1 / 23

From Last Time…

This lecture discusses the thin-lens approximation, ray tracing, focal point, image formation, and magnification in physics. It also explores the properties of real and virtual images formed by lenses.

rowsey
Download Presentation

From Last Time…

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


  1. Object Lens Image Image Object From Last Time… Lenses and image formation Physics 208, Lecture 5

  2. Thin-lens approximation: Ray tracing Image Optical Axis F Object F 1) Rays parallel to optical axis pass through focal point. 2) Rays through center of lens are not refracted. 3) Rays through F emerge parallel to optical axis. Here image is real, inverted, enlarged Physics 208, Lecture 5

  3. Object distance Image distance s s’ Object Image How are all these related? f f focal length Making an image Physics 208, Lecture 5

  4. Question A magnifying glass of diameter 2 cm and focal length 5 cm is used to form an image of the sun. Approximately what is the image distance? 0.2 cm 1 cm 2 cm 2.5 cm 5 cm Physics 208, Lecture 5

  5. Image size vs object size: Magnification = M = Magnification Objectheight Image distance Objectdist. Image height Physics 208, Lecture 5

  6. Far away objects The moon is 3.8x108 m away, and 3.5x106 m diameter. I use a 1 m focal length lens to make an image of the moon. About what diameter is this image of the moon? 0.5 cm 1 cm 2 cm 10 cm 1 m Physics 208, Lecture 5

  7. Image (real, inverted) Object Image (real, inverted) Image (virtual, upright) These rays seem to originatefrom tip of a ‘virtual’ arrow. Different object positions Physics 208, Lecture 5

  8. Image (virtual, upright) These rays seem to originatefrom tip of a ‘virtual’ arrow. Virtual images objects closer to a converging lens than the focal length form a virtual image • Virtual image • can’t be recorded on film, • Can’t be seen on a screen. • But rays can be focused by another lens • e.g. lens in your eye (focus on retina) • e.g. lens in a camera (focus on film plane) Physics 208, Lecture 5

  9. Do these rays come from real image, a virtual image, or an object? Can’t tell. Rays are exactly equivalent, and can be imaged by a lens in exactly the same way. Physics 208, Lecture 5

  10. Virtual Image : thin-lens equation Image (virtual) Object Object distance s Focal length f ( >0 for converging lens ) Image distance s’ Object distance < focal length Negative image distance: image on same side as object Physics 208, Lecture 5

  11. o  s’ s Magnifying glass Object at near point – biggest it can appear when in focus • Object closer than focal point • Lens produces virtual image • Light rays appear to originate from virtual image • Virtual image is used as object for eye lens. • Have moved object ‘closer’, while permitting eye to focus Image (virtual, upright) Physics 208, Lecture 5

  12. Magnifying glass A magnifying glass has a focal length of 8 cm. It is 1 cm in front of your eye. What is the closest that you can hold it to a bug so that the bug is in focus (your eye has a near point of 25cm). 4 cm 6 cm 8 cm 12 cm 25 cm s=? 1cm Physics 208, Lecture 5

  13. Magnifying glass: angular magnification • Without magnifying glass, • object is biggest at near point, ~ 25 cm. • Subtends angle • With magnifier • Object can be closer, at object distance s • Subtends angle • Angular magnification is • Increases with decreasing obj. dist. • Smallest obj. dist. is for image at near point: Physics 208, Lecture 5

  14. Diverging lens Optical Axis Object Image Focal length defined to be negative Then thin-lens equation can be used: Physics 208, Lecture 5

  15. Virtual image and diverging lens • Example: object at infinity • Rays appear to originate from focal point. • Result • Object has been (virtually) transported to a new location Physics 208, Lecture 5

  16. Nearsightedness Object I can’t focus on this This, I can see Physics 208, Lecture 5

  17. Fixing nearsightedness Object Physics 208, Lecture 5

  18. Reading glasses Without my glasses, my far point is about 25 cm. What is the weakest (longest focal length) corrective lens (located at my eye) would let me read a newspaper holding it 50 cm away? Lens should form a virtual image closer to my eye. I can focus on image only if it is less than 25 cm away. Weakest lens moves it least, so image distance = -25 cm. -25 cm +25 cm -50 cm +50 cm -100 cm +100 cm Physics 208, Lecture 5

  19. Diopters • Two lenses close together ~ • Single lens, “effective” focal length feff • Lens power P • Defined as with f in meters • Units of P are diopters • Two lenses close together Physics 208, Lecture 5

  20. Far away objects The moon is 3.8x108 m away, and 3.5x106 m diameter. I use a 1 m focal length lens to make an image of the moon. About what diameter is this image of the moon? 0.5 cm 1 cm 2 cm 10 cm 1 m Not a very big image. How can It be made ‘bigger’? Look at the image with a magnifying glass! Physics 208, Lecture 5

  21. Telescope: two lenses, object far away Objective: Forms real image of far-away object Eyepiece: Used as magnifying glass to examine image • Eyepiece forms virtual image • Real image formed on retina by your eye lens. Objective Eyepiece Physics 208, Lecture 5

  22. Telescope angular magnification • Without telescope, distant planet subtends angle • Objective lens forms real image • Height • Used as object by eyepiece lens • Eyepiece • Forms virtual image at infinity (for relaxed eye) • Object must be at focal point • Subtends angle • Angular mag: Physics 208, Lecture 5

  23. p q Virtual image ‘Object’ Eyepiece Compound Microscope Real, inverted, image Object Objective Object outside focal pointForms a real image Real image used as object for eyepiece.Eyepiece forms virtual image for eye. Physics 208, Lecture 5

More Related