May the nature of a EM wave be changed after interacting with some apparatus? In Young's double slit experience, if we assume plane waves to cross the holes, they continue to be plane waves? 
I ask that because on the internet I see plenty of images with spherical waves, I believe that is due to Huygens's principle.

source: wikipedia commons.
So I ask: may a plane wave become a spherical wave? As suggested in the picture?
A professor of mine did such approach in a class and got similar results as with planes waves.
I would answer: not because the first is not physical wave (the energy does not decrease with distance) the opposite of spherical waves. So believing that an unphysical thing cannot become a physical thing, a plane wave cannot be transformed into a spherical wave.
 A: 
may a plane wave become a spherical wave? As suggested in the picture?

Hope you have time forreading a little bit longer considerations.
Throw a stone into water. Why do you think waves are produced and why thy are expand in circles? What you need is a compressible medium [and for better visualization the observation happens on the boundary between this and a less dense medium (water to air for example)]. By compressing the medium in one direction — from the falling stone — the medium dodge around the falling stone. You get the first crest, not surprisingly in the form of a circle. But the medium has to be not only compressible but also elastical. For an elastical medium local compressions overgoing into relaxations with less density of the medium. Since there is a phase shift from the location of the thrown stone into the first crest this shift continues and the crest(s) expand always in circles. This expansion is called dissipation.
What changes if we throw a rod into the pound? The compression are following the shape of the rod and on both sides the crests are plane waves. At the end of the rod we still get (half)circular waves. So the dissipation follows the shape of the wave generator.
What a obstacle is doing with a wave dissipation? An obstacle is a medium without dissipation capabilities (at least theoretical). But the energy of the wave has to go somewhere and so the wave gets reflected. Behind the edge of an obstacle the wave can dissipate again and this happens again in circles, this time in a quarter circle.
For all this waves it is important to understand that the dissipation of a wave is a damping process, it will be no more observable nor existent if the dissipated energy in some volume is lower the thermal kinetic energy in this volume.
A good example for a long lasting plane wave is this of a plane wave inside a long canal. This wave can run for kilometers. Circular waves will occur only at positions the canal has edges.

So believing that an unphysical thing cannot become a physical thing, a plane wave cannot be transformed into a spherical wave.

That you are right. Proof it with the explanations I gave. Indeed in the above sketch the waves are running in circle left and right the slits, but between the edges the waves still plane waves.
