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C 2001 Wiley, Physics Cutnell & Johnson 5 th Ed.

Fig. 27–10 THIN-FILM INTERFERENCE. When light shines on a thin film of oil floating on a layer of water, two light waves (#1 and #2), enter the eye after reflection and refraction . The oil thickness t = several l n ’s of light in the oil. l oil = l vac / n. D path = 2t.

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C 2001 Wiley, Physics Cutnell & Johnson 5 th Ed.

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  1. Fig. 27–10 THIN-FILM INTERFERENCE When light shines on a thin film of oil floating on a layer of water, two light waves (#1 and #2), enter the eye after reflection and refraction. The oil thickness t = several ln’s of light in the oil loil = lvac/ n D path = 2t C 2001 Wiley, Physics Cutnell & Johnson 5th Ed.

  2. Fig. 27–11 NOTE: When a wave on a rope reflects from a wall, the wave undergoes a 180o phase change. Light waves can also undergo a 180o phase shift upon reflection depending on the reflecting surfaces. C 2001 Wiley, Physics Cutnell & Johnson 5th Ed.

  3. The 180o phase change depends on: • When light travels from a larger to a smaller index of refractive index, there is NO phase change upon reflection. • 2. When light travels from a smaller to a larger index of refractive index, there is a 180o phase change upon reflection. • Larger to a smaller index NO phase change • Smaller to a larger index180o phase change

  4. THIN-FILM INTERFERENCE There is NO phase change at air – oil interface. There is 180o phase change at oil – water interface. 180o loil = lvac/ n 0o D path = 2t C 2001 Wiley, Physics Cutnell & Johnson 5th Ed.

  5. If there are no phase changes upon reflection constructive (max intensity) interference will occur if the top film (oil) is: 2t = m ln2t = loil, 2 loil, 3 loil, … If there is a 180o phase change upon reflection constructive (max intensity) interference will occur if top film (oil) is 2 t = (m + ½) ln2t + loil/2 = loil, 2 loil, 3 loil, … IMPORTANT: loil = lvac/n For destructive (min intensity) interference, interchange the two equations!

  6. If there are no phase changes upon reflection destructive (min intensity) interference will occur if top film (oil) is: 2t = (m + ½) ln2t = ½loil, 1.5 loil, 2.5 loil, 3.5 loil, … If there is a 180o phase change upon reflection destructive (min intensity) interference will occur if top film (oil) is: 2 t = m ln2t + loil/2 = loil/2, 3 loil /2, 5 loil /2, … IMPORTANT: loil=lvac / n

  7. Fig. 37-12 Upper figures: Light waves striking an interface between optical materialsat (approximately) normal incidence. na > nb na = nb na < nb Mechanical wave pulses on ropes: NO PHASE NO REFLECTION 180o PHASECHANGECHANGE

  8. Fig. 37-15 A nonreflective coating has an index of refraction that is intermediatebetween those of glass and air. Destructive interference*occurs upon reflectionwhen film thickness is¼ lin the film. n = 1.00 1.38 1.50 180o 180o *No reflected light here!

  9. Fig. 37-16 A schematic diagram of a Michelson interferometer.

  10. Review

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