Question about Huygen's principle

[toesockshoe]

Huygens principle states that every point on a wave is a source of a secondary wavelet, so when you shine a light at a single slit, you get fringe patters. How come when you shine a normal light at a wall (without any slit), you don't get fringe patters? Surely, if every point on a wave is a source of a secondary wave, then pointing a light at a wall means these infinite number of waves will interfere and produce constructive and destructive interference... Why can't we see these bright and dark spots on the wall though like we do in a single slit?

 

[mfb]

You get the pattern that your source gives you: a laser pointer is like a very wide single slit, and will produce a very sharp single-slit pattern roughly as wide as the slit. Some approximations used for usual single slits are not valid here any more, but the idea is still the same.
A light bulb emits incoherent light, so you have the sum of many point-like emitters without any slit. The result is a uniform illumination.

A window is just a very wide single slit - so wide that the interference pattern is indistinguishable from classical optics with light rays.

 

[sophiecentaur]

infinite number of waves will interfere and produce constructive and destructive interference.

A large number of sources, with random phases, will not produce any pattern because the sum at any point will be totally random. If you take a pair of small slits and allow white light from only one direction to go through them, then there is some amount of coherence and you may get a couple of nulls on either side of the on axis peak. Use a laser source and the coherence of the light over its whole width will be very high and the resulting pattern may be extremely wide, with many peaks and nulls.
But there will be diffraction when any light passes through any a hole or obstruction - it's just a matter of degree.