The box of a pin hole camera, of length $L$ , has a hole of radius a. It is assumed that when the hole is illuminated by a parallel beam of light of wavelength $\lambda $ the spread of the spot (obtained on the opposite wall of the camera) is the sum of its geometrical spread and the spread due to diffraction. The spot would then have its minimum size (say $b_{min}$ ) when :

$a = \sqrt{\lambda L}$ and $ b_{min } = \sqrt{4 \lambda L}$

The box of a pin hole camera, of length $L$ , has a hole of radius a. It is assumed that when the hole is illuminated by a parallel beam of light of wavelength $\lambda $ the spread of the spot (obtained on the opposite wall of the camera) is the sum of its geometrical spread and the spread due to diffraction. The spot would then have its minimum size (say $b_{min}$ ) when :}

$a = \sqrt{\lambda L}$ and $ b_{min } = \sqrt{4 \lambda L}$

$a = \frac{\lambda^{2}}{L}$ and $ b_{min } = \left(\frac{2 \lambda^{2}}{L}\right) $

$a = \sqrt{\lambda L}$ and $ b_{min } = \left(\frac{2 \lambda^{2}}{L}\right) $

$a = \sqrt{\lambda L}$ and $ b_{min } = \sqrt{4 \lambda L}$

$a = \frac{\lambda^2}{ L}$ and $ b_{min } = \sqrt{4 \lambda L}$

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Considering two waves interfering at point P, having different distances. Consider a monochromatic light source ‘S’ kept at a relevant distance from two slits namely S₁ and S₂. S is at equal distance from S₁ and S₂. SO, we can assume that S₁ and S₂ are two coherent sources derived from S.
The light passes through these slits and falls on the screen that is kept at the distance D from both the slits S₁ and S₂. It is considered that d is the separation between both the slits. The S₁ is opened, S₂ is closed and the screen opposite to the S₁ is closed, but the screen opposite to S₂ is illuminating.
Thus, an interference pattern takes place when both the slits S₁ and S₂ are open. When the slit separation ‘d ‘and the screen distance D are kept unchanged, to reach point P the light waves from slits S₁ and S₂ must travel at different distances. It implies that there is a path difference in the Young double-slit experiment between the two slits S₁ and S₂.