Q. What do you mean by diffraction? Distinguish between Fresnel's and Fraunhofer's classes of diffraction.
Ans. The phenomena of diffraction in case of light was first observed by Grimaldil. According to this phenomenon when a obstacle is placed is path of lighr, the light waves bends around the obstacle and enter in the region of the geometrical shadow of the obstacle, is called diffraction of light. In other words, the deviation of light from its rectilinear propagation is called diffraction.
Diffraction of waves depends on the size of obstacle wavelength of waves.
When slit width is comparable to the wavelength of light, there is almost total diffraction condition.