Posted by: T. Boyd | September 25, 2012

Measuring the Speed of Light with a Television Set

Einstein said that nature’s speed limit is set at the speed of light which has been measured to be about 300,000,000 meters/sec.  Only electromagnetic waves travel that fast; everything that has mass cannot travel that fast even if shot by the most  powerful accelerator that has been made (or by a gigantic explosion, for example, of an exploding star or supernova).

Even though some scientists recently found evidence of particles breaking this speed limit, their evidence has since been discredited.  To get an idea of how fast this is, the time for a radar signal to travel from the earth to the  moon and back is a little over 2 seconds, compared to the time it took the astronauts to go to the moon and back in 7 days.

My son, David, measured the speed of light  in 8th grade, I think it was, and won some recognition for it at the county science fair, if I remember correctly.

The method uses a portable TV with a rabbit-ear antenna (or any small antenna) set to pick up a weak TV station in a known direction. A large metal reflector (or cardboard covered with aluminium foil) of about 2 feet by 2 feet or so is held in line with the TV set and the TV station, with the reflector on the side of the TV away from the station (so that it will reflect the TV signal back toward the TV as explained below).

The easiest channels to measure are the higher VHF channels (7 through 13) because their wavelengths are convenient sizes for this measurement.

To measure the wavelength, the reflector is moved slowly back and forth along the line which starts at the station, passes through the TV set, and extends beyond the TV set. This is so that the signal will be reflected from the metal sheet back to the TV set and cause an interference which will make the TV program fade out at certain distances.

The more positions that can be found that cause the fading to occur, the more accurate the speed of light measurement will be. The example that follows will make these ideas clearer.

How is the speed of light determined from these measurements? First of all, the speed of light is the same as the speed of TV signals since light and TV signals are both electromagnetic waves. And the relationship between wavelenth, frequency, and the speed of light is given by the equation:

frequency multiplied by wavelength = speed of light

The average distance between positions of cancellations is equal to twice the wavelength of the TV station’s signal, and the frequency of that signal is given by the table found at this web site.

For example, if channel 12 is used, with a frequency (middle of its range) of 207 Megahertz, and, for example, the experimenter measures an average distance between minimum signals of 0.75 meters ( wavelength = 2 x 0.75 = 1.5 meters), then he or she would calculate that the speed of light to be:

1.5 meters x 207,000,000/sec = 313,000,000 meters/sec

where I rounded off to 3 significant digits in the answer according to best scientific practices.  Since the accepted value for the speed of light is currently 299,792,458 meters/sec then the error in the student’s measurement is about 4%, which is considered good considering the simple apparatus used.

In my own son’s case, the impressive part of his science report was the large number of repeated measurements that he made (because his Dad kept insisting he do so!).  This allowed him to use statistics to determine the standard deviation of his average measurement, so that he could quote an experimental error along with his speed of light measurement.

As experimental scientists well know, determining the error of the measurement is usually as hard or harder than the actual measurement itself.  And having the student repeat the measurements so that he or she has, say, 10 values  for each position, then the confidence in the result will be much higher than if the student only measures the positions once or twice each.  And also, the statistical error will be an objective value rather than just a guess at the error.

I find it amazing that this universal constant, labeled “c” – the speed of light, and is the same c that is in Einsteins famous energy equation, E=mc2, can be so easily measured by a 13 year old student.  Some of the laws with which the Lord created and sustains the universe are very simple.  His word says in Deuteronomy 29:29, “The secret things belong to the Lord our God, but the things revealed belong to us and to our children forever, that we may follow all the words of this law.”

Every time we have a “revolutionary” breakthrough in science, it is because the Creator, our LORD (English Bibles use “LORD” in all caps for the Hebrew name YHWH whenever it appears), has given us the mental ability to discover what He has revealed.  There is much that remains hidden, but it is exciting to continue searching for the revealed truths that have not yet been discovered.


Responses

  1. […] Measuring the speed of light using radio or tv – see Measuring… […]


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