you observe a shadow
carefully, you will notice that its edges are extremely fuzzy, this is caused by wave
interference
This is often demonstrated in a Physics classroom with a laser light and penny
demonstration.
Light diffracting around the right edge of a penny can
constructively and destructively interfere (also wave like
property) with light diffracting around the left edge of the penny.
The result is that an interference pattern is created; the pattern
consists of alternating rings of light and darkness. Such a pattern is
only noticeable if a narrow beam of monochromatic light (i.e.,
single wavelength light) is passed directed at the penny.
Since, light waves are diffracting around the edges of the penny,
the waves are broken up into different wavefronts which converge
at a point on a screen to produce the interference pattern.
Can you explain this phenomenon with a strictly particle-view of light? This amazing
penny diffraction demonstration provides another reason why believing that light has a
wavelike nature makes cents (I mean "sense").
Polarization
A light wave is an electromagnetic wave which travels through the vacuum of
outer space. Light waves are produced by vibrating electric charges. A
simplification of an electromagnetic wave is a transverse wave which has both
an electric and a magnetic component.
The transverse nature of an electromagnetic wave is quite different from any
other type of wave which has been discussed.
Normally, the coils of the slinky might vibrate up and down or left and right,
moving along the same linear direction as you sighted along the slinky.
However, if a slinky wave were an electromagnetic wave, then the vibrations of
the slinky would occur in multiple planes. Unlike a usual slinky wave, the
electric and magnetic vibrations of an electromagnetic wave occur in numerous
planes.
A light wave which is vibrating in more than one plane is referred to as
unpolarized light. This concept of unpolarized light is rather difficult to visualize. In
general, it is helpful to picture unpolarized light as a wave which has an average of half
its vibrations in a horizontal plane and half of its vibrations in a vertical plane.
EXAMPLE: Light emitted by the sun, by a lamp in the classroom, or by a candle flame
is unpolarized light.
It is possible to transform unpolarized light into polarized light. Polarized light waves
are light waves in which the vibrations occur in a single plane.
There are a variety of methods of polarizing light. The four methods discussed on this
page are :
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


Polarization by Transmission
Polarization by Reflection
Polarization by Refraction
Polarization by Scattering
Polarization by Use of a Polaroid Filter
Polaroid filters are made of a special material which is capable of blocking one of the two
planes of vibration of an electromagnetic wave. (Remember, the notion of two planes or
directions of vibration is merely a simplification which helps us to visualize the wavelike
nature of the electromagnetic wave.)
In this sense, a Polaroid serves as a device which filters out one-half of the vibrations
upon transmission of the light through the filter. When unpolarized light is transmitted
through a Polaroid filter, it emerges with one-half the intensity and with vibrations in a
single plane; it emerges as polarized light.
Polarization of light by use of a Polaroid filter was is often demonstrated in a Physics
class through a variety of demonstrations.
Filters are used to look through and view objects.
The filter does not distort the shape or dimensions of the object; it merely serves
to produce a dimmer image of the object since one-half of the light is blocked as it
passed through the filter.
A pair of filters are often placed back to back in order to view objects looking
through two filters. By slowly rotating the second filter, an orientation can be
found in which all the light from an object is blocked and the object can no longer
be seen when viewed through two filters.
What happened?
In this demonstration, the light was polarized upon passage through the first filter;
perhaps only vertical vibrations were able to pass through. These vertical vibrations were
then blocked by the second filter since its polarization filter is aligned in a horizontal
direction. While you are unable to see the axes on the filter, you will know when the axes
are aligned perpendicular to each other because with this orientation, all light is blocked.
A picket-fence analogy is often used to explain how this dual-filter demonstration works.
Polarization by Reflection
Unpolarized light can also undergo polarization by reflection off of nonmetallic surfaces
(metallic surfaces = variety of vibrational directions – not polarized). The extent to which
polarization occurs is dependent upon the angle at which the light approaches the surface
and upon the material which the surface is made of, such as asphalt roadways, snow
fields and water which reflect a large concentration of vibrations in a plane parallel to the
reflecting surface causing a GLARE (polarization is large)
Fisherman are familiar with this glare since it prevents them from seeing fish which lie
below the water. Fisherman know that the use of glare-reducing sunglasses with the
proper polarization axis allows for the blocking of this partially polarized light and
reducing glare. This makes it possible to see the fish.
Polarization by Refraction
Polarization can also occur by the refraction of light. Refraction occurs when a beam of
light passes from one material into another material. At the surface of the two materials,
the path of the beam changes its direction.
The refracted beam acquires some degree of polarization.
Most often, the polarization occurs in a plane
perpendicular to the surface.
The polarization of refracted light is often demonstrated
in a Physics class using a unique crystal which serves as a
double-refracting crystal. Iceland Spar, a rather rare form
of the mineral calcite, refracts incident light into two
different paths. The light is split into two beams upon
entering the crystal. Subsequently, if an object is viewed
by looking through an Iceland Spar crystal, two images
will be seen. The two images are the result of the double
refraction of light.
Both refracted light beams are polarized - one in a direction parallel to the surface and the
other in a direction perpendicular to the surface.
Since these two refracted rays are polarized with a perpendicular orientation, a polarizing
filter can be used to completely block one of the images.
HOW?
If the polarization axis of the filter is aligned perpendicular to the plane of polarized light,
the light is completely blocked by the filter; meanwhile the second image is as bright as
can be. And if the filter is then turned 90-degrees in either direction, the second image
reappears and the first image disappears. Now that's pretty neat observation that could
never be observed if light did not exhibit any wavelike behavior.
Applications of Polarization
Polarization has a wealth of other applications besides their use in glare-reducing
sunglasses.
Polarization is also used in the entertainment industry to produce and show 3-D movies.
