1. Phys. 122: Thursday, 27 Aug. Written HW 1: due by 2:00 pm. (Turn in using envelope outside my office – Wk. 305 – if you don't have it ready to turn in now.) Written HW 2: ch. 20, probs. 12, 27, 38, 44, 51, and 72. Due in one week. Mast. Phys.: Assignment 1 available; due Tuesday, 08 Sept., by midnight. Ch. 20: we will skip section 20.7. Finish reading ch. 21 by Tues.; begin ch. 22 after that. (Can skip sections 22.5 and 22.6.) Office Hours (tentative): Mon. 1:30-2:30, Wed. 9-11, or by appointment.

2. Although it's called the “lens equation,” it works for mirrors too! Here, s and s' are the distances to the object and to the image (respectively).

3. For lenses instead of mirrors, the convention is different, since the rays go through instead of reflecting. Positive s' means an image on the other side of the lens from the object.

6. Clickers: can the reflected ray ever be not at the same angle as the incident ray? a) Yes, but we must remove certain parts of the mirror b) No – the laws of physics would be broken, leading to absolute chaos! c) Maybe, if you don't first read your physics book d) Ask again later (at the end of the semester). e) No reason - I just wanted to make sure my “e” voting button works.

7. Some review from Chap. 14: Oscillations (in preparation for ch. 20, Waves) Any motion which repeats itself (exactly, or nearly so) with the same time interval (T) is an example of Periodic Motion. The shortest time it takes to repeat is called the Period, denoted by T (for “Time”), measured in seconds (or minutes, hours, etc.)‏ From T, we can calculate the frequency f=1/T, and the angular frequency ω = 2 π f.

9. Clickers: An LED blinks once every 1/10 th of a second. The frequency of this blinking is... a) 1/10 Hz b) 1 Hz c) 10 Hz d) 1/10 th s e) 10 s

10. Chapter 20: Wave Motion Waves come in two basic types, depending upon the direction of the wiggles of the wave compared to the direction that the wave travels. A transverse wave has its wiggles at 90 degrees (perpendicular) to the wave travel direction. A longitudinal wave has its wiggles in the same direction that the wave is moving.

11. What's happening in a transverse wave?

12. What's happening in a longitudinal wave?

13. Clickers: Which type of wave are water waves on the ocean surface? a) Transverse b) Longitudinal c) A mixture of transverse and longitudinal d) Reverse longitudinal e) They don't count as they are too real to be covered in a physics book

15. The wavelength measures (nearest) distance between two identical points of the wave. It's the equivalent of period (T) for distance instead of for time.

16. ... but, what is this the solution of??? It is the solution of the wave equation: Harmonic wave motion: mathematical form (F = m a slightly disguised.) k is called the wavenumber, given by k = 2 π / λ. It converts distance into angle (just like ω =2 π /T does with time instead of distance).

17. Clickers: for the point labelled “A” on the string (with wave moving to the right) shown, what is the direction of the acceleration? A a) Upward b) Downward c) To the right d) To the left e) It is zero..

18. (Example: problem 20.13)

19. Properties: v = wave speed depends upon the properties of the material which carries the wave. For instance, v ² = T / μ for transverse string waves, where T= tension and μ = mass/length. Direction of wave is to the RIGHT with the – sign, and to the LEFT for +. (Also called D(x,t) sometimes in your book.)

20. Note: wave speed is also given by ω /k! Be careful: wave speed is NOT the same as speed of the particles doing the wiggling! The latter quantity is v y = dy/dt and wiggles with time; the wave speed is CONSTANT, and determined by properties of the thing that's wiggling.