Cerenkov radiation
electrically perpendicular to the direction of propagation, the electrical field of the wave produces an electrostatic force which affects the electrons within the atoms of the optical medium. These electrons are thus caused to move, which then creates a second oscillating electrical field. This second electrical field then interferes with the original light wave, and the superposition of the two waves produces a resultant wave, which travels at a slower rate than the original light wave, and has the result that the speed of light within optical media is lower than the speed of light within a vacuum. According to Maxwell’s equations of electromagnetism, an accelerating charged particle should emit electromagnetic waves. However, with our modern knowledge of quantum mechanics, we know this is not the full picture. Wave-particle duality tells us that, like light, every particle can be considered to behave as both a particle and a wave, with the length scale at which the wave-like behaviour becomes relevant known as the De Broglie wavelength, which, for an electron within a water molecule at room temperature which is vibrating randomly according to Brownian motion (and on average moves at around 590m/s ), would be a De Broglie wavelength of roughly 1200nm. This is important with respect to Cerenkov radiation because, as the charged particle moves through the optical medium, it interacts with the electric fields of the electrons surrounding the atoms of the optical medium (or the electrical field of the polar molecules that make up the optical medium, in the case of water), and excites the electrons to a higher energy level. These excited electrons return back to their ground state via spontaneous emission of a photon (whose energy is equal to the difference between the electron's ground state and excited state). The Huygens-Fresnel principle tells us that any point that emits waves will produce a spherical wave (Huygens, 1690) – which in the case of electromagnetic radiation will move at the speed of light in that medium (see figure 2 below). If the charged particle moving through the optical medium is moving more slowly than the emitted radiation (i.e., moving slower than light), then there will not be any crossing over of the waves. Cerenkov radiation is observed when the charged particle moves faster than light, and then the emitted radiation waves constructively interfere with one another, and they form a coherent wavefront, which propagates in the shape of a cone (as seen in Figure 2).
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Figure 3 clearly shows a bluish glow surrounding Penn State University's 1MW research reactor. The reactions taking place in the fissile rods of the reactor emit beta particles at 98% of the speed of light in a vacuum, which produces Cerenkov radiation because the phase velocity of light when travelling
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