What exactly are shockwaves and how come we can see them?

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I was browsing r/shockwaveporn and shockwaves look like just air? why can we see it tho. ??

In: Physics

They *are* just air, but they’re highly compressed air. Materials which let light pass through, like air or glass or water, have what’s called a refractive index. This is basically a number that describes how much light will bend when it passes from one type of material into another. It happens because the light moves through the two materials at two different speeds. With shockwaves, the energy of the blast compresses air enough to change it’s refractive index, so when light travels through normal air and then hits the shockwave it is bent slightly and is slowed down, then speeds up and bends back when it travels from the shockwave back into normal density air.

Shockwaves are formed when an moving air suddenly changes speed, resulting in a near instant increase in pressure, density, and temperature. During an explosion, either a significant amount of solid material is converted into gases (chemical explosives), pressurized gas released (a balloon popping), and/or energy is released super-heating the air (nuclear weapon explosions). This results in a forced rapid expansion of gases desperately trying to move away from the explosion center.

If the force and pressure is high enough, the air can get pushed away from the blast faster than the speed of sound in air, meaning that the surrounding air can’t get out of the way of the supersonic moving gases. They bunch up together forming the shockwave front that moves radially away from the explosion, visible by the significant change in air density right behind the shock per u/aseiden’s comment.