KEAM 2021 Physics and Chemistry Solved Paper

When a slit or hole size a is illuminated by a parallel beam, then it is diffracted at an angle of ‌

λ

a

.
In travelling a distance z, size of beam is ‌

zλ

a

.
So, ‌

zλ

a

≥a
or z≥‌

a2

λ

where, z is Fresnel's distance. So, image formation can be explained by ray optics for distance less than zf.