Physics 141 Lecture 26

Transcription

1 Physics 141 Lecture 26 Today s Concept: A) Lenses Electricity & Magne/sm Lecture 26, Slide 1

2 Mirrors in Movies Duck Soup (1933) The Lady rom Shanghai (1947) Enter the Dragon (1973) and many more

3 Reraction Snell s Law n 1 sin(θ 1 ) = n 2 sin(θ 2 ) θ 1 n 1 n 2 θ 2 That s all o the physics everything else is just geometry! Electricity & Magne/sm Lecture 26, Slide 3

4 Object Location Light rays rom sun bounce o object and go in all direc/ons! Some hits your eyes We know object s loca/on by where rays come rom. We will discuss eyes in lecture 28 Electricity & Magne/sm Lecture 26, Slide 4

5 Waves rom Objects are Focused by Lens Electricity & Magne/sm Lecture 26, Slide 5

6 Two Dierent Types o Lenses Electricity & Magne/sm Lecture 26, Slide 6

7 Converging Lens: Consider the case where the shape o the lens is such that light rays parallel to the axis o the mirror are all ocused to a common spot a distance behind the lens: Electricity & Magne/sm Lecture 26, Slide 7

8 Recipe or Finding Image: 1) Draw ray parallel to axis reracted ray goes through ocal point 2) Draw ray through center reracted ray is symmetric object image 3) Draw ray through ocal point reracted ray goes parallel to axis alternate or check You now know the posi/on o the same point on the image Electricity & Magne/sm Lecture 26, Slide 8

9 Example S > 2 image is: real inverted smaller object image > 0 S > 0 S > 0 Electricity & Magne/sm Lecture 26, Slide 9

10 image is: at ininity S = Example object > 0 S > 0 Electricity & Magne/sm Lecture 26, Slide 10

11 Example 0 < S < image is: virtual upright bigger image object S < 0 S > 0 > 0 Electricity & Magne/sm Lecture 26, Slide 11

12 Diverging Lens: Consider the case where the shape o the lens is such that light rays parallel to the axis o the lens all diverge but appear to come rom a common spot a distance in ront o the lens: Electricity & Magne/sm Lecture 26, Slide 12

13 Example image is: virtual upright smaller object image < 0 S > 0 S < 0 Electricity & Magne/sm Lecture 26, Slide 13

14 Executive Summary - Lenses S > 2 real inverted smaller 2 > S > real inverted bigger converging > S > 0 virtual upright bigger S > 0 virtual upright smaller diverging Electricity & Magne/sm Lecture 26, Slide 14

15 It s Always the Same: You just have to keep the signs straight: The sign convenkons S: posikve i object is upstream o lens S : posikve i image is downstream o lens : posikve i converging lens Electricity & Magne/sm Lecture 26, Slide 15

16 CheckPoints 2 & Image on screen MUST BE REAL Electricity & Magne/sm Lecture 26, Slide 16

17 CheckPoint 5 50 No light rom the top o the object can be transmived through the lens, so it will not be seen. the image is reversed so only the upper hal o the image will show The rays rom the bovom hal skll ocus The image is there, but it will be dimmer!! Electricity & Magne/sm Lecture 26, Slide 17

18 Cover top hal o lens Light rom top o object object image Cover top hal o lens Light rom bovom o object object image What s the Point? The rays rom the bovom hal s/ll ocus The image is there, but it will be dimmer! Electricity & Magne/sm Lecture 26, Slide 18

19 CheckPoint 7 The rays are bent more rom air to glass than rom water to glass Thereore, the ocal length in air is less than the ocal length in water We can see this also rom Lensmaker s Formula n lens n air 1 Electricity & Magne/sm Lecture 26, Slide 20

20 Checkpoint A converging lens is used to project the image o an arrow onto a screen as shown above. A piece o black tape is now placed over the upper hal o the lens. Which o the ollowing will be seen A) Only the lower hal o the object B) Only the upper hal o the object C) The whole object will skll show on the screen.

21 Air Bubble in water A Converging B Diverging C not a lens

22 Calculation A magniying glass is used to read the ine print on a document. The ocal length o the lens is 10mm. At what distance rom the lens must the document be placed in order to obtain an image magniied by a actor o 5 that is not inverted? Conceptual Analysis Lens EquaKon: 1/s + 1/sʹ = 1/ MagniicaKon: M = sʹ/s Strategic Analysis Consider nature o image (real or virtual?) to determine relakon between object posikon and ocal point Use magniicakon to determine object posikon Electricity & Magne/sm Lecture 26, Slide 21

23 A magniying glass is used to read the ine print on a document. The ocal length o the lens is 10mm. At what distance rom the lens must the document be placed in order to obtain an image magniied by a actor o 5 that is not inverted? Is the image real or virtual? A) REAL B) VIRTUAL A real image would be inverted A virtual image will be upright h h h h Electricity & Magne/sm Lecture 26, Slide 22

24 A magniying glass is used to read the ine print on a document. The ocal length o the lens is 10mm. At what distance rom the lens must the document be placed in order to obtain an image magniied by a actor o 5 that is not inverted? How does the object distance compare to the ocal length? A) B) C) Lens equakon Virtual Image Þ s < 0 Real object Þ s > 0 Converging lens Þ > 0 h s h s Electricity & Magne/sm Lecture 26, Slide 23

25 A magniying glass is used to read the ine print on a document. The ocal length o the lens is 10mm. At what distance rom the lens must the document be placed in order to obtain an image magniied by a actor o 5 that is not inverted? What is the magniica/on M in terms o s and? A) B) C) D) Lens equakon: MagniicaKon equakon: h s h s Electricity & Magne/sm Lecture 26, Slide 24

26 A magniying glass is used to read the ine print on a document. The ocal length o the lens is 10mm. At what distance rom the lens must the document be placed in order to obtain an image magniied by a actor o 5 that is not inverted? A) 1.7mm B) 6mm C) 8mm D) 40 mm E) 60 mm h h Electricity & Magne/sm Lecture 26, Slide 25

27 Follow Up Suppose we replace the converging lens with a diverging lens with ocal length o 10mm. I we skll want to get an image magniied by a actor o 5 that is not inverted, how does the object s div compare to the original object distance s conv? A) s div < s conv B) s div = s conv C) s div > s conv D) s div doesn t exist EQUATIONS PICTURES s nega/ve > not real object Draw the rays: s will always be smaller than s MagniicaKon will always be less than 1 Electricity & Magne/sm Lecture 26, Slide 26

28 Follow Up Suppose we replace the converging lens with a diverging lens with ocal length o 10mm. What is the magniicakon i we place the object at s = 8mm? A) B) C) D) E) EQUATIONS PICTURES h h Electricity & Magne/sm Lecture 26, Slide 27