== Lecture 17: Chapter 35 (Diffraction) == == Thin film interference == 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? <> <> <> A. spaced farther apart A. spaced closer together A. no change Ans: A. == Thin film interference == 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? <> <> <> A. spaced farther apart A. spaced closer together A. no change Ans: B. == Diffraction == The diffraction pattern below arises from a single slit. If we would like to sharpen the pattern, i.e., make the central bright spot narrower, what should we do to the slit width? <> <> <> A. narrow the slit A. widen the slit A. enlarge the screen A. close off the slit Ans: A. == Diffraction == Imagine holding a circular disk in a beam of monochromatic light. Diffraction occurs at the edge of the disk. The center of the shadow is <> <> <> A. darker than the rest of the shadow A. a bright spot A. bright or dark, depending on the wavelength A. bright or dark, depending on the distance to the screen Ans: B. == Diffraction? == Consider a beam of coherent monochromatic light, like a pencil shaped beam from a laser. At a certain time and position, the cross section of the beam is found to be a perfect circle of diameter D. However, it is moving in free space. Will this beam show a diffraction pattern at a screen some distance away later on? <> <> <> A. Yes, sure. A. No way. Ans: A.