Q:

Wavelets from a wavefront interfere to form original rays outside a slit but not inside?

If the wavelets interfere to form the original rays outside the slit, then why in slits so they get spread as per Huygens principle? Why don't the wavelets emerging from wavefront incident on the slit interface to form the original rays the same way as they interfere outside the slit resulting in fringes? And what is the intensity variation of wavelets as an angle between the wavelet and direction of propagation ?

A:

they always can be seen  as elementary waves (you call it wavelets?) they interfere so that straight wavefronts result before the slit. (at the border of your wide beam the do the same as at the slit) in the slit there are less  "wavelets" so you can see separate interference, if you make the slit wider and wider you have the same situation as on the other side of the slit, the minima and maxima get closer and closer till you don't have any.

