Lectures on diffraction

# Lectures on diffraction - Diffraction Light of wavelength...

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Diffraction Light of wavelength comparable to or larger than the width of a slit spreads out in all forward directions upon passing through the slit This phenomena is called iffraction Notes diffraction – This indicates that light spreads beyond the narrow path defined by the slit into regions that would be in shadow if light traveled in straight lines – This follows from the Huygens’s principle Narrow slit Slit width ~ λ Wave optics Wide slit Slit width >> λ Geometrical optics

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Diffraction Pattern • A single slit placed between a distant light source and a screen produces a diffraction pattern – It will have a broad, intense central band ( central maximum ) – The central band will be flanked by Notes a series of narrower, less intense secondary bands ( side or secondary maxima ) – The central band will also be flanked by a series of dark bands (minima)
Diffraction Pattern, Single Slit • The diffraction pattern consists of the central maximum and a series of secondary maxima and minima • The pattern is similar to an terference pattern Notes interference pattern • Where does the interference come from- there is only one source of light ?

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Poisson’s Spot Fresnel was the first person to argue that the wave nature of light will produce diffraction pattern When he gave a talk on the subject at the French Academy of Sciences, after a quick back-of-the-envelop calculation, Poisson pointed out that resnel’s theory is absurd as it Notes Fresnel’s theory is absurd as it predicts a bright spot in the center of the shadow of a disc . Later, when this bright spot was discovered by Arago, it was named “Poisson’s spot” Simeon Poisson 1781-1840 Augustin Fresnel 1788-1827 Shadow of a disc
Fraunhofer Diffraction Pattern A Fraunhofer diffraction pattern occurs when the rays leave the diffracting object in parallel directions – Screen very far from the slit Notes – Could be accomplished by a converging lens

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Fraunhofer Diffraction Pattern Photo • A bright fringe is seen along the axis ( θ = 0) • Alternating bright and dark fringes are Notes seen on each side
Diffraction vs. Diffraction Pattern Diffraction refers to the general behavior of waves spreading out as they pass through a slit (a consequence of the Huygens’s principle) • A diffraction pattern is actually a misnomer that is deeply ntrenched Notes entrenched – The pattern seen on the screen is actually another interference pattern – The interference is between parts of light waves originating from different regions of the slit

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Single-Slit Diffraction • The finite width of slits is the basis for understanding Fraunhofer diffraction • According to Huygens’s principle, each portion of the slit acts as a ource of light waves Notes source of light waves • Therefore, light from one portion of the slit can interfere with light from another portion
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Lectures on diffraction - Diffraction Light of wavelength...

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