DIFFRACTION Tran Thi Ngoc Dung – Huynh Quang Linh – Physics A2 HCMUT 2016 Contents • Diffraction by a single slit • Diffraction by N slits • Diffraction Grating • X ray diffraction DIFFRACTION • Diffr[.]
DIFFRACTION Tran Thi Ngoc Dung – Huynh Quang Linh – Physics A2 HCMUT 2016 Contents • Diffraction by a single slit • Diffraction by N slits • Diffraction Grating • X-ray diffraction DIFFRACTION • Diffraction is the deviation of light from a straight-line path when the light passes through an aperture or around an obstacle • Diffraction is due to the wave nature of light Huygens’s Principle M r O This principle states that we can consider every point of a wave front as a source of secondary wavelets The position of the wave front at any later time is the envelope of the secondary wavelets at that time To find the resultant displacement at any point, we combine all the individual displacements produced by these secondary waves, using the superposition principle and taking into account their amplitudes and relative phases M O d Diffraction BY A SINGLE SLIT Given a narrow slit of width a A parallel light beam of wavelength is radiated perpendicularly to the slit The beam is diffracted into many different directions + Consider the diffracted rays of angle =0 These rays are focused at F All these rays are in phase, constructive interference F is a bight central fringe + Consider the diffracted rays of angle We draw planes o, 1, 2, each of /2 apart , perpendicular to the diffracted rays These surfaces divide the slit into strips The width of each band is: sin The number of bands is: N width of slit width of band M slit /2 sin F 1 /2 a 2a sin sin o MQ S The optical path difference between the rays of adjacent strips is /2 There is destructive interference =>The combined light from adjacent strips is completely cancels at M M slit => If the number of strips is EVEN, M is DARK If the number of trips is ODD, M is BRIGHT /2 sin F 1 o /2 2asin λ 2m sin m λ a 2asin λ Bright Fringe : N 2m sin (2m 1) λ 2a Dark Fringe : N MQ S m 1;2;3 (2m 1) 3;5 Central Bright Fringe : sin Dark Fringe : Bright Fringe : λ m 1, 2, a λ sin (2m 1) 2m 3;5 2a sin m Intensity in the Single-Slit Diffraction Pattern sin / I Io /2 2 a sin Io 2 2 Io : I1 : I : I3 : : : : : 3 5 : 0.0472 : 0.0165 : 0.0083 I1 I2 + the central maximum is twice as wide as each side maximum + Central Bright Fringe : sin λ m 1, 2, a λ Bright Fringe : sin (2m 1) 2m 3;5 2a If a λ : the central fringe is very narrow no diffraction Dark Fringe : sin m Example 36.1 Single-slit diffraction You pass 633-nm laser light through a narrow slit and observe the diffraction pattern on a screen 6.0 m away The distance on the screen between the centers of the first minima on either side of the central bright fringe is 32 mm (Fig 36.7) How wide is the slit? d=6.0m L 2d tan 2d sin 2d a 2d 633 109 a 0.24mm L 32 10 ...Contents • Diffraction by a single slit • Diffraction by N slits • Diffraction Grating • X-ray diffraction DIFFRACTION • Diffraction is the deviation of light from... fringe is very narrow no diffraction Dark Fringe : sin m Example 36.1 Single-slit diffraction You pass 633-nm laser light through a narrow slit and observe the diffraction pattern on a screen... the superposition principle and taking into account their amplitudes and relative phases M O d Diffraction BY A SINGLE SLIT Given a narrow slit of width a A parallel light beam of wavelength