What will happen for single slit diffraction if?

What has been said is quite true.
As a good guide you will find that when the slit width = 1 wavelength the angle of the first minimum in the diffraction pattern is 90 degrees.
This means that a slit width of less than 1 wavelength is essentially a 'point source'

 

Hmm. A bit narrower than that, I think, or you will have a first minimum at +/- 90°, iirc, which is not omnidirectional.

 

If you use a slit that is 1 wavelength wide the first MINIMA !!!!! (please note MINIMA) occur at +/-90
There is only the central (0 degrees) maximum.
If the slit is 2 wavelengths wide then the first MINIMUM is at +/-30, the second MINIMUM is at +/-90 so there is a maximum between 30 and 90 (not half way between !!!)

 

Maxima occur with CONSTRUCTIVE interference.
minima occur with DESTRUCTIVE interference.
Are you familiar with 2 source interference....the conditions for the formation of MAX and MIN

 

Quite correct, 1 wavelength width should be seen as an upper limit. With this width some of the wave exists out to +/-90 degrees so wave effects can be detected.
Smaller than 1 wavelength and the intensity decreases, make it small enough to be a better point source and the intensity will be so low that detection of wave effects will be difficult.
Pay your money and make the choice.

 

straight through the slit is 0 degrees.
+/- 90 degrees means 90 degrees on each side of the straight through direction.

"pay your money and make the choice"....sorry....that is a colloquialism, more or less means that you can't have it both ways.
better point source means lower intensity; higher intensity means a less good point source.

 

Have you looked at the single slit diffraction pattern? The first nulls are at 90 to the straight through direction. Did you not write that you are familiar with Fraunhoffer?

 

If you have studied the theory for Fraunhofer's single slit pattern then you must be aware that it works for any width you want. If not, then you need to start with the simple, ideal two slit interference' situation and progress from that.
Do you appreciate that, for two slits of the same width, the resulting pattern can be calculated by 'multiplying' the single slit (wide) pattern by the double slit (fine) pattern? I googled "two slit diffraction pattern" and found an endless supply of hits which give diagrams and 'the maths', plus loads of diagrams - starting with wiki. I am not really sure what extra "visualisation" you are wanting. I can't think of anything simpler than what I have found.

 

You will generate an evanescent field. The geometry you describe is used a lot for near-field scanning microscopy, for example. The details were worked out in the 1940's, as I recall.