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Lecture 12: Diffraction and Diffraction Gratings
March 3, 2026
Reading Assignment
- Read: 32.6; Supplementary Reading Ch 1.4-1.7
- Study: Figs 32.24, 32.26; Supp Exs 1-5, 1-6, 1-7
Objectives
- (Continuing objective) Describe applications of the concepts of induction, waves, and light to everyday “real life” situations.
- Use the method of phasors to locate maxima and minima in a two-slit interference problem, multi-slit interference pattern, or for diffraction gratings.
- Explain how diffraction gratings can be used to separate colors.
- Using phasors, determine the phase difference corresponding to successive minima in a single slit diffraction pattern. From this, find the angles and screen locations of minima and determine the width of the “bright” central maximum.
- Describe diffraction, and explain how diffraction limits resolution. Use Rayleigh's criterion to solve resolution problems.
Homework
- Wednesday's Assigned Problems:
A45, A46, X11 (below), X12 (below); CH 32: 25, 29, 31; Supp CH 1: 7, 9
Problem X11 Light is normally incident on a grating with 10,000 lines/cm. (a) Find the maximum order in which 450 nm light will be visible, and (b) in which 650 nm light will be visible.
Problem X12 For visible light with wavelengths from 400 nm to 700 nm, show that the first-order spectrum is the only one that doesn't overlap with the next higher order.
Answers: X11 (a) 2, (b) 1
- Monday's Hand-In Problems:
A47, A105; CH 32: 18, 48; Supp CH 1: 8
Note: this is only the first half of the hand-in set.
Lecture Materials
- Click here for the Lecture overheads. Answers: CT1 - 2&5; CT2 - 3; CT3 - 5; CT4 - 1
- Spreadsheet showing phasor diagrams for 3-, 4-, and 20-source interference.
Videos of example problems
To see the problem statement, click on the link below. To play the video example, click on the underlined words "Video Demonstration" near the top of the page with the problem statement.- Example 1: Resolution limits.
- Example of three source interference.
- Example of single-slit diffraction problem.
Pre-Class Entertainment
- Back to California - Tom Ball and Kenny Sultan
- Mustang Sally - Wilson Pickett
- Save Me - Aretha Franklin
- Gimme Shelter - The Rolling Stones
- I Will Survive - Gloria Gaynor