Diffraction of light when two fingers brought close together infront of a light source

Diffraction by razor blade when illuminated by intense blue light

“light is never known to follow crooked passages nor to bend into the shadow”. Sir Isaac Newton ( )

“Any deviation of light rays from rectilinear path which is neither reflection nor refraction known as diffraction.’’ Arnold Johannes Wilhelm Sommerfeld ( )

Diffraction of Sound

Radio waves diffract around mountains. When the wavelength is km long, a mountain peak diffract the wave. Another effect that occurs is scattering – role of diffraction is not obvious.

Huygens’s Principle “Every point in a propagating wavefront serves as the source of spherical secondary wavelets, such that the wavefront at some later time is the envelope of these wavelets.” Huygens-Fresnel Principle Every unobstructed point of a wavefront, at a given instant, serves as a source of spherical secondary wavelets, The amplitude of the optical field at any point beyond is the superposition of all these wavelets.

Reflection and Refraction of waves © 2006 Walter Fendt

Christiaan Huygens ( ) Augustin Fresnel ( )

More refinement by Kirchhoff and Sommerfeld Gustav Robert Kirchhoff ( ) Arnold Johannes Wilhelm Sommerfeld ( )

Classical model of diffraction wavefront obstacle screen

On obstacle, the electron oscillator vibrating and reemittting at source frequency Incident field and field of all vibrating electrons superpose in such a way that there is zero field beyond the obstacle. Assume the mutual interaction between the oscillators are essentially negligible.

Classical model of diffraction

Diffraction of a wave by a slit Whether waves in water or electromagnetic radiation in air, passage through a slit yields a diffraction pattern that will appear more dramatic as the size of the slit approaches the wavelength of the wave. Narrower the slit, the wider the pattern A A A B B B >AB <AB

Fraunhofer and Fresnel Diffraction Joseph von Fraunhofer ( ) Augustin Jean Fresnel ( )

Fraunhofer vs. Fresnel diffraction In Fraunhofer diffraction, both incident and diffracted waves may be considered to be plane (i.e. both S and P are a large distance away) If either S or P are close enough that wavefront curvature is not negligible, then we have Fresnel diffraction P S S P

Fraunhofer limit diffraction If aperture (obstacle) has a width a Fresnel limit diffraction d is the smaller of the two distances from S and  and P

Fresnel diffraction pattern does change in shape as we move further away from the object (until, of course, we are so far away that the Fraunhofer condition is satisfied). The surface of calculation

Fraunhofer or far field diffraction Fresnel or near field diffraction

Fresnel –Fraunhofer Diffraction Far from the slit z Close to the slit Incident plane wave

Superposition of N Oscillators

1. Optics Author: Eugene Hecht Class no. 535 HEC/O Central library IIT KGP