Your Comments. That test was brutal, but this is the last physics course I have to take here WOOOOOO!!!!!

Transcription

1 Your Comments I'm kind o lost, this was a pretty heavy prelecture. I understand the equations and how we get them but I'm araid to say that I don't understand the concepts behind everything. Such as what exactly it means to be arsighted and how does the lens help. And I'm conused on approaching problems regarding a system o lenses. That test was brutal, but this is the last physics course I have to take here WOOOOOO!!!!! It was a good run. Thanks or the un and intriguing semester. Do we need to know how to calculate the ocal length or a lens or a given eye? I so, will we be given these equations on the inal, or are we expected to memorize the equations in this prelecture? And in regards to that exam, what the gravity-deying int(b*da) was that? do you actually read these comments? i don't believe you I've never heard o a telescope beore. Those pulsing arrows in the prelecture were extremely annoying... Electricity & Magnetism Lecture 28, Slide

2 End o Semester Logistics Check your grade book scores I anything is not correct appropriate person Remaining Assignments Prelecture 29 (Optional video o Tim solving problems) Homework (Mirrors) Final Exam 50 questions uniormly distributed over semester Combined is Monday May 2 th :30-4:30 PM Conlict is Wednesday May 4 th 8:00 :00 AM Electricity & Magnetism Lecture 28, Slide 2

3 Physics 22 Lecture 28 Today s Concept: A) Optical Devices Electricity & Magnetism Lecture 28, Slide 3

4 Executive Summary Mirrors & Lenses: S > 2 real inverted smaller > 0 2 > S > real inverted bigger Concave (Converging) Converging > S > 0 virtual upright bigger S S M S S < 0 S > 0 virtual upright smaller Convex (diverging) Diverging Electricity & Magnetism Lecture 28, Slide 4

5 It s Always the Same: S S M S S You just have to keep the signs straight: s is positive or a real image is positive when it can produce a real image Lens sign conventions S: positive i object is upstream o lens S : positive i image is downstream o lens : positive i converging lens Mirrors sign conventions S: positive i object is upstream o mirror S : positive i image is upstream o mirror : positive i converging mirror (concave) Electricity & Magnetism Lecture 28, Slide 5

6 System o Lenses Trace rays through lenses, beginning with most upstream lens Image rom irst lens Becomes object or second lens Electricity & Magnetism Lecture 28, Slide 6

7 Virtual Objects are Possible!! System o Lenses Object Distance is Negative! Image rom irst lens Becomes object or second lens Electricity & Magnetism Lecture 28, Slide 7

8 CheckPoint 3 Each lens is converging. The second lens has a larger ocal length than the irst ( 2 > ). What does the beam look like when it emerges rom the second lens? A B A. The beam is converging B. The beam is diverging C C. D The beam is parallel to the axis with a width < w D. E The beam is parallel to the axis with a width = w E. The beam is parallel to the axis with a width > w. Parallel rays are transmitted and pass through ocal point ( ) 2. Those rays also pass through ocal point o second lens ( 2 ) and thereore are transmitted parallel to the axis > implies that the width > w Electricity & Magnetism Lecture 28, Slide 8

9 Normal Eye Electricity & Magnetism Lecture 28, Slide 9

10 CheckPoint 2 A person with normal vision (near point 28 cm) is standing in ront o a plane mirror. What is the closest distance to the mirror the person can stand and still see hersel in ocus? A B A. 4 C cm B. 28 cm C. 56 cm q r q i The image is ormed an equal distance behind the mirror Thereore, i you stand a distance = ½ o your near point, the distance to the image will be the near point distance. d d Electricity & Magnetism Lecture 28, Slide 0

11 Far-Sighted Converging Lens creates virtual image at person s near point Electricity & Magnetism Lecture 28, Slide

12 Near-Sighted Fix with diverging lens that creates virtual image at ar point. Electricity & Magnetism Lecture 28, Slide 2

13 CheckPoint Two people who wear glasses are camping. One o them is nearsighted and the other is arsighted. Which person s glasses will be useul in starting a ire with the sun s rays? A. The arsighted person s glasses B. The nearsighted person s glasses Farsighted = Converging Lens Only Converging Lens can produce a real image! Electricity & Magnetism Lecture 28, Slide 3

