Information in Radio Waves
Using the “E.M. Spectrum” presentation:
Slide 1: Electromagnetic Radiation Spectrum
- Self - explanatory
Slide 2: Breakdown
- Give a brief rundown of the overall presentation. May ask students to try and answer the
questions and and try to name characteristics of a wave to get them thinking.
Slide 3: Terms to know
- A few options with this slide, 1) Can simply introduce these terms and read off the definitions,
2) Have students copy the words and definitions in their notes, or 3) have students copy the
words and define them on their own.
Slide 4: The E.M. Spectrum
- Introduce the electromagnetic spectrum. Make sure to point out to the students that
everything that they experience visually only exists in that very small sliver in the entire
spectrum shown.
Slide 5: The E.M. Spectrum (2)
- Ask students if they can truly picture how fast that is. When they say they can’t, as nobody
really can, have your students calculate / convert light speed into KPH and MPH. Have them
calculate a light year.
Points of interest that students enjoy:
○ point out that when students look out into space they are looking back in time
○ the light from the sun takes 8.3 minutes to get to Earth, therefore they see the
sun 8.3 minutes in the past
○ If an alien species 65 million light years away looked at our planet right now, they
wouldn’t see us, they would see dinosaurs.
■ Ask them why
Slide 6: How does light travel?
- Mention that since the time scientists first tried to describe light, there has been a flip-flopping
from waves to particles. Even now, depending on how light is studied (especially at the
quantum level) light exhibits properties of both a wave and a particle. Light can travel through a
vacuum unlike other wave motions like sound (sound works through continuous compressions
and rarefactions of matter, without matter therefore it can’t happen).
Slide 8: Longitudinal waves
- Waves that rely on subsequent compressions and rarefactions (relaxation) of matter, passing
the energy along parallel to the wave motion.
○ Easily demonstrated via a slinky
Information in Radio Waves
Slide 9: Transverse waves
- The displacement is perpendicular to the overall direction of motion.
○ Easily demonstrated via a length of string or rope
Slide 10: *Misconception
- Ask students what they think when they hear the words “radio waves” (you may want to ask
this before you get to this slide so students won’t just read the board
- Make it as clear as possible the students that radio waves do not equal sound waves. Explain
how the sounds they hear out of their radios comes from sound information being encoded into
a transmittable light wave which in turn is then decoded back into sound information.
Slide 11: Characteristics of a wave
- Briefly introduce the 4 characteristics of light waves. Mention that the properties and
information available in those waves rely on the specific properties of those characteristics.
○ Good way to explain it: The eye is the bodies tool of reading the information
inside the visible light spectrum. Amplitude is read as brightness (high amplitude
= very bright) and frequency is interpreted as different colors.
■ Phase and polarization carry information as well, but eyes have not
evolved to interpret them.
Slide 12: 1.) Amplitude
- In addition to what is on the slide, explain what a crest and a trough are. Go on to show what
a difference in amplitude would look like.
Slide 13: 2.) Frequency
- Some students may be slightly confused by the concept of cycles per second. Might be
clarified by comparing it to RPM.
○ Could also demonstrate it by moving your hand up at down slowly then very fast.
○ Reiterate that light travels at a constant speed. While moving at that speed it is
also Moving up and down at varying speeds
- The electromagnetic spectrum is divided based on this property of light. Wavelength and
frequency are directly related to one another (inverse relationship).
Slide 14: Animation
- Let students watch a few cycles of the animation. Ask students if the frequency is increasing
or decreasing as the animation goes on. Ask if the wavelength is increasing or decreasing.
Slide 15: 3.) Phase
- Phase can be the most difficult property of light to explain. Phase truly only comes into play
when two waves, of the same frequency, are being compared and interjected upon one another.
Information in Radio Waves
The amount they are offset from one another is measured and expressed in either degrees (0 to
360) or in radians (0 to 2𝜋).
Slide 16: 4.) Polarization
- The polarization of light has to do with the orientation of the fields in relation to the direction of
overall propagation. If the electric and magnetic field are oscillating in phase with one another,
then the resulting wave polarization will be linear. If they are out of phase from one another by
90 degrees it will have circular polarization. When the two components are not in phase and
either do not have the same amplitude or are not ninety degrees out of phase, though their
phase offset and their amplitude ratio are constant, the wave will have elliptical polarization.
Good speed of light demonstration:
Robert H. Stauffer, Jr., Cimarron-Memorial High School, Las Vegas, Nevada, USA
The activity requires a microwave oven, a microwave-safe casserole dish, a bag of
marshmallows, and a ruler. (The oven must be of the type that has no mechanical
motion-no turntable or rotating mirror. If there is a turn-table, remove it first.) First, open
the marshmallows and place them in the casserole dish, completely covering it with a
layer one marshmallow thick. Next, put the dish of marshmallows in the microwave and
cook on low heat. Microwaves do not cook evenly and the marshmallows will begin to
melt at the hottest spots in the microwave. (I learned this from our Food Science
teacher Anita Cornwall.) Heat the marshmallows until they begin to melt in four or five
different spots. Remove the dish from the microwave and observe the melted spots.
Take the ruler and measure the distance between the melted spots. You will find that
one distance repeats over and over. This distance will correspond to half the
wavelength of the microwave, about 6 cm. Now turn the oven around and look for a
small sign that gives you the frequency of the microwave. Most commercial microwaves
operate at 2450 MHz.
All you do now is multiply the frequency by the wavelength. The product is the speed of
light.
Example:
Velocity = Frequency ´ Wavelength
Velocity = 2450 MHz ´ 0.122 m
Velocity = 2.99 ´ 108 m/s
This works in my physics class, often with less than 5% error. Then the students can eat
the marshmallows.
(Reprinted with permission from The Physics Teacher, vol. 35, April 1997, p. 231.
Copyright 1997 American Association of Physics Teachers )