Back from Break and Back to Optics

Transcription

1 Back rom Break and Back to Optics Phys 1020, Day 21: Questions? Cameras, Blmld 15.1 Digital Cameras, Optical systems 15.2 Last lab this week Coming Up: Optical communication

2 What will happen to image i we increase ocal length o lens? a. Image is same size, same place b. Image is same size and urther rom lens c. Image is bigger and urther rom lens d. Image is smaller and closer to lens (o) (i) Lens equation: o i (o) (i) 2

3 What will happen to the brightness o the image? a. Less bright b. More bright c. Same brightness (o) (i) Lens equation: o i (o) (i) 3

4 How can we compensate or less light? 1. More light sensitive ilm 2. Larger lens / Open Iris all the way 3. Longer exposure time 4. Use Flash (o) (i) Lens equation: o i (o) (i) 4

5 Let s do an example I you use a 35-mm ocal length lens to take a photograph o lowers 2 m rom the lens, how ar rom that lens does the real image o the lowers orm? 1. Understand the problem and pick a model (physics) This is obviously an optics problem involving lenses and images We will use ray optics to solve; speciically the lens equation

6 Let s do an example I you use a 35-mm ocal length lens to take a photograph o lowers 2 m rom the lens, how ar rom that lens does the real image o the lowers orm? 2. Draw a picture Object Lens With ocal length Film Object Distance Image Distance 6

7 Let s do an example I you use a 35-mm ocal length lens to take a photograph o lowers 2 m rom the lens, how ar rom that lens does the real image o the lowers orm? 3. Draw a ray diagram Object Film Object Distance Image Distance 7

8 Let s do an example I you use a 35-mm ocal length lens to take a photograph o lowers 2 m rom the lens, how ar rom that lens does the real image o the lowers orm? 4. Solve the equation: 1 = 1 o + 1 i 1.035m = 1 2m + 1 i

9 Two objects dierent distances rom lens Screen at Focus o 2. What will happen at screen i close down iris in ront o lens? a. Images brighter and object 1 more in ocus b. Images dimmer and object 1 more in ocus c. Images brighter and object 1 more out o ocus d. Images dimmer and object 1 more out o ocus 9

10 F-number number / d d number Large: Focal length large (light spread over larger area) Small diameter lens (less light captured) Dimmer image. number Small: Focal length small (light spread over smaller area) Larger diameter lens (more light captured) Brighter image. 10

11 Depth o Focus: Sharp ocus! Rays diverge quickly! number / d Large diameter lens Small -number Bright image Small depth o ocus Small diameter lens large -number Dim image Large depth o ocus pretty sharp ocus or a large range o object distances 11 (squinting).

12 Next: Digital Cameras You have already seen same physics principles in other applications: Lens/Optics Color/Light Resolution/Pixels Semiconductors, P-N junctions, capacitors

13 Digital vs Film Cameras Mechanical Electrical Shutter Mechanical Iris Object Lens With ocal length Dark Box Film Electronic Light Sensor (semiconductor device) Battery Required! Object Distance Image Distance 13

14 Film vs Electronic Sensor 36 mm 24 mm FILM 4.4 mm 6.6 mm Electronic Light Detector (MUCH SMALLER) Object Object Distance Image Distance How should the lens system be set up in the digital camera so that the view captured is the same as the 35 mm camera when I am about the same distance away rom the subject: a. the lens and lens-detector distance must be identical to that o the 35mm b. the lens curved more and the lens-detector distance shorter c. the lens curved more and the lens-detector distance the same d. the lens should curved more and the lensdetector distance longer 14

15 The Electronic Detector Semiconductor Device (converts light to electrical charge) Most digital cameras use: charge-coupled device (CCD Array) 4.4 mm (1200 pixels) 6.6 mm (1600 pixels) Lots o tiny light detectors (each 1 pixel) The more light photons that hit, the more electrical charge builds up on the pixel Cannot discriminate between colors all colors treated equally 15

16 To capture inormation about color: put colored ilters in ront o each pixel 16

17 To capture inormation about color: put colored ilters in ront o each pixel Question: I the camera is taking a picture o an orange, light will be detected by: a. Blue pixels b. Red pixels c. Green pixels d. Both b and c e. Both b and a 17

18 Interpreting Color Computer determines true color at center pixel by looking at amount o light collected at surrounding pixels that way you have ino on red, green and blue.

19 36 mm (~3000 pixels) Resolution 24 mm ~2000 pixels FILM Traditional ilm (~200 iso) has eective resolution o ~3000 x 2000 pixels.captures more detail than most CCD arrays. 4.4 mm (1200 pixels) 6.6 mm (1600 pixels) Question: To improve the amount o detail captured by the digital camera, you need to: a. Increase number o pixels on the CCD array b. Increase the physical size o the CCD array, keeping the total number o pixels the same c. Both a and b. 2.1 million pixels 2.1 Megapixels 19

20 Optical Zoom vs Ditigal Zoom Optical Zoom: Real Image is larger on array Digital Zoom: Real Image is same size on array, uses subset o pixels and then guess at detail. NO EXTRA INFORMATION. 20

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22 Paul Revere's Ride Henry Wadsworth Longellow Listen my children and you shall hear O the midnight ride o Paul Revere, On the eighteenth o April, in Seventy-ive; Hardly a man is now alive Who remembers that amous day and year. He said to his riend, "I the British march By land or sea rom the town to-night, Hang a lantern alot in the belry arch O the North Church tower as a signal light,-- One i by land, and two i by sea; And I on the opposite shore will be, Ready to ride and spread the alarm Through every Middlesex village and arm, For the country olk to be up and to arm." next verses talk about moon, night wind, i. e. noise that could obscure signals 22

23 alternatives? (shows beneits o light used today)- 1) yell really loud at me - - Does rhyme with sea. but sound not carry as ar as light. 2) Row over and tell me - still rhymes, but slow. Light much aster. Uses another trick o modern optical communication- digital inormation. Flash a light. Just seeing it tells Paul his riend is there to send signal. All inormation contained in second pulse o light. I on = by sea, i o, no light= by land. binary digital signal - two choices (on/o). One bit o inormation. Inormation digital. Big Advantage-Very distinct and unambiguous. on or o -- can distinguish even i og, moonlight, etc. Now days- inormation coded as series o on and os that give numbers. Signal is insulated rom presence o noise. (same as hiss on tape recorder, etc.) We Shall return to this idea and how it happens... 23

24 Why use light? 1. light- ast. Transmit signals rapidly. 2. Can go long way without being absorbed. (under special conditions!) 24

25 Principles behind design o optical communication system Want to send telephone signals rom Boulder to Ft. Collins. Put sound inormation into light pulses. (lots o lantern lashes, a al Paul). What are some problems with sending this way? (Write down 3 per group.) Then be prepared to oer ways to get around the problems. 1. need lots o light lashes in a hurry. Need good way to turn light on and o very ast and detect pulses. 2. Light spreads out all over the place, not much gets to Ft. Collins. Also, clouds and dust block it. 3. Want to send many conversations, no way to separate one light rom another. 4. not be able to see light rom location

26 Solutions: 1. conine light to pipe, send it right where want, keeps clouds, dust etc. out, keeps light rom spreading out (laser). Send light pulses down thin glass optical ibers. ( light pipes ) Light does not spread out, can be directed to exactly right place, can have multiple ibers to separate conversations, no clouds, etc. 26