Question

Describe Young’s double-slit experiment with a neat diagram showing points of maximum and minimum intensity. 

Answer 1

1. In Young’s double-slit interference experiment, a plane wavefront is made to fall on an opaque screen AB having two similar narrow slits S₁ and S₂.
2. The figure below shows a cross-section of the experimental setup and the slits have their lengths perpendicular to the plane of the paper.
   
   Young’s double-slit experiment
3. The slits are about 2 - 4 mm apart from each other.
4. An observing screen PQ is placed behind AB.
5. Assuming that the slits S₁ and S₂ are equidistant from the S, the wavefronts starting from S and reaching the S₁ and S₂ at every instant of time are in phase.
6. When the rays fall on S₁ and S₂, the two slits act as secondary sources of light-emitting cylindrical wavelets (with axis along the slit length) to the right of AB.
7. The two secondary sources emit waves in phase with each other.
8. The crests/troughs of the secondary wavelets superpose as shown in the figure and interfere constructively high intensity giving rise to a bright band.
9. When the crest of one wave coincides with the trough of the other causing zero intensity, dark images of the slits are produced on the screen PQ.
10. The dark and bright regions are called fringes and the whole pattern is called an interference pattern.