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Name: Van
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Question:
At the risk of asking a question that cannot be answered briefly:
I am confused about light waves. I understand they are considered the only waves that can travel through a vacuum. If this is true, through what medium do they travel such that they can be considered "waves"? What is their frequency a frequency of? in what unit is it measured?

With sound waves it is easy to grasp: the "crest" of the wave corresponds to a compression of air. The frequency, in Hertz, is a measure of how many of these "crests" hit the eardrum in a second. With light waves I would like to know what corresponds to the crest, the trough, the amplitude, and the frequency of the light wave, and what happens in the physical universe the allows these wave properties of light to be measured.



Replies:
Dear Van,

As James Clerk Maxwell showed in 1864 when he published his four equations which describe all of electricity and magnetism, these equations predict a wave motion composed of electric and magnetic fields which travels at the speed which light had been measured to travel at. The conclusion seemed inescapable, and it has now been shown in innumerable ways, that light is indeed an electromagnetic wave in which the electric and magnetic fields of which the wave is composed travel at the speed of light (186,000 miles per second).

The frequency of light waves is the frequency at which the electric and magnetic fields oscillate. That light is a wave motion is easily demonstrated by performing experiments in which light waves interfere, such as the Young double slit experiment.

I like to prove that light is a wave motion by looking through my closely spaced fingers at a distant light. If you try this, you will see dark fringes in the space between your fingers which could be caused only by the destructive interference of the light waves.

The "crest" of a light wave is when the electric field is a maximum in one of the two the direction it points when it is maximum; the "trough" is when the electric field is a maximum in the opposite direction. The amplitude is the maximum value of the electric field and the frequency is the number of crests per second passing a given point.

The wavelength, by the way, is the distance between two crests. For visible light, the wavelength is around 5E-7 m and the frequency is about 6E14/sec. 6E14 means 6 followed by 14 zeroes (a very large number) and 5E-7 means 0.0000005 (a very small number). The speed of light in this notation is 3E8 m/s, where m stands for meters and s for seconds.

Electric and magnetic fields can exist in a vacuum just as gravitational fields can. Gravitational waves almost certainly also exist and also travel through vacuum at the speed of light, although they have not yet been observed due to the extreme weakness of the gravitational force; gravity is only important for us since we are so close to an enormous amount of matter.

Thanks for a very good question; feel free to ask more questions.

Best, Dick Plano, Professor of Physics emeritus, Rutgers University


Van,

Light does not have a medium. Light waves are oscillating electric and magnetic fields. When you make an electric current move back and forth in a wire, the current generates a magnetic field in loops around the wire. This field oscillates in time with the current in the wire. The back and forth change of the magnetic field generates an electric field around the magnetic field. This electric field also oscillates in time with the original current. The oscillating electric field generates a magnetic field. This happens back and forth. At some frequencies, these are called radio waves. At higher frequencies, they are called light waves. At very high frequencies, they are called gamma rays. They are all the same, except for frequency.

The frequency of the light wave is the frequency of oscillation of the electric and magnetic fields. A common unit for frequency is Hertz (i.e. cycles per second).

Dr. Mellendorf



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