Announcements 11/7/12

1. Announcements 11/7/12 • Prayer • Term project progress report due Saturday night • Evidence of progress • Text in body of email, list of group members, and group members CC’ed on the email Calvin & Hobbes

2. From warmup Extra time on? (nothing) Other comments? What does “subtends” mean? (2 people)

3. Clicker quiz Have you tried LearnSmart? yes no If you have tried it, how useful has it been? (only answer if you have tried it) Very Moderate Slightly Not at all

4. Summary of Near point vs. Far point Person A = nearsighted Can’t focus on object at infinity Use lens to bring object at infinite to image at “far point” image distance = negative! Person B = farsighted Can’t focus on object at 25 cm (comfortable reading distance) Use lens to bring object at 25 cm to image at “near point” image distance = negative!

5. From warmup: Compare the two situations: (1) You are using a magnifying glass to start a fire by focusing the sun's rays onto some straw. (2) You are using a magnifying glass to help you read small print. Qualitatively, what would "p" be for each situation? What would "q" be? 1)p is the distance to the sun from the magnifying glass, q is the distance of the magnifying glass to the straw 2)p is the distance from the letters to the magnifying glass, q is the distance of the magnifying glass to your eye Very common answer, but wrong! Quick writing: what is q for case 2? (talk to your neighbors; click in when you’re ready to answer)

6. From warmup: • Which will look bigger* to you, a 1 m tall object that’s 5 meters away from you, or a 10 m tall image that’s 50 meters away from you? • They will look the same size. They both are at an angle of arctan(5/1) with respect to the principal axis. * In the sense that it takes up more of your field of view “angular size”

7. Worked Problem • What is the angular size of a 0.1 m tall object that’s 5 meters away from you? Two methods! Thought Question • Which method should you use? • Method A • Method B

8. “Colton picture” r q q (in radians) = (section of arc)/r Quick writing • You are looking at an ant, h = 1 mm. What is the maximum viewing angle you can use to look at the ant, without any lenses?

9. Clicker question: • Which of the following is NOT true of angular magnification? • It is more useful than the absolute magnification when discussing telescopes • It is more useful than the abs. magnification when discussing magnifying glasses • It is given by the equation m = -q/p • It is likely to show up on an exam. m = q/q0 … where q0 = “the best you can do without magnification”

10. Magnifying Glass • The setup: f = 10 cm • Where would you like the image to be? • Let’s pick q = -50 cm. (q would generally be given in problem.) • What is m? (m = q/q0) • What is q? • What is q0? Answers: q = 6h/50 rad q0 = h/25 rad m = 3 Note: using formulas from book… mmax = 3.5 (for q = 25 cm) mmin = 2.5 (for q = infinity)

11. “Colton picture” r q q (in radians) = (section of arc)/r Quick writing • You are looking at the planet Mars, “h” (diameter, really) = 3.4  106m. The planet, as you are looking at it, is 2.5  1011 m away (this changes from month to month based on the relative positions of Mars and Earth). What is the maximum viewing angle you can use to look at Mars, without any lenses?

12. Telescope These focal spots should essentially overlap (not shown properly in this figure) • The setup: • Given details of setup, what is m? (m = q/q0) • What is q0? • What is q?

13. triangle: q (rad) = (intermed. height)/fe fe q If intermediate image were formed exactly at the focal point of the eyepiece, final image would be at . As it is, it will just be very far away. image Regardless of how far away it is, though, the angle is given by the blue ray. “Colton picture” for q r fo Answers: q0 = h/r q = foh/(rfe) m = fo/fe Because Mars is so far away, image is formed at the focal spot (essentially) Height of image = hfo/r (from M = -q/p)

14. From warmup Compare these two Wikipedia lists: Largest optical reflecting telescopes and Largest optical refracting telescopes. Which list contains the largest telescopes overall, and why are the largest telescopes all that variety? Reflecting is much larger. The glass in large refracting telescopes would sag so they use mirrors instead since they can support the back of them. reflecting telescopes are larger. this is because there are no aberration issues with reflecting telescopes so they can give clearer images. Also (my answer): only need to polish one side—saves a lot of money

15. eyepiece lens Incoming Light Curved Mirror Mirror Reflecting Telescope • A “Newtonian Reflector” http://lcogt.net/en/book/reflecting-telescopes

16. Compound Microscope I really dislike the eqn: “overall magnification” = Mobjective  meyepiece because it mixes absolute magnification with angular magnification (but apparently everyone does it that way)  Not on reading assignment, not on HW, not on exam, not especially interesting… let’s not bother with. Onward! http://en.wikipedia.org/wiki/Microscope

17. Chapter 37! • Interference effects • I.e. now returning to wave nature of light, instead of the ray approximation • Two mathematical facts we will use:

18. Interference... A single source Next few slides: credit Dr. Durfee

19. Interference... Two sources