5 Minutes Introduction to EM Wave

In the previous article, we mentioned that James Maxwell unified the electric and magnetic fields by combining the four Maxwell’s equations into a single wave equation which predicts the existence of the electromagnetic (EM) wave. The discovery of EM wave is a major breakthrough in modern physics because it let us understand the phenomena of electricity and magnetism more fundamentally and also inspires Albert Einstein to formulate the theory of general relativity.

The figure illustrates the EM Wave. It consists of magnetic and electric fields where both fields are perpendicular with each other

EM Wave is not the same as mechanical wave. Mechanical wave, for example sound wave can only propagate in a medium because it requires a collision of particles in a medium for propagation, which is why sound wave cannot travel through space. However, EM wave doesn’t need any medium to propagate, it can propagate in a vacuum at the speed of light, c = 3 x 108 ms-1 . Inside a medium on the other hand, it propagates less than the speed of light. This is because the atoms or molecules inside the material absorb and re-emit the EM wave, so there is a time delay in the propagation of EM wave.

EM wave can be described by the following simple relation: c = f λ , where f and λ are the frequency and wavelength of EM wave, respectively. Since the speed of light is a constant, then f and λ are inversely proportional to each other, which implies that the EM wave has shorter frequency with longer wavelength. Then, we can characterize the sources of EM wave with either their frequency or the wavelength, for example the light that we see in the daily life has the range of wavelength from to 400nm to 700nm. It is called visible light because we can see it. Other sources of EM wave that cannot be seen by our eyes are radiowave, microwave, infrared, ultraviolet, X-ray and gamma ray.

The Spectrum of the EM Wave

The EM wave exhibits the effect of diffraction and interference because it is a wave. Nevertheless, the emergence of quantum mechanics in the 20th century has redefined that EM wave has the properties of wave-particle duality in the microscopic world. The particle-like substance in EM wave is called photon. The particle property of EM wave can be observed from the photoelectric effect where when light shines on the surface of metal, the electrons can only be ejected from the surface of metal when the energy of light is discrete and the frequency of light greater than a threshold value.

The photoelectric effect

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