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Lab13_Slides Interference in Thin Films

Lab13_Slides Interference in Thin Films. When light travels through a very thin film of material – a few wavelengths thick – light will reflect from both the bottom and the top of the layer, causing interference. This can be seen in soap bubbles and oil slicks.

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Lab13_Slides Interference in Thin Films

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  1. Lab13_SlidesInterferencein Thin Films When light travels through a very thin film of material – a few wavelengths thick – light will reflect from both the bottom and the top of the layer, causing interference. This can be seen in soap bubbles and oil slicks.

  2. Interference of light from different layers of thin films such as air between glass, soap bubble, and nail polish on water

  3. Interferencein Thin Films The wavelength of the light will be different in the oil and the air, and the reflections at points A and B may or may not involve phase changes. These two rays interfere.

  4. Interferencein Thin Films A beam of light reflected by a material with index of refraction greater than that of the material in which it is traveling, changes phase by 180° or ½ cycle.

  5. Out of Phase by 180 degrees or p radians or l/2 In between In Phase Constructive interference Destructive interference

  6. Interferencein Thin Films Example : Thin film of air, wedge-shaped. A very fine wire 7.35 x 10-3 mm in diameter is placed between two flat glass plates. Light whose wavelength in air is 600 nm falls (and is viewed) perpendicular to the plates and a series of bright and dark bands is seen. How many light and dark bands will there be in this case? Will the area next to the wire be bright or dark?

  7. Interferencein Thin Films Example : Thickness of soap bubble skin. A soap bubble appears green (λ = 540 nm) at the point on its front surface nearest the viewer. What is the smallest thickness the soap bubble film could have? Assume n = 1.35.

  8. Interferencein Thin Films • Problem Solving: Interference • Interference occurs when two or more waves arrive simultaneously at the same point in space. • Constructive interference occurs when the waves are in phase. • Destructive interference occurs when the waves are out of phase. • An extra half-wavelength shift occurs when light reflects from a medium with higher refractive index.

  9. 1 2 What are the conditions for Constructive Interference in reflection from a soap bubble? First consider phase change upon reflection eye air 1.0 180 deg phase change for Ray 1when reflecting from the soap film of thickness L but not ray 2 Reflection Now consider the path length differences soap 1.30 L air 1.00 We must add a half wavelength to account for the 180 deg phase change for constructive interference. Path difference must = integral number of wavelengths plus 1/2 a wavelength.

  10. Interferencein Thin Films Example : Nonreflective coating. What is the thickness of an optical coating of MgF2 whose index of refraction is n = 1.38 and which is designed to eliminate reflected light at wavelengths (in air) around 550 nm when incident normally on glass for which n = 1.50?

  11. ConcepTest 34.6aParallel Slides I Consider two identical microscope slides in air illuminated with light from a laser. The slides are exactly parallel, and the top slide is moving slowly upward. What do you see when looking from the top view? 1) all black 2) all white 3) fringes moving apart 4) alternately all black, then all bright

  12. ConcepTest 34.6aParallel Slides I Consider two identical microscope slides in air illuminated with light from a laser. The slides are exactly parallel, and the top slide is moving slowly upward. What do you see when looking from the top view? 1) all black 2) all white 3) fringes moving apart 4) alternately all black, then all bright As the distance between the two slides decreases, the path difference between the interfering rays changes. Thus, the phase between the interfering rays keeps changing, alternately in phase (constructive) and out of phase (destructive) as the top slide moves.

  13. 1) 2) edge ConcepTest 34.6bParallel Slides II A laser shines on a pair of identical glass microscope slides that form a very narrow edge. The waves reflected from the top and the bottom slide interfere. What is the interference pattern from top view?

  14. 1) 2) edge ConcepTest 34.6bParallel Slides II A laser shines on a pair of identical glass microscope slides that form a very narrow edge. The waves reflected from the top and the bottom slide interfere. What is the interference pattern from top view? Right at the edge, the two reflected rays have no path length difference and therefore should interfere constructively. However, the light ray reflected at the lower surface (point E) changes phase by l/2 because the index of refraction of glass is larger than that of air.

  15. ray 2 ray 1 ray 3 t ConcepTest 34.6cParallel Slides III Consider two identical microscopic slides in air illuminated with light from a laser. The bottom slide is rotated upward so that the wedge angle gets a bit smaller. What happens to the interference fringes? 1) spaced farther apart 2) spaced closer together 3) no change

  16. ray 2 ray 1 ray 3 t ConcepTest 34.6cParallel Slides III Consider two identical microscopic slides in air illuminated with light from a laser. The bottom slide is rotated upward so that the wedge angle gets a bit smaller. What happens to the interference fringes? 1) spaced farther apart 2) spaced closer together 3) no change The path difference between ray 2 and ray 3 is 2t (in addition, ray 3 experiences a phase change of 180°). Thus, the dark fringes will occur for: 2t = mλ m = 0,1,2,… If t gets smaller, ray 2 and ray 3 have to be farther apart before they can interfere, so the fringes move apart.

  17. ray 1 ray 2 ray 3 t ConcepTest 34.6dParallel Slides IV 1) spaced farther apart 2) spaced closer together 3) no change Two identical microscope slides in air illuminated with light from a laser are creating an interference pattern. The space between the slides is now filled with water (n = 1.33). What happens to the interference fringes?

  18. The path difference between ray 2 and ray 3 is 2t (in addition, ray 3 experiences a phase change of 180°). Thus, the dark fringes will occur for: 2t = mλwater where λwater= λair/n Thus, the water has decreased the wavelength of the light. ray 1 ray 2 ray 3 t ConcepTest 34.6dParallel Slides IV 1) spaced farther apart 2) spaced closer together 3) no change Two identical microscope slides in air illuminated with light from a laser are creating an interference pattern. The space between the slides is now filled with water (n=1.33). What happens to the interference fringes?

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