Light and Mirrors MIRRORS

Transcription

1 Light and Mirrors MIRRORS 1

2 Polarized Sunglasses- How do they work? light waves vibrate in more than one plane light waves can be made to vibrate in a single plane by use of polarizing filters. 2

3 polarizing magic 3

4 Speed of Light The speed of all EM Waves (light) or c is 3 x 10 8 m/s. What is a light year? The distance light travels in one year Does light ever go slower? Yes, 3 x 10 8 m/s is the maximum and occurs in an vacuum. When it enters a medium, it slows down.

5 Light at a Boundary Absorbed ( Stuck inside ) Pass through the boundary (transmitted) Medium Changes Therefore Speed Changes What is this called? Refraction Bounce off the boundary Medium its traveling through does not change Therefore Speed remains same What is this called? Reflection 5

6 Reflection and Refraction at an Interface Why does the light bend?

7 Reflection and Mirrors 7

8 REFLECTION OF LIGHT Light obeys the law of refection that states that: "The angle of incidence is equal to the angle of reflection."

9 Angle of Reflection = Angle of Incidence Angles are measured with respect to the normal line

10 If all surfaces reflect, why can t I see my image?

11 Light reflection from a smooth surface is called regular or specular reflection. Light reflection from a rough or irregular surface is called diffuse reflection.

12 What type of reflection?

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14 Fiber Optics is also an example of reflection TOTAL INTERNAL REFLECTION

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16 Total Internal Reflection!!!

17 Mirrors

18 When we look at something in the mirror, the light wave we see is the reflected ray:

19 Object: source of rays Image: reproduction of object 19

20 Describing Images Size Same Reduced Enlarged Orientation Upright or inverted, Reversed Virtual or Real 20

21 Virtual reflected rays do not actually converge to form image. Cannot be projected. images form where light rays appear to have crossed. In mirrors, they form behind the mirror. are always upright. 21

22 Real images form where light rays actually cross. Can be projected. In mirrors, they form on the same side of the mirror as the object since light can not pass through a mirror. are always inverted (flipped upside down). 22

23 Types of Mirrors Flat or Plane Concave (Converging) Convex (Diverging) 23

24 FLAT MIRRORS Object Location: NA Virtual Erect Same Size Behind

25 FLAT MIRRORS A flat mirror reflects light rays in the same order as they approach it. Flat mirrors are made from pieces of plate glass that have been coated on the back with a reflecting material like silver or aluminum. The image is the same size as the object and the same distance behind the mirror as the object is in front of the mirror.

26 Notice that the images formed by a flat mirror are, in truth, reflections of real objects. The images themselves are not real because no light passes through them. These images which appear to the eye to be formed by rays of light but which in truth do not exist are called virtual images. On the other hand real images are formed when rays of light actually intersect at a single point.

27 Think about your image in a plane mirror? What is the same? What is different? 27

28 Images in a plane mirror are also reversed left to right. 28

29 Mirrors actually reverse you front to back, it is like turning a glove inside out

30 Plane Mirror In a plane mirror the object is the same size, upright, and the same distance behind the mirror as the object is in front of the mirror. Your mind extends the reflected ray beyond the mirror to form image. 30

31 Types of Mirrors Concave Mirrors Object Location: Near Virtual Erect Enlarged Behind Concave mirrors: Object Location: Far Real Inverted Reduced In Front Concave sphere whose inner surface was reflective aka converging mirrors since they bring light rays to a focus. ex. magnifying mirrors 31

32 Convex Mirrors Object Location: NA Virtual Erect Reduced Behind Convex mirrors: Convex sphere whose outer surface was reflective. aka diverging mirrors since spread out light rays. ex store security mirrors 32

33 Convex Mirrors

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36 What kind of mirror would be used Concave to focus a beam of light? 36

37 Object Location Real or Virtual Upright or Inverted Size Image Location Plane Mirror Converging Concave Mirror Diverging Convex Mirror 37

38 Ray Diagrams F: Focal Point, point where parallel light rays converge or appear to diverge. The focal point (F) is located halfway between the mirror s surface and the center of curvature. f: focal length, distance between F and mirror/lens C: Center of curvature, geometric center of sphere of radius. C = 2f =radius Principle axis: line that passes through both the center of curvature (C) and the focal point (F) and intersects the mirror at a right angle. 38

39 Concave Mirrors Light source Principle Axis C F Convex Mirrors Light source Principle Axis F C 39

40 A curved mirror has a geometric center or vertex A The center of curvature or radius C The focal length f of the mirror is half the radius f 1 2 R

41 C=2f f =? C/2 41

42 A concave mirror has a radius of curvature of 15 cm. What is the focal length of this mirror? A. 15 cm B. 30 cm C. 7.5 cm 42

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44 Ray Diagram Rules 1. Start at top of object. Light ray 1 travels parallel to the principle axis, strikes the mirror, and is reflected back through the focal point (F). 2. Light ray 2 travels from top of object and strikes the mirror, is in line with the focal point. It is reflected back parallel to the principle axis. 3. Where these 2 reflected light rays intersect is the location of the image. Sometimes it is necessary to extend reflected rays beyond mirror. If above axis, erect; if below, inverted. 44

