# Week 12

Read: Measuring light's wavelength (Chap. 20), Films, bubbles and rainbows (Chap. 21).
Quiz: Monday.

HW:
1. Fiber interference fringes (Ex. 20.1)
2. Two-slit interference pattern (Ex. 20.3)
3. Diffraction gratings (Ex. 20.4*)
4. Soap film interference (Ex. 21.1)
5. Optical path length (Ex. 21.2).
6. Thin film practice problem: A layer of iron oxide (refractive index 2.9) forms on the surface of a polished iron plate. When this layer is illuminated from above with white light, approximately how thick must the layer be in order for it to appear blueish in color?
7. Interference practice problem: Red light (713 nm in vacuum) is used in a Young's 2 slit experiment with slits separated by 120 microns. The screen is 2.75 meters from the slit plate. Find the distance, y, on the screen between the central bright fringe and the third order (n = 3) bright fringe. (answer: 4.9 cm)

Lab: Measuring hair diameter (Ex. 20.2). In this lab, we will use a Helium-Neon laser beam to illuminate a hair pressed between two microscope slides. By measuring the spacing of the interference fringes thus produced, we can determine the diameter of the hair. In the course of this experiment, you will need to measure and plot the angle, theta_n, at which the nth interference fringe occurs. Is sin(theta_n) proportional to n? If so, what is the slope of this plot? How can you determine the hair diameter from these measurements?

Chapter 18 (3 videos on Newton's Optics):

Chapter 20 (3 videos on Young's Lectures on Light):

Chapter 21 (2 more videos on Young's lectures on light):

Interesting related web content: The Rosetta stone, which Thomas Young helped translate in the 18th century after it was found by Napoleon's invading armies, now resides in the British Museum; Egypt still wants it back.
Physics 2