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Announcements 11/2/12

Announcements 11/2/12. Prayer Slinkies! (Seth, Ryan, William B, Clement) Progress Report due a week from Saturday. Pearls Before Swine. From warmup. Extra time on? (nothing in particular) Other comments?

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Announcements 11/2/12

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  1. Announcements 11/2/12 • Prayer • Slinkies! (Seth, Ryan, William B, Clement) • Progress Report due a week from Saturday Pearls Before Swine

  2. From warmup Extra time on? (nothing in particular) Other comments? Do astronomers have to worry about the fact that air doesn't have an n of exactly 1?

  3. Clicker question: • A converging lens should be: • convex • concave

  4. Convex/converging Lens at each interface, Snell’s law is satisfied f if not parallel, they converge (image formed) at a different point parallel rays converge to the focal point

  5. The equation: Where will image be formed? focus focus Ray diagram just like one for concave mirror, except rays continue to right instead of reflecting back! Magnification same: positive q = right hand side of lens

  6. From warmup The mirror eqn and the thin lens eqn look identical. How exactly are they different? The sign conventions are different, a positive focus means a focus that is "behind" the lens. The same is true of q. (from my answer) Also--how f is calculated is different. For mirrors, it's simply related to the radius of curvature, but for lenses it relates to the curvature as well as the index of refraction of the lens glass.

  7. Clicker quiz Just out of curiosity… have I ever used your warmup answer? yes no

  8. Concave/diverging Lens f parallel rays seem to be coming from the focal point

  9. The equation: Diverging lenses virtual image! (negative f) Negative q: means image on same side of lens as object (virtual)

  10. Demos • Demo: “Real image or not?” Class poll: What kind of image is this? • Real • Virtual

  11. Sign conventions: summary • Assuming that the light rays are coming from the left:

  12. “Lensmaker’s Eqn” Lens-makers’ eqn:

  13. From warmup • When you look straight down at an object under water, will it appear to be closer to you or farther away than actuality? Why? • Because you are looking at a flat refracting surface the image of the object forms closer to the surface than the actual object is (figure 36.18). This means the object appears closer to you than it actually is.

  14. Clicker Quiz Is that a real or a virtual image? real virtual

  15. Thick lenses and surfaces Careful with signs!!! (see table 36.2)

  16. Clicker question: • Can a converging lens ever produce a virtual image? • Yes • No

  17. Clicker question: • What happens if I cover up the upper half of lens? • Nothing • Upper half of image disappears • Lower half of image disappears • Intensity of image goes down by 50%

  18. Clicker question: • Can a diverging lens ever produce a real image? • Yes • No

  19. Multiple element problems • Image of one becomes object of next • Worked problem: Find qfinal, Mtot f = 20 cm f = -60 cm 60 cm 15 cm Answers: image is 20 cm to right of -60cm lens; M = -0.67

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