Single slit diffraction and fringe width I know that intensity of the bright fringes will decrease on moving away from centre and width of the central fringe is  2 times that of the second fringe.
Does the width of other bright fringes decrease too?
Mathematically:

That is all other bright fringes have same width.
But when we observe the diffraction pattern we see the bright fringes becoming narrower.
Why does this happen?
Is this because of decreasing intensity?
 A: 
I have superimposed a ruler on top of a photograph of the diffraction pattern due to a single slit.
To get visible images of fringes which are further out from the centre the central fringes have been overexposed and so the dark fringes look very narrow.  
You will note that the outer fringes do appear to be narrower but the fringe separation does not change.
So as you quite rightly suggested the outer fringes look narrower because they are less bright and the photographic plate / your eye can only get significant amounts of light from the region around the centre of an outer  fringe.  
There is another possible factor and that relates to the fact that the angular separation of the fringes is constant.  
When the diffraction patter is projected onto a flat screen the distance of a fringe from the central fringe will depend on the tangent of the diffracting angle.
if this was a significant effect then the linear separation of the fringes would increase as the order of the fringes increased.
However for most demonstrations like the one photographed the angular spread of the pattern is small ($0.1^c$) and so the approximation $\tan \theta \approx \theta$ is a good one and to a reasonable accuracy the linear separation of the fringes on the screen is constant 
A: This is because interference from a double slit is just a special case of diffraction and for diffraction the central maxima has the highest intensity and then the intensity falls of akin to the sinc function
