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Light as a Wave

The classical description of light as an electromagnetic wave makes some assumptions about its nature, based on what could be observed at the time. Although some of these assumptions have proven to be less than accurate as we learn more about electromagnetic phenomena, they make a good starting point for our discussion about light in particular and electromagnetic waves in general. The two basic assumptions were:

Of course, other properties of electromagnetic radiation have also been determined, and a number of theories and assumptions have been developed. These have been either confirmed or disproven by experiment. Before we look at the circumstances under which the wave model of light (or any electromagnetic wave) may break down, however, let's look at the wave model itself.



Since light was first recognized scientifically as a manifestation of electromagnetic energy, it can be represented as a waveform, like this:

Light as a wave.

If we think of this figure as representing the electrical energy present in the light waveform as it travels in the direction of the arrow, it looks as if the energy level is becoming alternately positive and negative, with momentary crossovers of zero electrical energy. In the basic model of light shown here, this is in fact the case; as with all electromagnetic waves, light energy is constantly changing its form between electrical energy and magnetic energy. The point of maximum magnetic energy coincides with the moment of zero electrical energy. Beyond that instant, energy shifts again from the magnetic field back into the electrical field, but with a reversal from the previous polarity. This continues as long as that particular ray of light exists.

There are other "modes" of propogation which involve more complex interactions between the electric and magnetic fields, but in all cases the Law of Conservation necessarily holds true: Energy is neither created nor destroyed as it is transformed from one form to another; the total energy in the wave must somehow remain constant throughout the full cycle.

NOTE: The sine wave shown here represents the strength and polarity of the electrical field associated with the motion of this ray of light. The light itself, assuming no outside influences, travels in the straight line indicated by the blue arrow. The light energy does not "wiggle" back and forth as it moves along its path.



As an electromagnetic wave, light has some characteristics in common with all forms of electromagnetic energy. These include wavelength, frequency, and speed of propogation. These characteristics are actually related to each other, so that any one can be calculated if the other two are known. Let's take a look at each of these characteristics:

Wavelength

Since light is a repeating waveform in motion, it is possible to measure the physical distance between matching points of adjacent cycles of the waveform. This is shown here:

Defining the wavelength of a light wave.

The symbol used to represent this distance is the Greek letter "Lambda" ().

The wavelength can actually be measured between any two corresponding points on the waveform. It is convenient to use the most positive point or the most negative point, both of which are shown above. However, we could have just as easily specified two zero-crossing points, so long as both crossed the zero line in the same direction.

Remember that the light itself does not wave back and forth along its path of travel. What we are actually measuring here is the distance traveled through space by this ray of light, while its electrical field goes from its maximum positive value, through zero to its maximum negative value, and then through zero again to once more reach its maximum positive value.

This distance is normally measured in meters (m) or some decimal fraction of a meter, such as centimeters (cm). The correct units of measurement are meters per cycle (m/cycle) or some appropriate derivation. In the case of light, the wavelength is so short that a specific distance, called the ångstrom (Å), has been defined.

One ångstrom = 10-10 m or 10-8 cm.

Visible light has a characteristic wavelength in the range of approximately 3900 Å to 7700 Å. Electromagnetic energy outside this range is no longer visible to the human eye.


Speed of Propogation

The speed at which light travels through any medium is determined by the density of that medium. The presence of matter, even transparent matter, will slow the light down. Even air will have some effect, and glass has a more significant effect on the speed at which light will travel through it.

Ever more sophisticated experiments have determined the speed of light quite accurately. According to current knowledge:

Speed of light in a vacuum = 2.997925 ± 0.000002 x 1010 cm/sec.

As made famous in Einstein's equation, the letter c is used as a general symbol for the speed of light.


Frequency and Period

In any electromagnetic wave, it takes time for the energy in the wave to change from electrical format to magnetic and then back again. The amount of time required to do this twice, covering one complete cycle, or wavelength of the signal is known as the period of the wave. Thus, the period of any wave, measured as some amount of time per cycle, is in fact the time interval that corresponds to the physical wavelength of the signal.

The frequency of the wave is the inverse or reciprocal of the period. That is, the frequency is the number of cycles of the waveform that occur in one second of time. For many years this was simply measured in units of cycles per second. Recently, however, the specific name hertz (abbreviated Hz) has been designated as the appropriate unit to indicate cycles per second.

In general equations, the letter f is used to indicate frequency in hertz.



The basic mathmatical formula that relates wavelength, frequency, and the speed of light is:

c = f x wavelength


The wave theory of light was happily adopted and accepted until it was found to fail to explain some observed and measured phenomena, in consistent and repeatable experiments. The two phenomena that upset this model are the photoelectric effect and blackbody radiation. These two effects could only be explained by assuming that light energy propogates as a series of independent "corpuscles," or bundles. This gave rise to the more recent particle theory of light.


Prev: What is Light? Next: Light as a Particle

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