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, 08Sept., 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.

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

For lenses instead of mirrors, the conventionis different, since the rays go through insteadof reflecting. Positive s' means an image onthe other side of the lens from the object.

Clickers: can the reflected ray ever benot 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.

Some review from Chap. 14: Oscillations(in preparation for ch. 20, Waves)

Any motion which repeats itself (exactly, ornearly 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.

Clickers: An LED blinks once every1/10th of a second. The frequency of this

blinking is...

• a) 1/10 Hz

• b) 1 Hz

• c) 10 Hz

• d) 1/10th s

• e) 10 s

Chapter 20: Wave Motion

Waves come in two basic types, dependingupon the direction of the wiggles of the wave

compared to the direction that the wavetravels.

A transverse wave has its wiggles at90 degrees (perpendicular) to the wave

travel direction.A longitudinal wave has its wiggles in thesame direction that the wave is moving.

What's happening in a transverse wave?

What's happening in a longitudinal wave?

Clickers: Which type of wave arewater 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

The wavelength measures (nearest) distancebetween two identical points of the wave. It'sthe equivalent of period (T) for distance insteadof for time.

... but, what is this the solution of???It is the solution of the wave equation:

Harmonic wave motion: mathematical form

∂2 y∂ x2

=( 1v2) ∂2 y

∂ t2

(F = m a slightly disguised.)k is called the wavenumber, given by k = 2 π/λ.It converts distance into angle (just like ω=2π/Tdoes with time instead of distance).

Clickers: for the point labelled “A” onthe string (with wave moving to the right)

shown, what is the direction of theacceleration?

A

a) Upwardb) Downwardc) To the rightd) To the lefte) It is zero.

.

(Example: problem 20.13)

Properties: v = wave speed dependsupon the properties of the materialwhich carries the wave. For instance,v² = T / μ for transverse string waves,where T= tension and μ= mass/length.

Direction of wave is to the RIGHT withthe – sign, and to the LEFT for +.

(Also called D(x,t) sometimes in your book.)

Note: wave speed is alsogiven by ω/k!

Be careful: wave speed is NOT the same asspeed of the particles doing the wiggling! Thelatter quantity is vy = dy/dt and wiggles withtime; the wave speed is CONSTANT, anddetermined by properties of the thing that'swiggling.