Two identical narrow slits ${\mathrm{S}}_1$ and ${\mathrm{S}}_2$ are illuminated by the light of a wavelength $\lambda$ from a point source $\mathrm{P}$.



If, as shown in the diagram above, the light is then allowed to fall on a screen, and if $n$ is a positive integer, the condition for destructive interference at $\mathrm{Q}$ is

• A

  $\left(l_1-l_2\right)=\left(2\text{n}+1\right)\lambda /2$
• B

  $\left(l_3-l_4\right)=\left(2\text{n}+1\right)\lambda /2$
• C

  $\left(l_1+l_2\right)-\left(l_3+l_4\right)=\text{n}\lambda$
• Correct answer
  D

  $\left(l_1+l_3\right)-\left(l_2+l_4\right)=\left(2\text{n}+1\right)\lambda /2$

Hence, the correct answer is option: 'D'.