A parallel beam of monochromatic light falls normally on a narrow slit and the light coming out of the slit is obtained on a screen kept behind parallel to the slit plane.

what kind of pattern do we observe on the screen and why?how does the (i)angular width (ii)linear width of the principal maximum in this pattern changes and the screen is moved ,parallel to itself,away from the slit plane?state two pointsof diffeerence b/w this pattern and the interference pattern observed in the young'double slit experiment .

We shall see a Fraunhoffer diffraction pattern on the screen.

1. Angular width of the central maximum is given by 

θ = 2λ/a

Since the angular width does not contain any screen distance D, so it does not change.

Again since θ is small thus

θ≈tanθ

2. Now linear width x of the central maximum is given by

x = 2λD/a

Thus if the screen is moved away the linear width of the central maximum will increase too.

3. Difference between interference and diffraction

1. In interference all the fringes will be of equal intensity but in diffraction the central maximum will have high intensity and in the rest of the fringes intensity falls rapidly.

2. In interference all the fringes will be of equal width but in diffraction the central maximum will have the highest width and for the other fringes width will diminish fast.