Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Physics 2112 Unit 28 Today’s Concept: A) Optical Devices - multiple lenses - human eye - telescope Example 28.1: Back to the spider Remember our spider that was 15cm to the left of a converging lens with a focal length of 20cm? A virtual image was formed 60cm upstream of the lens. (m=4) What if I put a second similar lens 10cm downstream of the first one. f Where is the final image of the two lens system? What is total magnification of the two lenses? Unit 28 - Slide2 Example 28.2: One final spider A converging lens with |f| = 25cm is located 12cm to the right of a diverging lens with |f| = 20cm. A 2cm tall spider is located 60cm to the left of the converging lens. Where is the final image of the two lens system? What is total magnification of the two lenses? Unit 28 - Slide3 System of Lenses Virtual Objects are Possible !! Object Distance is Negative! Image from first lens Becomes object for second lens Unit 28 - Slide4 It’s Always the Same: 1 1 1 S S f S M S You just have to keep the signs straight: s’ is positive for a real image f is positive when it can produce a real image Lens sign conventions S: positive if object is “upstream” of lens S’ : positive if image is “downstream” of lens f: positive if converging lens Mirrors sign conventions S: positive if object is “upstream” of mirror S’: positive if image is “upstream” of mirror f: positive if converging mirror (concave) Unit 28 - Slide5 Human Eye As always: 1/f = 1/S + 1/S’ S changes from object to object, but S’ is fixe by size of eye. What to do? Change f by changing R S’ Close object (small S) Small R Distant object (large S) Large R “accomodation” done by ciliary muscle Unit 28 - Slide6 Normal Eye Unit 28 - Slide7 Near-Sighted (myopia) Can’t focus beyond the “far point”. Fix with diverging lens that creates virtual image at far point. Unit 28 - Slide8 Example 28.1: Near Sighted Diana can’t focus on things that are farther that 40cm from her eye. What should the power of her corrective contact lenses be? Unit 28 - Slide9 Example 28.2: Near Sighted (II) Diana can’t focus on things that are farther that 40cm from her eye. Where would her image form for very distant objects if she didn’t have her contacts in? Unit 28 - Slide10 Far-Sighted (hyperopia) Eye’s lens gets rigid with age (presbyopia) Muscles can create small enough R. Converging Lens creates virtual image at person’s near point Unit 28 - Slide11 Example 28.3: Far-Sighted Frank can’t focus on anything closer than 1.0m from his eye. Assuming he wants to read books that are 15cm from his eye, what should the power of his corrective glasses be? Assume glasses are 2cm in front of his eye. 2cm Unit 28 - Slide12 Specific example: Telescope Angular magnification = b/a Unit 28 - Slide13 Telescope Key point: Looking at object far away, S M = Q2/Q1 Want big fo Long telescopes! hi/fe / hi/fo = fo/fe Unit 28 - Slide14 Example 28.4: Yerkes The Yerkes telescope in southern Wisconsin is the world’s largest refracting telescope. What is it’s magnification if fo = 19.8m and fe = 3.9cm? Unit 28 - Slide15 Real Modern Astronomical Telescope Astronomical telescopes don’t really care about magnification so much. Want large mirrors to collect light from faint objects. Large aperture to “resolve” close objects (more next week on resolution.) The Large Synoptic Survey Telescope (LSST) 8.4m mirror (27.5ft) Unit 28 - Slide16 How to Make a Big Telescope Mirror Melt it & Spin it. 52,000 lbs of borosilicate glass when filled Unit 28 - Slide17