• Tags: combined, interference, diffraction
• Difficulty: Advanced

In reality, YDSE fringes are modulated by the single-slit diffraction envelope. The intensity pattern is: I = $4I_{0}$c$os^{2}$(πdsinθ/λ) × [sin(πasinθ/λ)/(πasinθ/λ)]^{2}. The c$os^{2}$ factor gives the interference fringes; the si$nc^{2}$ factor gives the diffraction envelope. Missing orders occur when a diffraction minimum coincides with an interference maximum: d/a = integer. For example, if d = 3a, the 3rd, 6th, 9th... interference maxima are missing. This is an advanced JEE topic tested occasionally.