The Wave Nature of Light - Sections 24.4 - 24.6

Announcements Review Diffraction by a Single Slit Final QuestionsYoung’s double-slit experiment: interference patternThe slits S 1 and S 2 are separated by a distance d, and the source is monochromaticAssume that the viewing screen is located a perpendicular distance L ≫ d from the barrier containingthese slitsTo reach any arbitrary point P in the upper half of the screen, a wave from the lower slit must travelfarther than a wave from the upper slit by a distance (called the path distance ∆D)∆D = D 1 − D 2 = d sinθThe value of ∆D determines whether the two waves are in phase when they arrive at point PIf ∆D is zero or some integer multiple of the wavelength, the two waves are in phase at point P andconstructive interference occursTherefore, the condition for bright fringes (or constructive interference) at point P isd sinθ bright = mλ m = 0,±1,±2,...When ∆D is an odd multiple of λ/2, the two waves arriving at point P are 180 ◦ out of phase and giverise to destructive interferenceTherefore, the condition for dark fringes (or destructive interference) at point P is(d sinθ dark = m + 1 )λ m = 0,±1,±2,...2The Wave Nature of Light