My question is basically just what’s in the title.
In: Technology
Just like an optical lens focuses light, an explosive lens focuses an explosion. It’s essentially a series of explosives that, added together, cause the detonation wave to converge on a point, rather than expand all over.
I find it easier to explain less with the light analogy, and more if you think of the explosive as an expanding wave of pressure. Imagine you had a basketball of TNT, and you started the detonation right in the center of it. You’d expect it to radiate outward more or less symmetrically as an expanding sphere, [like this](https://imgur.com/sBJjqT4).
OK, now imagine that we cut our basketball of TNT in half, and join it up with an explosive that is like TNT but has a slower detonation velocity. Now we detonate it at the enter again. In practice it would be pretty messy but we could imagine that the detonation wave is going to move through the basketball in a lopsided way, [like this](https://imgur.com/iUyQUGr).
This is basically the principle that explosive lenses use. With different velocity explosives, you can shape the detonation wave. In a nuclear bomb, each lens is a combination of fast and slow detonation velocity explosives, so that when you detonate them at the top, the explosive force warps [like this](https://imgur.com/krUpxkS).
That’s only one lens, so it’s incomplete and wouldn’t work quite right (the detonation waves on the side would in reality also be hitting the detonation waves created by the other lenses), but you can see that by putting a high angle of slow velocity explosives in there, you are causing the entire detonation wave to warp around and sort of invert itself. The goal here is to make the blast pressure push down on a sphere in the center of the bomb equally from all sides, and so this “inversion” allows that to happen. You’re basically slowing down the center of the expanding explosive sphere, and letting the edges speed ahead, so that it flips around. [Here’s a kind of cruddy diagram](https://nuclear-knowledge.com/wsimages/04_explosives_09.jpg) that shows how these waves would interact and convert on the center core. Again, they start out as explosions on the edge of the bomb, but the explosive lens are what make them converge spherically on the center, using these different detonation velocities to slow or speed up the detonation depending on where it is.
Strictly speaking one should just call this something other than an “implosion” because you’re really just shaping an “explosion.” (A true implosion would be somehow “sucking” something from within, like if you suck all of the air out of a vessel and it internally collapses.)
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