momentum change of phase of light in lense I am wondering about the following question: 
If a perfect plane wave passes through a biconvex lense, then in the middle where the lense is the biggest, the light passes the longest time in the medium whereas on the upper/lower part of the lense, the light still passes some time in air where it is quicker than in the lense. Therefore, if one looks at the plane of constant phase, this plane will be tilted a little.
So, I get why the shape after the lense looks like a "spherical wave". But what I don't get is why the whole wave then converges to one focal point. Why doesn't it propagate through space straight without converging to one point keeping the shape that it directly had after the lense. I added a picture:

So why does the wave converge to one point? Why is the momentum changed within the lense? I get why the shape of the planes with constant phase change, but not why they should converge.
I got the picture from here http://www.schoolphysics.co.uk/age16-19/Optics/Refraction/text/Lenses_and_waves/index.html
 A: This is a good question: after some thought, I can't think of a more "everyday" or "intuitive" explanation than the following.
Once you know Huygens' Principle, you can reason the convergence / divergence intuitively: destructive interference prevents the kind of translational motion of wavefronts that you propose. If you put any significant amount of spherical emitters along a spherical wavefront and sum up their contributions coherently, you get converging, not translating, spherical wavefronts. 
You can also take the behavior as following from the uniqueness of solution theorems (or principles) for certain kinds of partial differential equations with appropriate boundary conditions. In this case, the equation is D'Alembert's wave equation - or, in a one-frequency, monochromatic case, Helmholtz's equation. Field arrangements with spherical phase fronts, even with wildly varying intensities along the phase fronts, give rise to converging or diverging spherical wavefronts, depending on the direction of motion.
The above is actually Huygens' principle in disguise: one of the ways of solving the wave equation is through a Green's function method: a Green's function is roughly a kind of "atomic" solution to a linear differential equation which all solutions can be made up of. In the case of the Helmholtz's wave equation, the Green's function is a spherical wave point source, with amplitude distribution of the form $e^{i\,k\,r}/r$. Using this Green's function, one can show that Huygens' principle is true from the mathematical properties of the Helmholtz wave equation. Both the texts Born&Wolf and Hecht&Zajac do this. 
