Measuring the Speed of Light
Sarah Hansen & Matthew Kyle Sharp
Goal
Historically, measuring the speed of light was very difficult. Because light
travels so quickly, it covers large distances in very short times, and it is hard to
measure such distances and times precisely. Yet, this number is one of the most
important numbers in science. Einstein’s theories involved ideas about how fast
light can move (the speed of light is the c in E=mc2); in fact, lightspeed is the
fastest anything can move!
In this lab we will take advantage of something that is considered a “problem” with
old-fashioned TVs that use antennas to measure just how fast light is traveling across the lawn of
Yerkes.
Units of Speed
Speed describes how fast something moves. We express a speed as how long it takes to travel a certain
distance. That is,
Speed = Distance / Time
The most familiar units of speed are probably miles per hour
(mi/hr). You can also express speed as kilometers per second
(km/s), or yards per minute (yd/min) or in whatever units of
distance and time are most convenient for you. When stating a
speed, it is very important to always include the units – if you
just say “she can run 5” people will not know if you mean 5 mi/hr or 5 km/s… and
there’s a big difference!
Speed of a Human
From the units of speed, we know that to measure a speed, you need
to know a distance and a time. Choose a member of your group
who likes to run. How fast do you think this person can run?
In your notebook, write the person’s name and your guess at
their speed (don’t forget units!).
How can you measure how fast this person can run? HINT: you’ll be
provided with a stopwatch and a ruler. Discuss with your group and
come up with a plan to measure how fast your groupmate can run.
http://www.atletismoweb.com/
curiosidades/
In your notebook, write your procedure for measuring the
person’s speed.
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Following your procedure, make the measurement. Then,
Record the distance, time and calculated speed in your notebook.
How close was your guess? To compare, make sure that both the guess and the measurement are in the
same units!
Write your Guessed and Measured speeds expressed in the same units in your notebook.
How sure are you of your result? Can you think of some ways to make your result more precise and
accurate?
In your notebook, list at least three things that could be done to improve the measurement.
Speed of Light
How fast do you think light travels?
In your notebook, write your guess at lightspeed.
Can you use the same methods to measure the speed of light that you used to measure the speed of
your classmate? If so, try it. If not, why not?
In your notebook, write the results of your experiment and/or why it doesn’t work.
Measuring the speed of light is tough! Many smart people came up with complicated ways of trying to
measure lightspeed……in the end, the problem comes down to finding a way to measure small periods
of time and large distances. In this lab we are going to use a television to measure a very short time
very accurately.
How a TV works
In the days before cable television and widespread use of satellite dishes, most
people used antennas on their roofs to receive television signals. Often these
antennas do not produce a clear picture for any number of reasons, hence their
declining popularity. In this lab we are going to look for an unclear signal on
purpose. The signal that we are seeking is called a ghost or a reflection. We will
use this signal and what we know about how TVs work to measure the fastest thing in
the universe!
A TV creates the impression of moving images by slightly changing the picture on the screen
continually at a rate faster than the human eye can detect. That rate is 30 times a second in the United
States. If the refresh rate was much slower we might notice the small changes and see a flicker. The
entire image is not changed at once; rather, a new image is produced as the TV scans through each line
of the image. This is like when a person reads printed text. They read from left to right from top to
bottom, character by character, line by line. The image on a TV is produced by an electron beam
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scanning across the screen. Electrons hit the screen, which is coated with a special chemical called
phosphor, causing it to glow. Things get a bit more complicated with color TV, but it is essentially the
same except instead of just one spot being hit, three different ones are hit each corresponding to a
specific color (cyan, magenta and yellow).
TVs in the US conform to the National Television System Committee (NTSC) standard. This
regulation means that the entire screen is refreshed 30 times a second. Each TV screen has 525
horizontal lines.
To refresh all 525 lines 30 times in one second, how many lines must the TV be able to scan in
one second? Do the calculations in your notebook.
Scanning an individual line takes slightly less than 1/15,750 second because there is a small lag time to
get from one part of the screen to another, so one scan across one line on the screen takes 1/15,735
of a second, or 0.0000635525897680331 seconds.
Specific bands in the radio frequency range of the electromagnetic spectrum have
been assigned as carrier frequencies (or channels) for TV broadcasts. They are:
Channels 2-6
Channels 7-13
Channels 14-69
54-88 MHz (VHF, Very High Frequency)
174-216 MHz (VHF)
470-806 MHz (UHF, Ultra High Frequency)
The TV’s scan rate is fixed and independent of the frequency of the carrier signal. The frequency
range assigned to broadcast TV is a portion of the radio part of the electromagnetic spectrum. Light
in the radio range has very long wavelength waves, on the order of centimeters to meters, compared to
visible light (which has wavelengths only millionths to billionths of a meter long). The TV set coverts
the information contained in these radio waves to voltage changes that correspond to a specific spot or
pixel on the screen.
