CBSE NOTES CLASS 12 PHYSICS
CHAPTER 10 WAVE OPTICS
The angle of diffraction due to an aperture (i.e., slit or hole) of size a illuminated by a parallel beam is . This is the angular size of the bright central maximum. In travelling a distance z, the diffracted beam acquires a width due to diffraction.
The distance beyond which the divergence of beam of width ‘a’ becomes significant is called Fresnel distance, at which point, is denoted by zF
For distances much smaller than zF, the spreading due to diffraction is smaller compared to the size of the beam. It becomes comparable when the distance is approximately zF. For distances much greater than zF, the spreading due to diffraction dominates over that due to ray optics (i.e., the size ‘a’ of the aperture). The ray optics is valid in the limit of wavelength tending to zero.