1 / 27

MSTC Physic B

MSTC Physic B. Chapter 24 Section 2 and 3. Thin film interference. Commonly observed in thin layers of oil on water and soap bubbles Various colors result from the interference of waves reflected from the opposite surfaces of the film. Thin film interference.

naava
Download Presentation

MSTC Physic B

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. MSTC Physic B Chapter 24 Section 2 and 3

  2. Thin film interference • Commonly observed in thin layers of oil on water and soap bubbles • Various colors result from the interference of waves reflected from the opposite surfaces of the film

  3. Thin film interference • A wave traveling in a medium of low refractive index undergoes a 180 degree phase change upon reflection from a medium of higher refractive index • There is no phase change in the reflected wave if it reflects from a medium of lower refractive index

  4. Thin film interference 1 2 n t • When ray 1 reflects from the upper surface it undergoes a phase change of 180 degrees with respect to the incident ray

  5. Thin film interference 1 2 n t • Ray 2 undergoes no phase change with respect to the incident ray when it reflects from the lower surface • It is now 180 degrees out of phase with initial

  6. Thin film interference 1 2 n t • Ray 2 travels an extra distance equal to 2t before the waves recombine

  7. Thin film interference 1 2 n t • If 2t = λ/2 rays 1 and 2 will recombine in phase and constructive interference will result

  8. Thin film interference 1 2 n t • In general, constructive interference will occur if 2t = (m + ½) λ where m = 0,1,2,…

  9. Thin film interference 1 2 n t • If the extra distance traveled by ray 2 corresponds to a multiple of λ, the 2 waves will combine out of phase and destructive interference will occur

  10. Thin film interference 1 2 n t • In general, destructive interference will occur if 2t = mλ where m = 0,1,2,…

  11. Thin film interference 1 2 n t • These conditions for constructive and destructive interference are only valid when the film is surrounded by a common medium

  12. Thin film interference 1 2 n t • If the film is located between 2 different media (one higher n and one lower n) the conditions for constructive and destructive interference are reversed

  13. Sample problem Calculate the minimum thickness of a soap bubble film (n=1.33) that will result in constructive interference in the reflected light if the film is illuminated with light whose wavelength in air is 600 nm.

  14. Sample Problem Solar cells are often coated with transparent thin film, such as silicon monoxide (n=1.45) in order to minimize reflective losses from the surface. A silicon solar cell (n=3.5) is coated with a thin film of silicon monoxide for this purpose. Determine the minimum thickness of the film that will produce the least reflection at a wavelength of 550 nm, which is the center of the visible spectrum.

  15. Sample Problem A thin, wedge-shaped film of refractive index n is illuminated with monochromatic light of wavelength λ. Describe the interference pattern observed for this case.

  16. Newton’s rings • If place a plano-convex lens on top of a plane glass surface a pattern of light and dark rings is observed

  17. Newton’s rings • The air film between the glass surfaces varies in thickness from zero at the point of contact • System must be viewed from above using light of a particular λ

  18. Newton’s rings 2 1 • Ray 1 reflects from the glass plate and has a 180 degree phase change • Ray 2 reflects from the lower part of the lens with no phase change

  19. Newton’s rings 2 1 • Use same equations we used for thin films (film is now air) • The contact point is dark because ray 1 undergoes a 180 degree phase change with respect to ray 2

  20. Newton’s rings 2 1 • Can obtain expressions for the radii of the bright and dark bands in terms of the radius of curvature of the lens • Used to test optical lenses

  21. Diffraction • Spreading of waves into a region behind an obstruction • Occurs when waves pass through small openings, around obstacles, or by sharp edges

  22. Diffraction • Longer the wavelength, compared to the width of the opening, the greater the diffraction

  23. Diffraction • Resemble interference patterns because they result from constructive and destructive interference

  24. Diffraction • According to Huygen’s principle, each portion of a slit acts as a source of waves • Hence, one portion of a slit can interfere with light from another portion

  25. Diffraction • Pattern consists of a broad, intense central band • On either side are narrower, less intense secondary bands and a series of dark bands • Central bright is about twice as wide as the next brightest max and about 22 times as bright

  26. Diffraction • For destructive interference, w sinΘ = m λ m=1,2,3,….. where w=width of slit and Θ=angle of particular minimum

  27. Sample problem Light of wavelength 580 nm is incident on a slit of width 0.3 mm. The observing screen is placed 2 m from the slit. Find the positions of the first dark fringes and the width of the central bright fringe. What is the width of the first order bright fringe?

More Related