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 Z_F called the Fresenl's distance by the following equation Z_F= a²/y.

For distance greater than Z_F 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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