Two narrow slits are illuminated by a single monochromatic source. Name the pattern obtained on the screen. One of the slits is now completely covered. What is the name of the pattern obtained now on the screen ? Draw intensity pattern obtanied in the two cases. Also, write two difference between the pattern obtained in the above two cases.

Explanation

When two narrow slits are illuminated by a single monochromatic source, the pattern obtained on the screen is interference pattern consisting of alternate bright and dark fringes.
When one of the slits is covered completely, no interference occurs. What we obtain is diffraction pattern. Intensity patterns in the two cases are shwon in Fig. `(a)` and `(b)`, respectively.
INTERFERENCE PATTERN :
(1) All bright fringes are equally bright.
(2) Dark fringes are completely dark.
DI FFRACTION PATTERN :
(1) The brightness of successive bright fringes from the centre goes on decreasing.
(2) Dark fringes are not completely dark.