Question
What is the effect on the interference fringes in Young’s double slit experiment due to each of the following operations? Justify your answers.
  1. The screen is moved away from the plane of the slits.
  2. The separation between slits is increased.
  3. The source slit is moved closer to the plane of double slit.

Answer

  1. Linear fringe width increases $\beta=\frac{\lambda\text{D}}{\text{d}}$
$\Rightarrow\beta\propto\text{D}$

No effect on angular fringe width $\Big(\text{Q}=\frac{\lambda}{\text{d}}\Big).$
  1. Both linear fringe width & angular fringe width decrease $\Big(\beta\propto\frac{1}{\text{d}},\text{Q}\propto\frac{1}{\text{d}}\Big).$
  2. If condition $\frac{\text{s}}{\text{S}}<\frac{\lambda}{\text{d}}$ is satisfied, interference will be obtained otherwise, no interference will be obtained.

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