TV antennas are designed to interact with the radio waves that carry the TV signal. Most TV antennas
are placed on roofs and orientated in specific directions for the best reception. There are a number of
reasons that a TV connected to an antenna will display a poor image.
Assuming that the TV set is working fine, make a list in your notebook of some possible reasons
for a poor signal.
Modeling a Television: Signals and Echoes
This part of the lab will be a physical demonstration of the principles behind how a TV works. Follow
the directions of your lab instructors, and then answer the following in your notebook:
Describe what each member of the group did, and what that person represented in the model
(e.g. the TV, the antenna, the signal, etc….). Do the same for any props you may have
used.
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What we want to know is how fast the signal is traveling. If you knew the scan speed of the TV,
describe how you could determine the speed of the TV signal.
Copy the image of a TV screen below into your lab notebook. On the same sketch, draw what it
would look like if the TV signal was bouncing off of a nearby building and interfering
with your reception.
Now draw what it would look like if the TV signal was traveling half the speed as before. Write
whether it will look the same as the previous drawing and why or why not.
Measuring the Speed of Light
It is now time to make our own measurement of the speed of light. We will use a similar procedure to
what we discussed during the Modeling a TV activity. There will be two groups: one for measuring the
distance, once for measuring the time. You will get a chance to do both parts. Record your data and
answer the questions below in your notebook as you go.
Distance: Just How Far?
Make a sketch of the showing the South Building, the Observatory, where the TV antenna is,
and where the TV signal is traveling:
What do you think the TV signal is bouncing off of? Why? Make sure this object is included in
your sketch and label it ‘Reflector.’
What distance do you need to measure?
How far do you think it is?
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Your method for measuring this distance.
We want to make as good a measurement as possible, so you need to measure carefully. What
will you do to make sure that your measurement is as accurate as possible?
Now measure the distance and record your results in your notebook.
Time: Measuring the Ghost
We will use the primary and ghost images on the TV as our stopwatch. The primary image
(the brighter one) is caused by the signal when it is first received by the TV antenna. The faint
ghost image is caused by the signal after it has reflected off some object and bounced back to
the antenna. Since we know how fast the TV scans, we can use the separation between the
primary and the ghost to figure out how much time elapsed between the TV receiving
the signal the first time and receiving the reflected signal.
Example: We have a television that takes 10 seconds to do one scan. In your notebook,
answer the following:
How long would it take to cross half of the screen?
How long would it take to cross a quarter of the screen?
Explain how you would calculate the time elapsed for the signal to cross some fraction, f, of
the screen.
So, if we know the scan speed of the television and what fraction of the screen the TV scans while
the signal is traveling to the reflector and back, then we can calculate the elapsed time. The scan speed
is noted in the How a TV Works section, and we will measure the fraction of the screen between the
primary and ghost images. To do so, do the following:
1. In your lab notebook, make a data table like the one below and use it to record your results.
2. Measure the width of the screen in mm and record.
3. Pick a place to measure the ghost (think about it!), measure the separation between primary and
ghost images, and record.
4. Calculate the fraction of the screen between the primary and ghost images (a decimal is fine).
5. Use the screen fraction to calculate the time elapsed between the primary and reflected (ghost)
signals and record this lag time lag.
Screen Width (mm)
Separation Between Ghost and Primary (mm)
Screen Fraction Between Ghost and Primary
Lag Time (s)
6. In your notebook, sketch what you measured.
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Putting It All Together
Make the following data table in your notebook and fill it in.
SPEED of
LIGHT
TV Channel
City of Channel’s Origin
Calculated Lag Time
(in seconds)
Reflector
Distance traveled When Reflecting
(in m)
Calculated Speed of Broadcast TV Signal
(in m/s)
You now have measured the time it takes for the signal to pass the antenna, reflect, and return to the
antenna. You also have measured the distance traveled during that time. Remember,
speed = distance/time.
So, calculate the speed of the TV signal (i.e., the speed of light) in meters/second and record it in your
notebook.
In 1983 the General Conference on Weights and Measures officially redefined the meter as the
distance that light travels in vacuum in 1/299,792,458 of a second. That means we have a universal
standard for the speed of light (c):
c = 299,792,458 meters/second
How does your measurement compare?
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