Interference and Diffraction Physics 014 Interference and Diffraction

Topics Light as a wave Diffraction Young’s interference experiment Intensity & Double slit interference

Light as a wave Huygen’s Principle: All points on a wavefront serve as point sources of sperical secondary wavelets. After a time t, the new position of the wavefront will be that of a surface tangent to these secondary wavelets.

Light as a wave

Light as a wave

Light as a wave

Light as a wave The time taken to go from point e to point c EQUALS the time taken to go from point h to point g, so

Light as a wave Remember that for the index of refraction c=speed of light in vacuum, v=speed of light in material

Light as a wave Using these expressions we can rederive Snell’s law of refraction.

??? The figure shows a monochromatic ray of light traveling across parallel interfaces, from an original a, through layers of materials b and c, and then back into a. Rank the materials according to the speed of light in them.

Light as a wave Notice that although the wavelength changes between two media, The frequency does NOT change between two media.

Light as a wave The wavelength change in media means that a phase difference between two light waves can change if they travel through two different media.

Diffraction If a wave hits a barrier with an opening, the part of the wave that passes through the barrier will flare out. (diffract)

Diffraction

Diffraction

Diffraction

Young’s Interference Exp. Young’s double slit experiment displayed a characteristic interference pattern

Young’s Interference Exp. Maxima: bright bands, bright fringes Minima: dark bands, dark fringes

Young’s Interference Exp.

Young’s Interference Exp. Path difference Maxima Minima m=0,1,2,…

Young’s Interference Exp.

??? For the diagram, what are ΔL (as a multiple of wavelength) and the phase difference (in wavelengths) for the two rays if point P is (a) a third side maximum and (b) a third minimum

Single slit diffraction Let’s consider plane waves diffracted by a single long, narrow slit of width a on a screen

Single Slit Diffraction

Single Slit Diffraction

Single Slit Diffraction The minima may be found using, a sinθ = m λ for m = 1,2,3, …

Single slit diffraction The intensity for the single slit diffraction pattern is

Diffraction by circular apperature

Double Slit Diffraction

Diffraction Grating

Interferometers

Interferometers