When waves encounter obstacles like slits, they diffract or bend around the edges. Diffraction can be explained by Huygens' principle, which says each point on a wavefront acts as a new source. For a single slit, the new wavefront shape is determined by combining spherical wavelets from points across the slit. There are two types of diffraction: Fresnel, where distances are finite, and Fraunhofer, where incident waves are plane waves. X-ray diffraction uses wavelengths comparable to atomic sizes to determine crystal and molecular structures.

1. Diffraction

3. DIFFRACTION S S A B FIG 1.1

6. Diffraction by single slit,& its pattern.

11. Fresnel diffraction Fraunhofer diffraction S S

16. Bragg Diffraction

20. Single Slit Diffraction Notice that central maximum is twice as wide as secondary maxima Sin  = m  / W, Destructive Dark Fringes on screen y = L tan  L (m  W  Maxima occur for y= 0 and, y  L (m  1/2)(  W  m=1 m=  1 L