The Electromagnetic Spectrum
Background Information
The electromagnetic spectrum includes all the waves shown in the chart below. All these waves are the same oscillating electromagnetic disturbance. Note the tremendous variation in wavelength and frequency. The wavelength and frequency are related by:
wavelength X frequency = speed
= 3 X 108 m/s
The frequencies allocated for television lie on either side of the FM band. You may have seen a shortwave or ham operator with a long wire antenna, usually at least 15 m long. These antennas are long because shortwave signals are typically weak, and the longer antenna provides a stronger signal. Compare this length with the wavelength, which is hundreds or thousands of meters.
In a commercial radio receiver, a typical AM/FM radio antenna is one meter or less in length, still considerably less than the shortest FM wavelength. What would happen if an AM/FM antenna were made much longer, so it was about the same length as the wavelength? In that case, the antenna would be excited by both the crest and the trough of the radio wave, which would cancel each other out, and the extra length would have no effect. So the antenna length of a commercial radio provides approximately an upper limit on the range of wavelengths that can be received. In Steps 1 and 2 the students use the antenna length or enclosure size to estimate the wavelength of several kinds of radiation. The microwave oven and radar detector resonate for electromagnetic waves in the same way a pipe resonates for sound waves (See Background Information for Chapter 4, Activity 3). The students take the length of the resonant enclosure as a first approximation of the wavelength.
The first to measure the speed of light was Ole Roemer. Roemer noticed that the periods of the moons of Jupiter depended on where the Earth was in its orbit during the measurements. If the Earth was moving away from Jupiter, the period was longer than if the Earth was moving toward Jupiter.
Roemer concluded that the Earth’s motion in its orbit changed the distance the light from Jupiter’s moons had to travel, and this different distance accounted for the differences in measured period.
For the two observations shown in the drawing, the motion of the Earth added about eight minutes (radius of the Earth’s orbit divided by the speed of light) to the travel-time of the light from Jupiter.