Describe Young's double slit experiment with a labelled diagram. Write the expression for fringe width and state how it depends on the wavelength, the screen distance and the slit separation.

1. 1 · State the principle. Light from two coherent sources superposes and forms interference fringes.
2. 2 · Draw the set-up. [Source slit S; double slit S₁ and S₂ with separation d; screen at distance D; overlapping wavefronts; point P at height x; alternate bright and dark bands with the central bright fringe at O]
3. 3 · Explain the working. S₁ and S₂ take light from one wavefront, so they are coherent. Their waves overlap, and the path difference decides bright or dark.
4. 4 · Write the conditions.
   • Bright: path difference nλ, xₙ = nλD/d
   • Dark: path difference (n + ½)λ, xₙ = (n + ½)λD/d
5. 5 · Give the fringe width. β = λD/d (final expression); all fringes are equally wide.
6. 6 · How the fringe width changes. β ∝ λ and β ∝ D, but β ∝ 1/d: red light, a farther screen or closer slits give wider fringes.
7. 7 · Real life. A street lamp seen through muslin cloth shows a similar bright pattern.
8. 8 · Close in one line. So, Young's experiment proves the wave nature of light.

Scoring tip: Label d and D clearly on the diagram, and write β = λD/d with each symbol named.