How does the shock wave traveling from ground zero of a nuclear weapon block out the light behind it from the perspective an observer looking at the explosion?

Question

Essentially the title, how does the shock wave from a nuclear weapon block the light behind it producing the characteristic double flash of a nuclear detonation. I don’t understand how the compression of air renders it opaque to the light behind it.

In: Physics

Answer 1

It’s not that it’s become opaque, but that the light gets refracted in a particular way. For example why is the sky blue? The light coming from the sun isn’t blue, but the atmosphere interacts with the light in such a way that it appears blue to us, meaning air/atmosphere can affect light.

In a normal just sitting there everyday view you wouldn’t see this be cause all the air is evenly distributed with no crazy pressure waves affecting it. But then you have the crazy pressure wave of a nuclear explosion and it causes light refraction that happens to direct it away from the observer, momentarily. It’s that “wavy lines above a hot road” thing taken to a particular extreme.

Answer 2

At high enough densities/temperatures, air becomes opaque to visible light. The densities/temperatures necessary to do this are _ridiculously_ high — hence the “dimming” effect only lasts for a second or so at most, even for a huge nuke, and we don’t see it in other “very hot” phenomena (like the surface of a Sun, which is a lot cooler than the nuked air at that moment).