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Interference of Light Waves and Young’s Experiment

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Estimated time: 9 minutes
CBSE: Class 12

Key Points: Interference of Light Waves and Young's Experiment

  • Interference = redistribution of energy when two coherent waves superpose.
  • Based on energy conservation, total energy remains constant, only redistributed.
  • Constructive: I > (I1 + I2) → bright fringe
  • Destructive: I < (I1 + I2) → dark fringe

Conditions for Sustained Interference:

  • Sources must be coherent.
  • Separation between sources must be small.
  • Distance of screen from sources must be large.
  • For good contrast: amplitudes of the two waves should be nearly equal.
  • Two sources must propagate along same line.

Young's Double Slit Experiment (YDSE):

Setup: Light source → single slit → double slit (S₁ and S₂, separation d) → screen (distance D).

Path difference at point P: \[\delta=S_2P-S_1P=\frac{x_n\cdot d}{D}\]

Bright Fringe (Constructive): Path difference = even multiple of λ/2

δ = nλ, n = 0,1,2,3...

Dark Fringe (Destructive): Path difference = odd multiple of λ/2

\[\delta=(2m-1)\frac{\lambda}{2},\quad m=1,2,3...\]
CBSE: Class 12

Formula: Position of n-th bright fringe

\[\begin{array} {c}x_n=\frac{n\lambda D}{d}=n\beta \end{array}\]

CBSE: Class 12

Formula: Position of m-th dark fringe

\[\begin{array} {cc} & x_m=\frac{(2m-1)\lambda D}{2d} \end{array}\]

CBSE: Class 12

Formula: Fringe Width (β)

\[\beta=\frac{\lambda D}{d}\]

CBSE: Class 12

Formula: Angular fringe width (α)

\[\alpha=\frac{\beta}{D}=\frac{\lambda}{d}\]

CBSE: Class 12

Formula: Distance between n-th bright and m-th dark

\[x_n-x_m=\left[n-\frac{(2m-1)}{2}\right]\beta\]

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