CBSE Class 12-science Answered
An aperature of size a illluminated by a parallel beam sends diffracted light into a angle of approximately ~y/a. This is the angular size of the bright central maximum. In travelling a distance z, the diffracted beam therefore acquires a width zy/a due to diffraction.The distance beyond which divergence of the beam becomes significant is approximately, z ~ a2/y. This is defined as ZF called the Fresenl's distance by the following equation ZF= a2/y.
For distance greater than ZF the spreading due to diffraction cannot be neglected.
Normally for most slit widths used in experiments, the distance beyond which the effects of diffraction can be detected is very large( may be a few metres). Now you are making most observations within a distance where you cannot detect these effects of diffraction. So neglecting diffraction effects and using ray optics does not give us wrong results.
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