14 Angular Magniication: Telescope How does this apply to things ar away? E.g. the moon Your eye can ocus rays that are parallel or slightly diverging - Assume or simplicity that the rays rom the eyepiece are parallel The math: First, what is the approximate image distance or the objective, s? A) s s s s s B) C) s no approximation Physics 22 Lecture 28, Slide 4

15 Angular Magniication: Telescope How does this apply to things ar away? E.g. the moon Your eye can ocus rays that are parallel or slightly diverging - Assume or simplicity that the rays rom the eyepiece are parallel The math: Objective: Eyepiece: 2 M s s s ' s s2 2 s2 M 2 s s ' ' 2 2 s 2 2 Geometry h' h' ; ; M M 2 2 M M 2 ' 2 s s 2 2 Physics 22 Lecture 28, Slide 5

16 Multiple Lenses Exercises Two converging lenses are set up as shown. The ocal length o each lens is 47 cm. The object is a light bulb located 70 cm in ront o the irst lens. s 70 cm 47 cm What is the nature o the image rom the irst lens alone? A) REAL B) REAL C) VIRTUAL D) VIRTUAL UPRIGHT INVERTED UPRIGHT INVERTED EQUATIONS PICTURES S S S S S Draw Rays as above. s > s > 0 real image M S S M < 0 inverted image Electricity & Magnetism Lecture 28, Slide 6

17 Multiple Lenses Exercises Two converging lenses are set up as shown. The ocal length o each lens is 47 cm. The object is a light bulb located 70 cm in ront o the irst lens. Lens separation 2 m What is the object distance s 2 or lens 2? A) s 2 = -.43 m B) s 2 = +.43 m C) s 2 = m D) s 2 = m E) s 2 = +2.7 m THE OBJECT FOR THE SECOND LENS IS THE IMAGE OF THE FIRST LENS s 70 cm 47 cm s.43 m s OR s Image o irst lens is a REAL object or the second lens Electricity & Magnetism Lecture 28, Slide 7

18 Multiple Lenses Exercises Two converging lenses are set up as shown. The ocal length o each lens is 47 cm. The object is a light bulb located 70 cm in ront o the irst lens. Lens separation 2 m s 70 cm 47 cm s.43 m s m What is the nature o the FINAL image in terms o the ORIGINAL object? A) REAL B) REAL C) VIRTUAL D) VIRTUAL UPRIGHT INVERTED UPRIGHT INVERTED S 2 M 2 s 2 S2 S EQUATIONS s 2 s 2 > s 2 > 0 real image 2 M 2 < 0 M M M 2 > 0 upright image PICTURES Draw Rays as above. RESULTS s m M 9.6 Electricity & Magnetism Lecture 28, Slide 8

19 Multiple Lenses Exercises Suppose we now decrease the initial object distance to 58 cm. Applying the lens equation, we ind s 2.48m Lens separation 2 m s 58 cm 47 cm s 2.48 m What is the object distance s 2 or lens 2? A) s m B) s m C) s m D) s m E) s m THE OBJECT FOR THE SECOND LENS IS THE IMAGE OF THE FIRST LENS s OR s Image o irst lens is a VIRTUAL object or the second lens Electricity & Magnetism Lecture 28, Slide 20

20 Multiple Lenses Exercises Suppose we now decrease the initial object distance to 58 cm. Applying the lens equation, we ind s 2.48m Lens separation 2 m s 58 cm 47 cm s 2.48 m s m What is the nature o the inal image in terms o the original object? A) REAL B) REAL C) VIRTUAL D) VIRTUAL UPRIGHT INVERTED UPRIGHT INVERTED EQUATIONS PICTURES s2 S2 s2 s 2 < 0 M 2 S2 S 2 s 2 > 0 M 2 > 0 real image M = M M 2 < 0 inverted image Draw Rays as above. RESULTS s m M 2. Electricity & Magnetism Lecture 28, Slide 2

21 Thank you! Thanks or a antastic semester! Study hard or your inals! Electricity & Magnetism Lecture 28, Slide 22