Tag Archives: light

Cherenkov Radiation (or the Photonic Boom)

I’m assuming many of you will know what a sonic boom is, if you travel fast enough through a medium sound waves will compress in front of you and produce a sonic boom. Of course, this only happens if you are traveling at greater than the speed of sound in a given medium. Cherenkov Radiation can be described as the light equivalent of the sonic boom. So here’s (a simplified version of) what happens. Nothing can travel at speeds greater than c, the speed of light in a vacuum, but light in a medium will travel slower than c and in that sense a particle could travel faster than light through that medium, and when a charged particle travels through a medium at a speed greater than light travels through that medium it excites nearby molecules, which in turn emit light. Doing this requires lots of energy, which is why it’s commonly seen in the cooling tanks of nuclear reactors.

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Speeding and Blue Shift

Okay, so I’m assuming speeding on roadways is illegal in most countries, and that driving through a red light is also illegal. Okay? Okay. Let’s talk about blue shift. Blue shift is an application of the Doppler effect to light. Basically, if you travel in the direction opposite a wave, the apparent frequency of said wave increases, and the opposite is true if you travel in the same direction as the wave, the apparent frequency decreases. Since blue has a high frequency and red has a low frequency (as far as light goes) light is blue shifted when moving toward the light source. So now that that’s taken care of, how fast would you need to be going to see a red light at an intersecti0n as a green light. Using the speed of light, a frequency for red light (4.4*10^14 Hz), a frequency for green light (6*10^14 Hz), and Wolfram|Alpha, we get a speed of about 3/10 of the speed of light. That’s fast.

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Ultimate Hot

Let’s start this fun fact with something else, black-body radiation. Black-body radiation is light given off by any object with a temperature of more than 0K, or absolute zero (very,very cold). Basically, everything glows, but not necessarily in visible light. Humans glow in infrared, the sun’s surface glows in the visible to ultraviolet range, and the sun’s core glows in the x-ray to gamma radiation range. We can extrapolate on these observations (I’ll be making extensive reference to Wien’s displacement law) to find out when something is absolutely hot, that is, when things are so hot they make no sense in our current models of the physical universe. This point occurs when the black-body radiation that is given off by an object with a temperature around 10^32 Kelvin. For the rest of you out there, that’s so hot your molecules will break apart, the atoms in them will break apart, the nuclei in those will break apart, the protons and neutrons in those will break apart, and that’s it. Physics can predict nothing beyond what is now know as absolute hot.

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