What is an electromagnetic wave? Describe some evidence supporting the idea that

Question

What is an electromagnetic wave? Describe some evidence supporting the idea that light is an electromagnetic wave. Review and re-read sections 32.1, 32.3, and 32.4 in University Physics, 13th ed. You do not need to go over all the mathematical details, but you will need to understand EM waves conceptually. Now, answer these questions: What is an electromagnetic wave? Describe some evidence supporting the idea that light is an electromagnetic wave. Figure 32.13 on pg. 1061 in the textbook shows a representation of a simple EM wave, similar to one that would be produced by a laser. Draw your own version of this diagram and clearly add the following to it: (If any of these can't be added to the diagram explain why they can't.) The amplitude of the EM wave. The wavelength of the EM wave. The frequency of the EM wave. What everyday property of light is most directly related to the amplitude of an EM wave? What everyday property of light is most directly related to the frequency of an EM wave? How is the wavelength of an EM wave related to this same everyday property? Based on your answers to questions 3 and 4, which physical quantity is related to the energy carried by an EM wave, the amplitude of the wave or the frequency/wavelength of the wave. Explain your reasoning.

Explanation / Answer

1)   Electromagnetic waves are waves which can travel through the vacuum of outer space. Electromagnetic waves are created by the vibration of an electric charge. This vibration creates a wave which has both an electric and a magnetic component. An electromagnetic wave transports its energy through a vacuum at a speed of 3.00 x 108 m/s (a speed value commonly represented by the symbol c). The propagation of an electromagnetic wave through a material medium occurs at a net speed which is less than 3.00 x 108 m/s
The mechanism of energy transport through a medium involves the absorption and reemission of the wave energy by the atoms of the material. When an electromagnetic wave impinges upon the atoms of a material, the energy of that wave is absorbed. The absorption of energy causes the electrons within the atoms to undergo vibrations. After a short period of vibrational motion, the vibrating electrons create a new electromagnetic wave with the same frequency as the first electromagnetic wave. While these vibrations occur for only a very short time, they delay the motion of the wave through the medium. Once the energy of the electromagnetic wave is reemitted by an atom, it travels through a small region of space between atoms. Once it reaches the next atom, the electromagnetic wave is absorbed, transformed into electron vibrations and then reemitted as an electromagnetic wave. While the electromagnetic wave will travel at a speed of c (3 x 108 m/s) through the vacuum of interatomic space, the absorption and reemission process causes the net speed of the electromagnetic wave to be less than c.
We have strong reason to conclude that light itself—including radiant heat and other radiation, if any—is an electromagnetic disturbance in the form of waves propagated through the electro-magnetic field according to electro-magnetic laws.
Maxwell’s achievement ranks as one of the greatest advances of physics. For the physicist of the late 19th century, the study of light became a study of an electromagnetic phenomenon—the fields of electricity, magnetism, and optics were unified in one grand design. While an understanding of light has undergone some profound changes since the 1860s as a result of the discovery of light’s quantum mechanical nature, Maxwell’s electromagnetic wave model remains completely adequate for many purposes.
Light polarisation rotates in a magnetic field (Faraday rotation), i.e. light is connected and reacts to magnetism. Maxwell's argument is of course no experimental proof, it's a theory, but all its predictions match the properties of light very well. Maxwell knew of Faraday rotation and his prediction of electromagnetic waves saw the speed close to the speed of light, so he just conjectured that they should be the same.
Today, there are many more things that explicitly tell you light must be electromagnetic: Absorption and Emission of electromagnetic radiation can only be explained by quantum electrodynamics and the carrier particles are photons, the quantizations of the electromagnetic field. Since we can see atoms emitting visible light (some of them), this has to be electromagnetic (see e.g. LEDs).

Some information is missing in question 2.

3,4 ) Amplitude
The amplitude of electromagnetic waves relates to its intensity or brightness (as in the case of visible light).
With visible light, the brightness is usually measured in lumens. With other wavelengths the intensity of the radiation, which is power per unit area or watts per square meter is used. The square of the amplitude of a wave is the intensity.

Frequency
The frequency of any waveform equals the velocity divided by the wavelength. The units of measurement are in cycles per second or Hertz.

Wavelength
The wavelengths of electromagnetic waves go from extremely long to extremely short and everything in between. The wavelengths determine how matter responds to the electromagnetic wave, and those characteristics determine the name we give that particular group of wavelengths.

Electromagnetic waves can be classified and arranged according to their various wavelengths/frequencies; this classification is known as the electromagnetic spectrum.

5) Conclusion
Electromagnetic radiation can be described by its amplitude (brightness), wavelength, frequency, and period. By the equation E=hE=h uE=h, we have seen how the frequency of a light wave is proportional to its energy. At the beginning of the twentieth century, the discovery that energy is quantized led to the revelation that light is not only a wave, but can also be described as a collection of particles known as photons. Photons carry discrete amounts of energy called quanta. This energy can be transferred to atoms and molecules when photons are absorbed. Atoms and molecules can also lose energy by emitting photons.

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