Phys. 122: Thursday, 20 Aug. Written HW 1: ch. 23, probs. 8, 12, 15, 34, 53, and 58(b). Due a week from today. NO labs nor recitations this week. Both begin next week. Finish reading ch. 23 and begin ch. 20 for next week. Update: the mobile app for iClicker is now called “REEF Polling.” You can purchase a subscription at the iClicker website. (If you already have an iClickerGo subscription 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.

iClicker Website: (Visit thiswww.iclicker.com site to register. If you use the REEF app, please me the registration directly as well.) Online HW: Make sure your license is for our book!! (Knight Phys. for Sci. & Eng., 3 rd edition.) When prompted for the COURSE ID, enter MPARENDT122FALL15. Please be sure to enter this EXACTLY, or you may enroll in the wrong course.

Light Reflection: Diffuse vs. Specular Notice that the angle reflected equals the angle in! We'll see why shortly.

Images from flat mirrors: always on the other side, the same size, and right-side-up. This symbol means a clicker question is on the next slide!!

Clickers: The reflection produced by a single flat mirror is similar to the reflection produced by two flat mirrors that... a) form a 45 degree angle between them b) form a 90 degree angle between them, provided you look along the symmetry axis c) form a 30 degree angle between them d) form a 60 degree angle between them, provided you look along the symmetry axis e) face each other directly (at 180 degrees)

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What happens with a 90 ° angle?

Curved mirrors have more possibilities. Here, a parabola shape reflects parallel rays (from an object that's very far away) all to the same point, called the focal point of the mirror.

If the object is not infinitely far away, then the image is not at the focal point. Rays 1 and 2 help us find the image...

For the general case, we can use the “lens equation” to locate an image (distance s') for a given object (distance s). Magnification is the ratio of the image size to the object size. This is also the ratio of their distances, so M = s'/s.

Geometry of similar triangles is how we can derive the lens (and mirror) equation.

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).

Clickers: Snell's Law should actually (more properly) be called: a) Snell's Helpful Suggestion b) Snel's Law or Sahl's Law c) Snell Slaw d) Newton's, Leibniz', and Hooke's Law e) Huygen's Principle

Clickers: what causes Snell's Law to be true – where does it come from? (a) International agreement or conspiracy (b) Omniscient textbook authors, like Knight (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 the wave speed slows down in certain materials (e) The fear of getting an F in this class (“Snell” was really Willebrord Snel van Royen!)

Why does light behave this way? We'll see in ch. 34 that light is a transverse wave. Many properties of light can be traced to this fact. Fundamental is that light slows down in certain materials.

Example: problem 23.10

Clickers: Snell's Law is... a)... a property of light waves only b)... a property of light waves AND soldiers marching from land onto mud only c)... not really a Law, since if you break it there's no fine nor jail time d)... a property of ALL waves (sound, ocean, radio, etc.) e)... already making my brain hurt

Clickers: when is total internal reflection (NO refracted wave) possible? a) When going into a material with the same index of refraction b) When going from a smaller to a larger index of refraction c) When going into a material in which the light must slow down d) When going into a material in which the light must speed up e) I can't vote; I don't yet have my iClicker.

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

Clickers: air has an index of refraction really close to 1. Can we tell that it's not exactly 1 without using sensitive laboratory equipment? a) No b) Yes c) Can we leave yet?