Physics Honors/AP 1 and 2

Homework

# Physics 24: 6-12 Diffraction and Interference

## Homework

Text Reading: Giancoli, Physics - Principles with Applications, Chapter 24: Sections 6 to 12

##### Study Points
• Section 6: Diffraction grating is the impression of narrowly-spaced lines on film, so that each space between a pair of lines is effectively a single slit. Principle maxima for a thin slit will be governed by the relationship sin θ = mλ/d, where m is an integer (0, 1, 2....), and λ is the wavelength of light passing through a slit of width d.
• Section 7: Spectroscopy uses diffraction gratings to spread light, similar to a prism. Light from a dense gas like the sun will provide a continuous spectrum; light from a hot, rare gas of a single element will provide the characteristic emission spectrum for that element, and light passing through a cool gase will be absorbed at specific wavelengths.
• Section 8: Thin Films provide two close interfaces, each of which reflects some of the incident light. The slight difference in distance between the two surfaces creates two light waves slightly out of phase, and therefore able to interfere with one another, creating maxima and minima. Thin film interference provides a way to inspect optical elements for precision.
• Section 9: Michelson's Interferometer provides a similar means of making very precise measurements of distances on the order of ¼ the wavelength of light used.
• Section 10: Polarization occurs because light waves travel at right angles to the direction of propagation of the wave. When only those waves which are in the correct plane can pass through a filter. Polarizing filters can be used to precisely control the intensity of light of specific wavelengths that can reach an observer, allowing the observer to block "bright" light and see wavelengths of dimmer lighter more easily.
• Section 11: Liquid crystal technology makes possible the thin computer screens of laptop computers and flat panel TVs. Electrodes embedded in glass stimulate small areas (pixels) containing molecular crystals, which then change orientation, creating polarizing filters that can transmit or intercept light. By stimulating individual pixels differently, LCDs can build up pictures with rapid response times.
• Section 12: Atmospheric scattering demonstrates the dependence of light on the size of particles in the medium. Particles very much larger than the wavelength of light will absorb or reflect it. Particles very much smaller than the wavelength of light will affect it only slightly. Longer wavelengths of light are thus less likely to be scattered than shorter ones.

### Key Equations

• Diffraction grating principle maxima:
• Brewster's angle (polarization)

### Web Lecture

Read the following weblecture before chat: Diffraction and Interference

### Study Activity

Use the simulation below to explore diffraction.

• Open the Introduction.
• Turn on the spigot to drop water into the water tank.
• View the wave dispersion from the top. What do you notice? What do you notice about the shape of the wave as it travels from the source?
• View the wave dispersion from the side. What do you notice about the shape of the wave as it travels from the source?
• Increase and decrease the frequence and amplitudes, and observe how wave shapes change. Use the graph to visualize wave "mapping".
• Change the source to a sound wave and repeat your observations. How do sound waves differ from water waves? Use the graph, waves, and particles visualizations to help you understand how sound waves propagate.
• Change the source to a laser repeat your observations. Use the graph and screen to help you understand how light waves propagate How do light waves differ from water waves? from sound waves?
• Repeat your observations using the Interference button to see how each type of wave behaves when there are two sources.
• Repeat your observations usiung the Slits button to see how each type of wave behaves on passing through a slit.

Physics simulation Java Applets are the product of the PHET Interactive Simulations project at the University of Colorado, Boulder.

### Chat Preparation Activities

• Forum question: The Moodle forum for the session will assign a specific study question for you to prepare for chat. You need to read this question and post your answer before chat starts for this session.
• Mastery Exercise: The Moodle Mastery exercise for the chapter will contain sections related to our chat topic. Try to complete these before the chat starts, so that you can ask questions.

### Chapter Quiz

• Required: Complete the Mastery exercise with a passing score of 85% or better.
• Go to the Moodle and take the quiz for this chat session to see how much you already know about astronomy!

### Lab Work

If you want lab credit for this course, you must complete at least 12 labs (honors course) or 18 labs (AP students). One or more lab exercises are posted for each chapter as part of the homework assignment. We will be reviewing lab work at regular intervals, so do not get behind!