Three-dimensional movies are actually two movies being shown at the same time through
two projectors. The two movies are filmed from two slightly different camera locations.
Each individual movie is then projected from different sides of the audience onto a screen
through a polarizing filter. The polarizing filter used for the projector on the left may
have its polarization axis aligned horizontally while the polarizing filter used for the
projector on the right would have its polarization axis aligned vertically.
The audience then wears glasses which have two Polaroid filters. Each filter has a
different polarization axis - one is horizontal and the other is vertical. The result of this
arrangement of projectors and filters is that the left eye sees the movie which is projected
from the right projector while the right eye sees the movie which is projected from the
left projector. This gives the viewer a perception of depth.
The polarization of light provides some substantial support for the wavelike nature of
light. It would be extremely difficult to explain polarization phenomenon using a particle
view of light. Polarization would only occur with a transverse wave. For this reason,
polarization is one more reason why scientists believe that light exhibits wavelike
behavior.
WAVE LESSON 5 HOMEWORK
1. Constructive interference of waves occurs when two crests meet.
a. True
b. False
2. Light can be considered a wave because four of the behaviors we observe light
exhibiting are also observed in waves. What are these four behaviors? What happens to
the wave in each case with regard to velocity, frequency, wavelength and amplitude?
3. The behavior of light reflecting off of a transparent material (such as glass or water) is
dependent upon the optical density of the material. The reflection behavior of a rope also
depends on the nature of the material in which it is reflecting off of. State the relationship
and draw the reflected and transmitted wave pulse.
a. From Low to Higher density:
Incident Pulse
Reflected Pulse
Transmitted Pulse
Reflected Pulse
Transmitted Pulse
velocity
frequency
wavelength
amplitude
inversion
b. From High to Lower density:
Incident Pulse
velocity
frequency
wavelength
amplitude
inversion
4. Two waves travel through one another. How do you know if the resultant amplitude
will be greater or smaller than the individual amplitudes.
5. Suppose that light passes through two Polaroid filters whose polarization axes are
parallel to each other. What would be the result?
6. Light becomes partially polarized as it reflects off nonmetallic surfaces such as glass,
water, or a road surface. The polarized light consists of waves vibrate in a plane which is
____________ (parallel, perpendicular) to the reflecting surface.
7. Consider the three pairs of sunglasses below. Identify the pair of glasses is capable of
eliminating the glare resulting from sunlight reflecting off the calm waters of a lake?
_________ Explain. (The polarization axes are shown by the straight lines.)
WAVE LESSON 5 HOMEWORK ANSWERS
1. Constructive interference of waves occurs when two crests meet.
a. True
b. False
2. Light can be considered a wave because four of the behaviors we observe light
exhibiting are also observed in waves. What are these four behaviors? What happens to
the wave in each case with regard to velocity, frequency, wavelength and amplitude?
a. Reflection:
Same velocity, frequency, and wavelength in the same
medium. Reduced amplitude due to some energy
transmitting into other medium.
b. Refraction:
Same frequency with velocity and wavelength increasing or
decreasing. Amplitude in new medium would represent less
energy being transmitted due to some energy being
reflected.
c. Diffraction:
Waves both bend around barriers in a single medium. The
amount of diffraction depends on wavelength. The longer
the wavelength the greater the diffraction. Since it is the
same medium, the velocity , frequency, and wavelength
stay the same.
d. Interference:
Light, sound and waves both exhibit the phenomena of
constructively and destructively interfering when waves
from two separate sources attempt to occupy the same point
in the medium at the same time. This means amplitude may
be greater or smaller depending on the type of interference.
3. The behavior of light reflecting off of a transparent material (such as glass or water) is
dependent upon the optical density of the material. The reflection behavior of a rope also
depends on the nature of the material in which it is reflecting off of. State the relationship
and draw the reflected and transmitted wave pulse.
a. From Low to Higher density:
Incident Pulse
Reflected Pulse
Transmitted Pulse
velocity
same
decreases
frequency
same
same
wavelength
same
decreases
amplitude
decreases
Not Applicable
inversion
entering more dense medium acts like a fixed end
none
b. From High to Lower density:
Incident Pulse
Reflected Pulse
Transmitted Pulse
velocity
same
increases
frequency
same
same
wavelength
same
increases
amplitude
decreases
Not Applicable
inversion
entering less dense medium acts like a free end
none
4. Two waves travel through one another. How do you know if the resultant amplitude
will be greater or smaller than the individual amplitudes.
It depends on whether constructive or destructive interference occurs.
Constructive interference allows for crest on crest situations which results
in greater amplitudes. Destructive interference allows for crest on trough
situations which results in lesser amplitudes. Therefore the waves cancel
each other out.
5. Suppose that light passes through two Polaroid filters whose polarization axes are
parallel to each other. What would be the result?
Answer: The first filter will polarize the light, blocking one-half of its vibrations. The
second filter will have no affect on the light. Being aligned parallel to the first filter, the
second filter will let the same light waves through.
6. Light becomes partially polarized as it reflects off nonmetallic surfaces such as glass,
water, or a road surface. The polarized light consists of waves vibrate in a plane which is
____________ (parallel, perpendicular) to the reflecting surface.
Answer: Parallel
Reflected light becomes partially polarized in a plane which is parallel to the reflecting
surface.
7. Consider the three pairs of sunglasses below. Identify the pair of glasses is capable of
eliminating the glare resulting from sunlight reflecting off the calm waters of a lake?
_________ Explain. (The polarization axes are shown by the straight lines.)
Answer: A
Referring to the above question, the glare is the result of a large concentration of light
aligned parallel to the water surface. To block such plane-polarized light, a filter with a
vertically aligned polarization axis must be used.