WAVE PROPERTIES, BEHAVIORS, AND
INTERACTIONS
Unit Overview
Various forms of waves
have been studied previously, as well as how waves travel. The next step is to
understand how waves interact with other waves, in addition to various
obstacles with which waves come into contact. Anytime there is a change in the
medium the wave is traveling in, the wave itself will be affected.
Recall, the medium refers to what the wave is traveling through. A medium can
be a vacuum, solid, liquid or gas.
Wave Interactions
If light is traveling
through air (gas) and hits a mirror (solid) it may get bounced back to the
source. Light is described as being reflected when it returns back to
the source.
When light hits a rough
object, like crinkled foil, the reflection is diffuse. A smooth
object, like a mirror, can make a reflection of a clear image. A mirror
reflects all light waves back out at the exact same angle that they arrived;
this is called a true image.
When light reflects off
an object some of the wavelengths are absorbed and some are reflected; the
color we see is the wavelengths that are reflected. A blue object
absorbs all wavelengths except blue.
A black object absorbs
all wavelengths of light. This explains why black objects, like a
shirt, are hot in the sun. Transparent materials will transmit most of the energy
through the material but some may be absorbed or reflected.
Light also may change
direction when it changes mediums without being sent back to the source.
Anytime a light wave is bent, it is said to be refracted. When the sun’s
rays travel through space and hit the medium of the earth’s atmosphere, they
are both slowed down and bent. If the light waves continue to travel and hit a
lake, they are refracted or bent again. A classic example of bending or
refracting light is a prism.
A prism separates all of
the colors of light by bending them different amounts. Each color of light has
a different frequency, which means that each color of light vibrates a
different amount of times per defined period. As light travels through a prism,
it is easier for the light that has the lowest frequency to pass through. A
color with a higher frequency vibrates more per defined period, causing it to
take longer to pass through the prism.
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BELOW ARE COMMON LIGHT PRISMS. |
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Rainbow |
Oil Slick |
Diamond |
Diffraction happens when waves pass through an opening or around
an obstacle, and change direction. If the wave has a higher wavelength, it
results in a larger diffraction. The size of the opening or obstacle will also
determine the amount of diffraction.
The absorption of light
happens when a light wave goes into the object and does not bounce off. It
depends on the frequency of the light being transmitted, and the nature of the
atoms of the object. If they are complementary, light will be absorbed; if not,
light will be reflected or transmitted.
Light transmission
happens when light strikes a transparent object and passes through to the other
side.
Video Clip: Real
World Science: Light
This video investigates
the features and characteristics of light, which is a visible form of energy.
The program describes some of the properties of light, the components and
behavior of light waves, reflection and refraction, and explores the color
spectrum.
Real World Science: Light
Properties of Visible Light: Combining
Colors
The fact that waves have
different frequencies is a good thing. If there were only one frequency, your
radio could only play one station. When you change the dial on your radio to a
different station, you are allowing it to pick up a different frequency.
Different frequencies can occupy the same space at the same time, which allows
many radio stations to be available for your radio at the same time. A stage
play often uses many color spotlights to highlight the actors. In this example,
the left side stagehand operates a yellow spotlight and the right side
stagehand operates a blue spotlight. Each has an actor to follow. What will
happen if the two actors switch sides of the stage and the
spotlights beams have to cross one another? Everyone knows that when
yellow and blue are mixed, they make green. Will the actors both become green?
Of course, the answer is no. The actors will still remain lit by their original
colors, yellow and blue. This is because the two colors of light have different
frequencies and do not interfere with one another.
However, when 2 colors of
light are mixed a new color is formed, as shown below. If several
colors of light are mixed, white light is formed. If no light is
present, black is formed. Colored light mixtures are shown below.
Mixing paint is very
different than mixing light. Remember that colored objects absorb
particular wavelengths of light. Thus, when yellow paint and blue
paint are mixed green paint is made. When many colors of paint are
mixed black or brown is formed, as shown below.
Visible
Light
Visible light is made up
of electromagnetic waves, vibrations of electric and magnetic fields that
propagate through space. In contrast to slow-moving ocean waves to which they
are similar, electromagnetic waves travel at the speed of light: 300 million
meters per second, or 669.6 million miles per hour!
Every electromagnetic
wave exhibits a unique frequency, and wavelength associated with that
frequency. For instance, the image below represents an electromagnetic wave
corresponding to the color red.
Its frequency is 428,570
GHz (pronounced gigahertz), which can also be stated as 428 trillion, 570
billion cycles per second. So when you look at red light, your eye receives
over four hundred trillion waves every second!
The wavelength of such
light is but 700 nanometers long, which means that one wave spans only
7/10,000,000, or 7 ten-millionths of a meter. Measuring a single wavelength of
this light against the length of a meter is like comparing a thumbtack's
diameter to the distance across the United States.
Just as red light has its
own distinct frequency and wavelength, so do all the other colors. Orange,
yellow, green, and blue each exhibit unique frequencies and consequently,
wavelengths. While we can perceive these electromagnetic waves in their
corresponding colors, we cannot see the rest of the electromagnetic
spectrum.
Electromagnetic Spectrum: A Review
Recall that most of the
electromagnetic spectrum is invisible. Exhibiting the highest frequencies are
gamma rays, x-rays and ultraviolet light. Infrared radiation, microwaves, and
radio waves occupy the lower frequencies of the spectrum. Visible light falls
within a very narrow range in between.
So radio waves, such as
those that FM radio stations broadcast, are simply electromagnetic waves with a
much lower frequency and longer wavelength than visible light. The millimeter
radio waves, also called microwaves, possess a slightly higher frequency and
shorter wavelength than the FM radio waves.
Wave Interference
Certain waves can
interfere with one another at times. Interference occurs when crossing wave
patterns result in a loss of energy for some waves and an increase of energy
for other waves.
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This interference can be
observed on your car radio. Suppose a person is leaving on a long trip in their
car, and the car radio is set on their favorite local station as they leave
home. As the car gets some distance away from home, the local station begins to
fade out. A radio station from another city operates at the same frequency as
the local station. When the car is between the two stations, the radio will
experience interference and, both stations will fade in and out. When the car
travels far enough from home, the radio will then pick up only the new station.
Waves of different frequencies are capable of traveling in the same medium at
the same time without interfering with one another. Waves of the same frequency
will interfere with one another if they are in the same medium at the same
time.
There are two types of
wave interference. One is known as constructive interference and the other is
called destructive interference. Constructive interference happens at
any location along a medium where the displacement of the interfering waves is
in the same direction. The interference increases the displacement where the
waves meet.
Destructive interference occurs at any location along a medium where the
displacement of the interfering waves is in opposite directions. The pulses
have the same maximum displacement but in opposite directions. In the instance
of complete overlap, the displacement by one pulse cancels out the other.
Destructive interference is a temporary state of displacement being less than
that of the wave with the highest amplitude.
An interesting fact is
that the interaction of waves along a medium does not cause them to deviate
from their path. The waves continue on as if nothing happened after the point
of interference.
Video Clip: Wave
Interference
This clip discusses the
various types of wave interference that can occur. Longer wavelengths mean that
the waves are of a lower frequency. This clip also discusses refraction.
Complete the guided notes as you watch.
Watch the following video clip and complete the guided notes.
Wave
Interference
PhET Simulation:
Light and Sound Wave Simulation
Run the PhET Simulation
and answer the questions in the attached document. Go to question 10 to attach and
send to your teacher.
Light and Sound Wave PhET Simulation Student copy
Quizlet Vocabulary