Download Electromagnetic Waves Materials

Physics PHYS 102
General Astronomy
Electromagnetic Waves
Materials
1 Microwave Transmitter
1 Microwave Receiver
2 Rods and stands
2 Meter sticks
Aluminum foil
Laser pointer
Slit film
Optical rail and mounts
Large tape measure
Scissors
Purpose
Aside from a few notable exceptions, like neutrinos and
possibly gravity waves, all of the information about the
universe that astronomers have gathered travels to us via
electromagnetic radiation. Electromagnetic radiation
consists of perpendicular oscillating electric and
magnetic fields, and travels at the speed of light,
. The frequency (f) and wavelength ()
of electromagnetic waves are related by
The electromagnetic spectrum is shown in Figure 1. The
longest electromagnetic waves are radio; then, in order
of decreasing wavelength come microwaves, infrared,
visible, ultraviolet and x-rays. The shortest waves are
gamma rays.
In this lab you will be exploring the properties of
electromagnetic radiation at two wavelengths:
microwaves and visible light.
Part I – Microwaves
Figure 1. The electromagnetic spectrum.
(http://commons.wikimedia.org/wiki/File:ElectromagneticSpectrum.png)
In this part of the activity, you will be using the
microwave transmitter and receiver.
CAUTION: The output power of the Microwave Transmitter is well within standard safety levels.
Nevertheless, never look directly into the microwave horn at close range when the Transmitter is on. Avoid
standing in front of the microwave source when possible.
Experiment 1: Transparent and Opaque
1.
Set up the transmitter and receiver on the meter stick as shown in Figure 2, separated by a distance of
approximately 80 cm.
Figure 2. Set up for Experiment #1. The microwave transmitter and receiver are aligned using the meter stick.
2. Turn on the transmitter and receiver, and make sure that a useable signal strength is obtained.
3. What do you think will happen if a sheet of paper is inserted between the source and receiver?
4. Try it. Record the result below.
5. Do some experiments in order to make a list of objects that are transparent to microwaves. List those that
are opaque.
2
Experiment 2: Apertures
1. Insert an aluminum foil barrier using two rod stands, as shown in Figure 3.
Figure 3. Aluminum barrier placed between the transmitter and receiver.
2. Is the foil transparent to microwaves?
3. Directly between the transmitting and receiving antennas, poke a small hole in the aluminum foil with a
pencil to let the microwaves pass through. Do you detect microwaves with the receiver?
4. Enlarge the hole incrementally until you begin to detect microwaves. How big is the hole?
3
5. In order for EM waves to travel through a hole, the size of the hole must be at least approximately the same
order of magnitude as the wavelength. Approximately what is the order of magnitude of the wavelength?
6. Do these waves lie in the microwave region of the electromagnetic spectrum?
Experiment 3: Double slit
1. Now replace the foil with one in which two rectangular openings, approximately 4-5 cm wide, have been
cut, as shown in Figure 4.
Figure 4. Setup for the double slit experiment. Two rectangular apertures are cut into the aluminum foil.
2. Adjust the positions of the transmitter and receiver to obtain the maximum signal strength when the receiver
is equidistant from the slits. This should occur somewhere around 50 cm.
4
3. Now move the receiver to the left or right. As you do this, the signal will decrease and then increase due to
destructive and constructive interferences of the EM waves coming from each slit.
4. Place the receiver at the appropriate distance on a second meter stick which has one end directly beneath the
foil. Hold the meter stick end beneath the foil so that the entire meter stick rotates around this point. Locate
the minimum on each side of the center, and measure the distances y and L in Figure 5.
Slit
s
r1
Microwave
Source
y
r2
d/2
L
Figure 5. Diagram of the double slit experiment. Microwaves passing through two rectangular slots in the
aluminum foil interfere to create interference fringes.
5. Use the Pythagorean theorem to determine the distances
√
(
) and
and
√
:
(
) .
5
6. In order for destructive interference to occur to create a minimum intensity, the path difference for waves
coming from each slot must only be ½ of a wavelength. Therefore
(
).
7. What is the wavelength of the microwave transmitter?
8. What frequency is the microwave transmitter?
6
Part II – Visible Light
In this part of the laboratory activity, you will be using a diode laser pointer and a film with very fine slits
etched into it. A diagram of the slit film is shown in Figure 6. The slits you will be using in the experiment are
marked.
#1
#2
Figure 6. Diagram of slits on the film used in this experiment. The slit marked #1 is a single slit with a
width of 0.08786 mm. The slit marked #2 consists of two slits, each 0.08786 mm wide and separated
(center-to-center) by 0.35 mm.
Experiment 1: Single slit and diffraction
1.
Set up the laser and slit film as shown in Figure 7. Let the laser beam strike the slit marked #1 in Figure 6.
This is a single slit. Allow the laser beam to travel to a distant wall after passing through the film.
7
Figure 7. Setup up of single slit experiment showing laser and slit film mounted on optical rail.
2. Draw what you see projected on the distant wall. This effect is called “single slit diffraction” and results
from destructive and constructive interference between waves passing through different parts of the slit.
8
3. It can be shown that the first minimum in the light distribution hitting the wall gives the wavelength of the
light according to
where a is the width of the slit and y and L are defined as in Figure 8. Measure y and L to find the
wavelength.
Slits
Laser
r1
d/2
y
r2

a
L
Figure 8. Setup for the laser diffraction experiment. In this diagram, the double slit is shown. L is the distance
from the slits to the observing screen, and y is the distance from the center line to the first minimum in light
intensity.
4. What is the wavelength?
9
5. In Figure 9, diffraction rings are clearly visible is a photograph of two stars taken though a telescope. Can
you explain the appearance of these rings? Why is diffraction a problem for astronomers?
Figure 9. A high magnification photograph of two stars, made through a telescope. Diffraction rings are
clearly visible around the stars.
10
Experiment 2: Double slit
6. Now move the laser so that it passes through the slits marked #2 in Figure 6. This experiment is the same as
the microwave double slot experiment, except using laser light instead of microwaves.
7. Measure the quantities L and y as defined in Figure 8. Obtain the slit separation d from Figure 6.
8. Use the Pythagorean theorem to determine the distances
√
(
and
√
) and
:
(
) .
9. In order for destructive interference to occur to create a minimum intensity, the path difference for waves
coming from each slot must only be ½ of a wavelength. Therefore
(
).
What is the wavelength of the laser? Does this agree with Figure 1?
11
10. What is the frequency of the laser light?
12