Phys. 122: Tuesday, 25 Aug.

Written HW 1: due Thursday. Labs and recitations begin this week! Finish reading ch. 20 by Thurs.; begin ch. 21 after that. Update: the mobile app for iClicker is now called“REEF Polling.” You can purchase a subscription atthe iClicker website. (If you already have an iClickerGosubscription it should automatically update to REEF.)• Obtain an iClicker (or REEF app) and bring it to class

every day. Set up a Mastering Physics account and do the practice problems if it's new to you. First actual MP

assignment will be available Thursday!

iClicker Website: www.iclicker.com (Visit this

site to register. If you use the REEF app, please

email me the registration directly as well.)

Online HW: www.masteringphysics.com

• Make sure your license is for our book!!

(Knight Phys. for Sci. & Eng., 3rd edition.) •When prompted for the COURSE ID, enter

MPARENDT122FALL15.

Please be sure to enter this EXACTLY,

or you may enroll in the wrong course.

Clickers: In one nanosecond, light travelsabout how far?

a) one foot

b) one mile

c) a tenth of a micrometer

d) a few Ångstroms (tenths of nanometers)

e) Kajillions of miles

Clicker Question 6• As you walk away from a plane mirror on a wall, your

image willA.get smaller.

A.may or may not get smaller, depending on

where the observer is positioned.

A.always be a real image, no matter how far you

are from the mirror.

A.change from being a virtual image to a real

image as you pass the focal point.

A.always be the same size.

Images fromflat mirrors:

always on theother side, thesame size, andright-side-up.

Clicker Question 1

• The figure shows the path of a light ray through three different media. Rank the media in order of their refractive indices.

A.

A.

1 2 3n n n

3 1 2n n n

3 2 1n n n

2 1 3n n n

Clicker Question 4• When light travels from air into water

A. its velocity, wavelength, and frequency all change.

A. its velocity changes, but its frequency and wavelength do not change.

A. its frequency changes, but its velocity and wavelength do not change.

A. its velocity and wavelength change, but its frequency does not change.

A. its wavelength changes, but its velocity and frequency do not change.

Clickers: Snell's Law comes frommatching wave phase at interfaces. It can

also be derived from... (a) wanting to get there as fast as possible

(b) asking the Ouija board physics questions

(c) The fact that light is made of photons, and their frequency changes in certain materials

(d) The fact that light is a wave, and waves rarely go in straight lines

(e) pressing the “e” button on your clicker repeatedly

What about the second (fainter) outerrainbow that's often visible? It comesfrom a double reflection insidethe drops, and that's whatreverses the colorscompared to the innerone.

Total internal reflection: trig says that sinesof angles cannot be greater than 1!

Clickers: Let's do an easy one first! If wehave a flat window, angle α is zero. Whathappens to angle δ in that case?

a) It's zero also b) It goes to 90° c) It goes to 180° d) It becomes θ₁ e) Total int. reflection, so no ray comes out

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).

Clickers: An object moves from far awaytoward a concave spherical mirror, untilpassing its focal point. Its image will... a) disappear completely after passing the focus b) continue to become smaller while this happens c) continue to become larger while this happens d) be right on top of the object the whole time e) become huge while zooming off to infinity, then flip over and come in from infinity in the opposite direction while shrinking

Convex spherical mirrorsare simpler! In that case,f is negative, so s' mustalways be negative also.This means the image ison the opposite side, usingthe convention for mirrors.

For the general case, we can use the “lensequation” to locate an image (distance s')for a given object (distance s).

Magnification is the ratio of the image sizeto the object size. This is also the ratio oftheir distances, so M = s'/s.

Focusing with a lens:

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: To use a magnifying glass (converginglens) to properly magnify an object, the objectshould be....

a) Closer than the focal length f to the lens b) Between f and 2f from the lens c) Greater than 2f from the lens d) a virtual object e) on the same side of the lens as your eye