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 now obtained on the screen? Draw the intensity pattern obtained in the two cases. Also write two differences between the patterns obtained in the above two cases.
Answer: With both slits open: an interference pattern. With one slit covered: a single-slit diffraction pattern.
Solution
With both narrow slits open and fed by one monochromatic source, the two slits act as coherent sources and an interference pattern is obtained. When one slit is covered, only one slit remains and a single-slit diffraction pattern is obtained. The two intensity distributions are shown in the figures: interference gives equally spaced fringes of equal brightness, while diffraction gives a broad bright central maximum with weaker side maxima. Two differences: (i) Interference fringes are equally spaced and of equal width; diffraction fringes are not equally spaced (the central band is twice as wide). (ii) In interference all bright fringes have the same intensity and the minima are perfectly dark; in diffraction the intensity of the maxima falls off rapidly and the minima are not perfectly dark.
