What’s in a
light wave?
What doe
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wave loo
k like?
What is fre
quency?
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What am I going to learn?
What are the properties of waves?
What is a sine wave?
What is the relationship between the speed of a
wave, its wavelength, and its frequency?
How are waves created?
Why do we study waves? Have you ever seen the sun? Heard
music? Watched ripples in a pond? Then you were observing
waves! All of these waves carry energy in different forms by light,
sound and motion. We study wave properties to understand all of
the different types of wave energy we see in the natural world.
In this lab, you will create lines of sand from a swinging cup and
study the properties of the wave trace you create as you slide a
sheet of paper under the sand at different speeds. Through
quantitative measurements, you will determine a relationship
between the wavelength, frequency and speed of a wave.
Some useful background!
Energy often travels in waves. Waves can take on many shapes. One of the most
common wave shapes is called a sine wave, shown below.
Sine Wave
Light waves are one of many types of waves in nature shaped like sine waves.
Their repeating pattern allows us to classify them based on their wavelength.
The properties of a wave are shown in the figure below. Besides light, what
other types of waves have you seen in nature that are shaped like sine waves?
1
Wavelength
Crest
P
Trough
Direction of
Motion
Crest
Trough
Amplitude
The crest of wave is the highest point on the wave, and the trough is the lowest
point. Wavelength is the distance measured from one crest to the next crest or
from one trough to the next trough. The amplitude of the wave is its height,
measured from the center dotted line. The frequency of a wave is the number
of crests that pass a fixed point (“P” in the diagram) in a given amount of time
as the wave moves. For example, if 3 crests pass point P in one second, the
frequency is three cycles per second.
Since waves are usually moving, we can also measure their speed.
Where have I heard frequency before?
You’ve probably heard of frequency on the radio
station or the channel on a walkie-talkie. This is
the same frequency as described above! The
radio waves used in these types of
communication also have a frequency. It’s the
number of times the radio wave oscillates per
second. So 98.1 FM really means 98.1 MHz, or 98.1
million oscillations per second!
Pre-lab Activity and Predictions
Do this pre-lab activity before you do the lab
Steps:
1. With your pen, draw lines repeatedly back and forth starting at the top of a
sheet of paper. Make sure your pen moves along the same straight line back
and forth.
2. Have your partner pull the paper away from you as shown in the photo.
3. Experiment with the speed your partner moves the paper. What happens to
the wavelength of the wave you draw when you pull the paper faster or
slower? Experiment with how fast you move your hand back and forth.
What happens to the wavelength when you move your pen back and forth
with a higher or lower frequency?
4. What relationship did you see between frequency, wavelength, and
speed?
Pre-lab Activity and Predictions
(continued)
5. Consider two waves, A and B, that have different properties.
If wave A and wave B have the same wavelength and same speed, how
do their frequencies compare (same, A greater, B greater) and why? Justify
your answer with a picture.
If wave A and wave B have the same wavelength, but A moves faster, how
do their frequencies compare (same, A greater, B greater) and why? Justify
your answer with a picture.
If wave A and wave B have the same speed, but A has a larger
wavelength, how do their frequencies compare (same, A greater, B
greater) and why? Justify your answer with a picture.
Materials
sand
long roll of paper,
paper towels, or
sheets of paper taped
together
plastic cup
tape
calculator
scissors
chair or
other
hanging measuring
tape
support
pen
stopwatch
string
Let’s experiment!
Steps:
1. Poke a hole in the bottom of a cup so
that sand will flow freely from it.
2. Tie the cup to a low hanging chair and
place the long paper below it as shown
in the photo.
3. Fill the cup with sand. Plug or tape over the hole until ready to start your experiment.
4. Measure the frequency of the swinging cup by timing its swing for 10 cycles and
dividing the time by 10. Record this in the chart below.
5. One person in your group will hold the stopwatch and swing the cup while the other
person is in charge of pulling the paper. When you’re ready, pull the cup to one
side, open the hole in the cup, and let the cup start swinging.
6. Simultaneously, start the stopwatch as your partner
begins to pull the paper at a slow constant speed as
shown in the photo.
7. When the paper-puller gets near the end of the
paper, stop pulling, and at the same time, stop the
stopwatch.
8. Measure the distance from start to end as
indicated in the photo, and count the
number of wavelengths. Record these
numbers and the time on the stopwatch in
the table provided.
9. Pour the sand back into your cup and repeat steps 4-8, pulling the paper at slower
and faster speeds.
10. Re-hang the cup making the string longer so that it swings at a different frequency.
Repeat steps 4-9 for this length and record your data on the second chart provided.
11. Calculate and record the average wavelength and average
speed in the columns indicated in the table.
Length of
String
Units____
Frequency for
10 Cycles
Units____
Distance
Pulled
Units____
Time of Pull
Units____
Number of
Wavelengths
Speed of
Wave
Units____
Wavelength Size
Units____
Results
Short String Length
Length of
String
Units____
Frequency for
10 Cycles
Units____
Distance
Pulled
Units____
Time of Pull
Units____
Number of
Wavelengths
Speed of
Wave
Units____
Wavelength Size
Units____
Results
Long String Length
Let’s Find a Relationship
With your data, plot the points of wavelength vs. speed on the
chart below. Draw a different line for each frequency/string length
you recorded.
(units)
Wavelength ___
Label each line with the average frequency from that string height
Speed ___
(units)
Play some more!
We learned about the shape of light - a sine wave. But waves can come in
other shapes such as a triangle wave and a square wave, shown below.
Triangle Wave
Square Wave
Using your set up, try to make waves that look like these, or other types of
waves besides sine waves. Draw the shapes you made in the space below
and describe how you made them. As a bonus, try these other shapes:
1
What happened?
Post-lab questions:
1. Did you find the relationship between speed, wavelength and frequency that you
predicted above? If not, how did it differ? What surprised you about this result?
2. What are some sources of error that might have affected your experiment?
3. The speed of light is constant, no matter the wavelength or frequency. Based on
your experiment, what is the relationship between wavelength and frequency for
light?
4. When we created waves on paper in the pre-lab and then with sand during the
lab, we were combining a left-to-right oscillating motion with a perpendicular
forward motion. Can you think of a type of wave that oscillates in the same
direction as it travels? Try making this kind of wave with the swinging cup.
More fun physics!
The electromagnetic (EM) spectrum, shown below, describes different
types of light. Light is referred to as an EM wave because it is made up
of both electric and magnetic waves moving together through space.
The EM spectrum is arranged by wavelength––longer wavelengths to
the left, shorter wavelengths to the right. Though we are most familiar
with visible light, all EM waves in the spectrum are types of light. Did
you know X-rays were a type of light?
The longest light waves are radio waves. The shortest waves are
gamma rays. No matter what the frequency or wavelength, all light
travels at 299.8 million m/s. Speed (c) is related to frequency (f) and
wavelength (λ) by the equation:
c=λ*f
Since the speed on the left side of the equation is always constant,
that means that as wavelength goes down, the frequency must go up.
The shorter the wavelength (as we move to the right on the spectrum)
the higher the frequency.