Light As A Wave

Most of what we can directly observe about light is best explained by waves, so for a long time light was thought to be a wave. Before we can jump into the wave properties of light, we need to clarify what waves are.

Waves: Moving Disturbances

Remember the last time you walk past a pond? You probably grabbed a pebble and tossed it in. Right at that point, you created a wave. The rock hitting the water pushed some water down, creating a disturbance in the water. When the water recoiled back up, it caused water near it to go down. This continues until the energy of the falling rock is finally disapated through out the pond. The ripple you created moved across the pond, but the water that formed it didn't. The water moved up and down, but did not travel horizontally very far. That is the main idea of a wave. It moves, but the medium it passes through stays pretty much stationary. Waves are disturbances and mediums are what is disturbed. In the case of the pond, the medium moved perpendicular to the direction of wave travel.

This is a transverse wave. The other type of wave is one in which the medium moves parallel to the direction of wave travel.

This is a longitudinal wave. Sound is an example of a longitudinal wave travelling through the medium of air. For most of our discussion of waves, we will focus of transverse waves because their properties are easier to see.

Properties of Waves

Waves have four major properties that distinguish one from another:

Amplitude
Frequency
Wavelength
Speed

Amplitude

Amplitude is the height of the wave. It shows the energy of the event that started the wave. In the previous pond example, if we had hoisted a large boulder and (with the help of a friend) launched it into the pond, we would have seen much higher waves come crashing toward us.

When dealing with sound waves, loud sounds have high amplitude.

Frequency

Frequency is a measure of the number of waves that pass a point in a given amount of time. It is usually measured in Hertz (Hz>. One hertz means one wave passes every second. When we count waves, we have to divide the whole waveform into parts. The easiest way to do this is to go from one crest to another. This is one wave. We can now count the number of crests passing in a second to find our frequency.

Wavelength

Wavelength is just that: the length of the wave. It is measured in units of distance, which can be anything from meters to nanometers.

Speed

The speed of a wave is how fast it is travelling. In a wave, this can be determined by multiplying the frequency times the wavelength. Physicists write the formula:

In many cases the speed of a type of wave through a medium is constant. The speed of sound is constant through air (at a given pressure) and and light travels the same speed through water. This means that, in order to make the equation still correct, an increase in frequency results in a decrease in wavelength. (Some numbers may make this more clear: Suppose we fix the speed at 12 meters per second. If the frequency is 6 Hz then the wavelength must be 2 meters. Decrease the frequency to 4 Hz and the wavelength increases to 3 meters.) An easy way to remember this is with the phrase "lower, longer". Lower frequencies have longer wavelengths.

Electromagnetic Waves

In the late 1800's, physicists knew about radio waves and light, but considered them different things. James Clerk Maxwell, in formulating his equations explaining the two, showed that both are the same thing. They are all examples of electromagnetic radiation. They differ only in frequency. The chart below shows the spectrum of electromagnetic radiation.

Electromagnetic waves are unique since they are actually two waves. They are composed of an electric field waving perpendicular to a magnetic field.

What's Waving?

There is one rather critical problem with the wave theory of light. What is the medium for light? (More simply put: What's waving?) We can hear sound waves in air and underwater because their is air and water present to carry the sound. In a vacuum there is no sound because there is nothing to wave. Sound may not be able to pass through a vacuum, but light can. This presented quite a problem to physicists. Everything they knew about waves said there had to be something in the vacuum to carry light, but they couldn't detect it. Some scientists (notably Isaac Newton) theorized in the existence of an aether. The aether was said to fill the vacuum and could not be removed. Light was a wave in the aether. For all the problems it solved, there were some problems with the aether. First of all, the aether had to be very stiff (far stronger than steel) in order to be able to move light at the speed it was measured to go. A stiff medium can carry a wave at a faster speed than a soft one, and light goes really fast! At the same time, this unbelievebly stiff aether had to allow objects to move through it without any resistance. This made the aether seem to be more an invention of physicists than a property of nature. It wasn't until the early 1900's that Einstien showed that the aether didn't exist.

A Self-Supporting Wave

The question still remains: What's waving? In a way, the wave is waving itself. Remember that light is an electric field and a magnetic field perpendicular to each other. Michael Faraday and Hans Christian Oersted showed that a magnetic field can induce an electric field and vice versa. In a light wave, the electric field is causing the magnetic field, which in turn causes the electric field. They support each other. Remove one, and the other collapses.