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Diffraction and Interference • A wave generated by a single point source spreads in all directions. After traveling a large distance, the arriving wave fronts may appear to be flat. How did a wave that “flattened out” begin to spread again after passing through an opening? Huygens’ Principle • Wave Fronts are made up of tinier wave fronts. • http://www.launc.tased.edu.au/on http://www.launc.tased.edu.au/online/sciences/Physics/diffrac.html line/sciences/Physics/diffrac.html Huygen’s Principle • Waves spreading out from a point source may be regarded as the overlapping of tiny secondary wavelets, and that every point on any wave may be regarded as a new point source of secondary waves. Huygens’ Principle • Think: Can two or more small waves combine into one big wave? Huygens’ Principle • Think: Can two or more small waves combine into one big wave? Huygens’ Principle • So can’t you think of one big wave as the combination of two or more small waves? THAT’S HUYGENS’ PRINCIPLE! Diffraction • A result of Huygens’ principle is that waves are constantly regenerating themselves. This becomes apparent when there is an obstacle or barrier. Diffraction with water waves • Notice with the water waves that the diffraction was more obvious when the opening was small compared to the wavelength • “Kissing Fingers Demo” Helpful Diffraction • The longer the wave compared with the obstruction, the greater the diffraction. • This is why AM radio stations get better reception in the mountains! They are longer wavelength, so bend around mountains. Annoying Diffraction • Ever wonder why you can’t make a microscope to “see” an atom in visible light? Annoying Diffraction • Ever wonder why you can’t make a microscope to “see” an atom in visible light? • If the size of object is the same as the wavelength of light, the image will be blurred by diffraction. Smaller objects won’t be seen at all Annoying Diffraction • Ever wonder why you can’t make a microscope to “see” an atom in visible light? • If the size of object is the same as the wavelength of light, the image will be blurred by diffraction. Smaller objects won’t be seen at all • No optical microscope can be built big enough or designed well enough to overcome this diffraction limit. • Electron microscopes used to “see” very small things Check your understanding • Why is blue light used to view tiny objects in an optical microscope? Check your understanding • Why is blue light used to view tiny objects in an optical microscope? • Blue light is shorter wavelength, so diffracts less compared to other colors of visible light. Young’s Interference Experiment • Is Light a wave or particle? All waves reflect, refract, diffract, and interfere. • We’ve seen everything but interference so far. Can Light interfere and cancel out? Young’s Interference Experiement • Used monochromatic light (light of single color • Passed the light through two narrow slits An Interference pattern!! Yet another demonstration of light’s wave properties! Diffraction Grating • Thousands of tiny slits. Diffracts light. Colors are produced by interference between light beams. • So if you see red, what color is being cancelled? • x = • L d • d = distance between slits • x = distance between central and 1st order • L = distance from center of slits to 1st order • Three of these quantities can be measured in an experiment. What 3? • X, L and d so with that we can calculate the • =(xd)/L • ex • d=0.02cm x=0.184cm L=80cm • =(x*d)/L • = (0.184*0.02)/80 = 4.6*10-5 cm • Find freq. • f = c/ = (3*108m/s)/(8.6*105cm*(1m/100cm)) = 6.5*1014 • CD’s and DVD’s are like diffraction gratings. Thin-Film Interference • Have you ever wondered about these? • Reflections off two surfaces put light waves out of phase. Some colors are cancelled. What’s left is what you see. • In the case of oil and soap bubbles, the thickness varies throughout, so different colors cancel in different places