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Published byCornelia Harrison Modified over 8 years ago
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Thin Film Interference Interference + Light
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Superposition t +1 t +1 t +2 -2 + Destructive Interference Out of Phase 180 degrees
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Superposition t +1 t +1 t +2 -2 + Constructive Interference In Phase
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Electromagnetic Waves Light, Radio, TV, Microwaves, Satellites, X-Rays
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Electromagnetic Waves x z y Transverse (vs. sound waves – longitudinal) E perpendicular to B and always in phase E & B increase and decrease at same times Can travel in empty space (sound waves can’t!) “Speed of light”: v = c = 3 x 10 8 m/s(186,000 miles/second!) Frequency: f = v/ = c/ Period: T = 1/f wave travels one wavelength in one period T
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Speed of light in medium Index of refraction Speed of light in vacuum so Index of Refraction 186,000 miles/second: it’s not just a good idea, it’s the law! always! In general n increases with density of the medium.
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Index of Refraction 1.Speed changes when moving to a new medium. 2.Frequency of the light remains constant. 3.Therefore, the wavelength changes. v old f * old and v new f * new v new / v old new old n old / n new = new old
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Thin Film Interference n 1 (thin film) n2n2 n 0 =1.0 (air) t 1 2 Get two waves by reflection off two different interfaces. Ray 2 travels approximately 2t further than ray 1. 37
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Reflection + Phase Shifts n1n1 n2n2 Upon reflection from a boundary between two transparent materials, the phase of the reflected light may change. If n 1 > n 2 - no phase change upon reflection. If n 1 < n 2 - phase change of 180º upon reflection. (equivalent to the wave shifting by /2.) Incident wave Reflected wave 39
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Thin Film Summary n 1 (thin film) n2n2 n = 1.0 (air) t 1 2 Ray 1: 1 = 0 or ½ Determine number of extra wavelengths for each ray. If |( 2 – 1 )| = ½, 1 ½, 2 ½ …. (m + ½) destructive If |( 2 – 1 )| = 0, 1, 2, 3 …. (m) constructive Note: this is wavelength in film! ( film = o /n 1 ) 42 + 2 t/ film ReflectionDistance Ray 2: 2 = 0 or ½ + 0 This is important!
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Thin Film Practice n glass = 1.5 n water = 1.3 n = 1.0 (air) t 1 2 Blue light ( = 500 nm) incident on a glass (n glass = 1.5) cover slip (t = 167 nm) floating on top of water (n water = 1.3). Is the interference constructive or destructive or neither? 45 1 = 0B) 1 = ½C) 1 = 1 What is 1, the total phase shift for ray 1
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Thin Film Practice n glass = 1.5 n water = 1.3 n = 1.0 (air) t 1 2 Blue light ( = 500 nm) incident on a glass (n glass = 1.5) cover slip (t = 167 nm) floating on top of water (n water = 1.3). Is the interference constructive or destructive or neither? 45
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Thin Film n glass =1.5 n plastic =1.8 n=1 (air) t 1 2 Blue light = 500 nm incident on a thin film (t = 167 nm) of glass on top of plastic. The interference is: (A) constructive (B) destructive (C) neither 48
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The gas looks: bright dark A thin film of gasoline (n gas =1.20) and a thin film of oil (n oil =1.45) are floating on water (n water =1.33). When the thickness of the two films is exactly one wavelength… t = n water =1.3 n gas =1.20 n air =1.0 n oil =1.45 The oil looks: bright dark 50
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Thin Film Practice n glass = 1.5 n water = 1.3 n = 1.0 (air) t 1 2 Blue light ( = 500 nm) incident on a glass (n glass = 1.5) cover slip (t = 167 nm) floating on top of water (n water = 1.3). Is the interference constructive or destructive or neither? 45
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Thin Film Practice n glass = 1.5 n water = 1.3 n = 1.0 (air) t 1 2 Blue light ( = 500 nm) incident on a glass (n glass = 1.5) cover slip (t = 167 nm) floating on top of water (n water = 1.3). Is the interference constructive or destructive or neither? 45
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