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InterviewSolution
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Explain that for a greater distance, the spreading due to diffraction dominates over due to ray optics. |
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Answer» Solution :An APERTURE of size .a. illuminated by a parallel beam sends diffracted light into an angle `theta` , then angular width `theta~~(lamda)/(a)` In travelling a distance z, the diffracted beam acquires a width `=ztheta` `=(zlamda)/(a)` DUE to diffraction The value of z the spreading due to diffraction is equal to the size of a of the aperture. `:.a=(zlamda)/(a)` `:.z=(a^(2))/(lamda)` This quantity is called the Fresnel distance `z_(F)`. `:z_(F)=(a^(2))/(lamda)` This equation shows that for distances much smaller than `z_(F)` the spreading due to diffraction is smaller compared to the size of the beam (That means moving in a straight line). Hence, spreading due to diffraction DOMINATES over that due to RAY optics and it shows that ray optics is VALID in the limit of wavelength tending to zero. |
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