Question

We obtained alternate dark and bright regions if we look at the shadow by an obstacle closed to geometrical shadow.
a) Mention the phenomenon behind it.
b) Differentiate the interference pattern with a coherently illuminated single slit diffraction pattern.

Answer 1

a) We obtained alternate dark and bright regions if we look at the shadow by an obstacle closed to geometrical shadow. This phenomenon is known as interference. This was first shown in 1801 by Thomas Young, who sent sunlight through two narrow slits and showed that an interference pattern could be seen on a screen placed behind the two slits. The interference pattern was a set of alternating bright and dark lines, corresponding to where the light from one slit was alternately constructively and destructively interfering with the light from the second slit.
b) Difference between the interference pattern with a coherently illuminated single slit diffraction pattern:

1. The region of minima is perfectly dark in interference while in diffraction it is not perfectly dark.
   
2. Intensity of all maxima and minima is same in interference while it is not true in diffraction, it gradually decreases towards 2nd and 3rd.
3. All fringes are of equal width in interference but in diffraction it is not so.