Question

Why all points on a wavefront producing secondary wavelets interfere and produce a pattern ONLY when they diffract (via single slit or something) .. Why don't those secondary wavelets interfere just like that always? cause Huygen says, that principle is always true.. so everything should produce interference pattern :P

Answer 1

i think it this way,,, if u have a line of sources (like a miniature tube light) ,then we know the wavefronts look like , cylindrical wavefronts ,,,,but i have considered a line source that is INFINITELY long ,, so we must imagine that we have HUGE cylinders that seem to have INFINITE HEIGHT , travelling radially outwards,,,,,,,,


BUT what happens when the lenght is NOT so long but SMALL,,, then we apply huygens principle,,,,,,,,,,when PLANE wavefronts arrive the slit ,, the wavefront (which is not AS LONG AS ITS other wavefronts ,,, bcos it is between the slits ,,while others are not bound by any slits) ,,, so this shortened wavefront bcomes the source of secondary wavelets
so u see , this is NOT an INFINITELY long source ,,but has its end points,,,,
The points in the middle emit wavefronts(secondary) as usual ,,just like that in the INFINITE case,,,,,but the 2 sources that lie in the END POINTS OF THE WAVEFRONT cause the whole change,,,,,,u see , every point on the wavefront is egoistic and WANTS to emit SPERICAL wavefronts ,,irrespective of its position ,,,,, When multiple spherical wavefronts overlap in the MIDDLE, they look like CYLINDERS ,,,bu at the ENDS u see , they ARE SPHERICAL,,,,, this is how light bends ,,,,, (i feel :D)