Introduction:

When we discuss electromagnetic waves we usually think of visible light, but visible light actually makes up only a small fraction of the electromagnetic spectrum. The electromagnetic spectrum is the total range of frequency or wavelengths of electromagnetic waves. The spectrum’s range extends from the long wavelengths of radio waves to the short wavelengths of gamma waves. Below is a diagram of the electromagnetic spectrum.

Electromagnetic waves are similar in may ways to the mechanical waves you have probably studied in class, therefore many of the terms and mathematical equations used for mechanical waves can be used in the study of electromagnetic waves.

For simplicity we can think of an electromagnetic wave as energy that is moving from place to place and travels in the form of a transverse wave. Illustrated below is the relationship of wavelength and frequency of a transverse wave.

Wavelength:  The distance from one crest of a wave to the next and is denoted by the Greek letter lambda (l). In the image above the wavelength is about 7.5 units.

Frequency:  The number of crests, troughs, or any other point on the wave that passes a given point in a unit time interval. (f)

Amplitude:  The maximum displacement of the wave from an equilibrium position.  In the image above the equilibrium position is 0 so the amplitude is 1. There is positive and negative displacement for each wavelength. The displacement is proportional to the amount of energy, the greater the displacement the larger the amount of energy associated with the wave.

Wavelength and frequency are inversely proportional, meaning that if the frequency goes up, the wavelength goes down. The same is true vice versa. Also, all forms of electromagnetic radiation travel at the same high velocity, the speed of light (c).  The current accepted value for the speed of light is 2.997992458 x 10⁸ meters per second (m/s). For our lab the rounded value of 3.00 x 10 ⁸ m/s is acceptable.

The relationship between frequency, wavelength, and the speed of electromagnetic radiation is given by:

c=f*l

In the lab we will be using Smiley (4.6m radio telescope) to measure the radio emission of two astronomical radio sources, but a different frequencies.  We will calculate the wavelengths of the two frequencies of radio waves emitted from the radio sources.

Prelab Questions:

1. A source emits radio waves with a wavelength of 6 cm.  What is the frequency of the radio emission?

2. Radio waves travel at the speed of light.  The Galileo spacecraft orbiting Jupiter sends a signal 670 million km to earth.  How long does the signal take the reach earth?  Remember that velocity = distance/time.

3. Compare the wavelength of a radio wave with a frequency of 1.42 GHz to the wavelength of a visible light wave with a frequency of 6 x 10¹⁴ Hz.

4. Curious question:  Why can your radio pick up radio waves though walls, yet you can not see through walls?

Procedure:

1. Log into the Smiley Observation Control Room.  For information on how to use Smiley please see the Smiley Users Manual.
2. With Map selected select the Sun by selecting the Sun from Sources tab and click on GO.
3. In Continuum mode choose the Base Frequency to be 1.42 GHz. Also choose your IF GAIN to be 20 (when looking at the Sun you may need to set a lower IF GAIN) and your PLOT RATE to be 1x.
4. Now click on Begin Scan.  Use HandPaddle to find the maximum intensity.
5. Record the maximum intensity (the peak in your intensity) in the data table below. To find the maximum intensity click on Save Scan and save your scan. Then click on Open Data File and click on List on the file you just saved. Look through the data points and find your maximum intensity.
6. While on the same object choose the Base Frequency to be 4.8 GHz.  Do not readjust the PLOT RATE but you may need to change the IF GAIN to 10.  Again use HandPaddle to find the maximum intensity.
7. Record the maximum intensity in the data table below by following the procedure you used in Lab 1.  This procedure entailed saving your scan, then opening the data file, then listing the points to find your peak intensity.
8. Next, calculate the wavelength using the equation above. Also take the ratio of the maximum intensity at 1.42 GHz to the maximum intensity at 4.8 GHz (divide the 1.42 GHz maximum signal by the 4.8 GHz maximum signal)and record in the data table below.
9. Now multiply your ratio by a correction factor of .1 and record in the table below. This correction factor is needed because we are using different IF GAIN values for the 1.42 GHz signal and 4.8 GHz signal.
10. Repeat steps 2-9 for two more objects above the horizon. An object is below the horizon if its altitude is negative. If you see this, stop the telescope and choose another object.

Conclusion:

This section is your opportunity to describe the observations you made.  Feel free to comment on any aspect, including problems, how difficult/easy the observations were, and what makes sense and what does not.