45 Hints Images in front of mirror are always real Real Images are always inverted Inverted images are always below the principle axis If reflected rays do not cross, extend them behind mirror Images behind mirror are always virtual Virtual Images are always erect Erect images are always above the principle axis 45

46 Perform Ray Diagram Use Table in notes to summarize 46

47 Object Location Real or Virtual Upright or Inverted Size Image Location Plane Mirror Converging Concave Mirror Diverging Convex Mirror 47

48 Locating images in concave mirrors Fill in hypertextbook.com 48

49 Concave Mirror Describe image when Object located beyond C Real or Virtual? Real Erect or Inverted? Inverted Same size, Enlarged or Reduced? Reduced Location? Between f and C 49

50 Concave Mirror with the Object located beyond C 50

51 Concave Mirror Object beyond C Light rays that travel parallel to the principle axis, strike the mirror, and are reflected back through the focal point (f). 51

52 Concave Mirror Object beyond C Light rays that travel through the focal point (f), strike the mirror, and are reflected back parallel to the principle axis. 52

53 Concave Mirror Object beyond C Image: Real Inverted Smaller Between f and C The image is located where the reflected light rays intersect 53

54 Concave Mirror with the Object located at C 54

55 Concave Mirror Object at C Light rays that travel parallel to the principle axis, strike the mirror, and are reflected back through the focal point (f). 55

56 Concave Mirror Object at C Light rays that travel through the focal point (f), strike the mirror, and are reflected back parallel to the principle axis. 56

57 Concave Mirror Object at C Image: Real Inverted Same Size At C The image is located where the reflected light rays 57 intersect

58 Concave Mirror with the Object located between f and C 58

59 Concave Mirror Describe image when Object located between f and C Real or Virtual? Real Erect or Inverted? Inverted Same size, Enlarged or Reduced? Larger Location? Beyond C 59

60 Concave Mirror Object between f and C f C Light rays that travel parallel to the principle axis, strike the mirror, and are reflected back through the focal point (f). 60

61 Concave Mirror Object between f and C f C Light rays that travel through the focal point (f), strike the mirror, and are reflected back parallel to the principle axis. 61

62 Concave Mirror Object between f and C Image: Real Inverted Larger Beyond C f C The image is located where the reflected light rays 62 intersect

63 Concave Mirror with the Object located at f 63

64 Concave Mirror Object at f Light rays that pass through the center of curvature hit the mirror and are reflected back along the same path. 64

65 Concave Mirror Object at f Light rays that travel parallel to the principle axis, strike the mirror, and are reflected back through the focal point (f). 65

66 Concave Mirror Object at f No image is formed. All reflected light rays are parallel and do not cross 66

67 Concave Mirror with the Object located between f and the mirror 67

68 Concave Mirror Describe image when Object located between f and mirror Real or Virtual? Virtual Erect or Inverted? Erect Same size, Enlarged or Reduced? Larger Location? Further away, behind mirror 68

69 Concave Mirror Object between f and the mirror Light rays that travel through the focal point (f), strike the mirror, and are reflected back parallel to the principle axis. 69

70 Concave Mirror Object between f and the mirror Light rays that travel parallel to the principle axis, strike the mirror, and are reflected back through the focal point (f). 70

71 Concave Mirror Object between f and the mirror Image: Virtual Upright Larger Further away The image is located where the reflected light rays 71 intersect

72 Locating images in convex mirrors 72

73 Convex Mirror with the Object located anywhere in front of the mirror 73

74 Convex Mirror Object located anywhere f C Light rays that travel parallel to the principle axis, strike the mirror, and are reflected back through 74 the focal point (f). f is negative.

75 Convex Mirror Object located anywhere f C Light rays that travel through (toward) the focal point (f), strike the mirror, and are reflected back parallel to the principle axis. 75

76 Convex Mirror Object located anywhere Image: Virtual Upright Smaller Behind mirror, inside f d i is negative f C The image is located where the reflected light rays 76 intersect

77 4/11 Friday we drew ray diagrams. If you were absent, you will have to complete these and show them to me. You will need a calculator today. Have notes out Houses Due Wed, Test on Friday Warm up #2 What part of the EM spectrum causes sunburn? Can it penetrate glass?

78 THE MIRROR EQUATION The mirror equation can be used to locate the image: d d f o i Where d o is the object s distance, d i is the image distance and f is the focal length. M h h i o The ratio M is called the magnification, h o is the object s size and h i is the image size. d d i o

79 F = focal point f =focal length = C/2 or r/2 C = center of curvature, radius d o = distance between object and mirror/lens d i = distance between image and mirror/lens h o = height of object h i = height of image m = optical enlargement of an object, ratio of size of image to size of object 79

80 R radius of curvature C + converging Concave Mirror - diverging Convex Mirror f focal length + converging - diverging d o object distance + real object + real object d i image distance + real images - virtual images h o object size + if upright + if upright h i image size + if upright - if inverted

81 Magnification + Upright Virtual - Inverted Real > 1 Enlarged < 1 Reduced 81

82 Ex 2 A concave mirror has a focal length of 25 cm. An object is placed 32 cm from the mirror. f = 25 cm d o = 32 cm What kind of image? Why? Real, object beyond F in concave mirror 1 d i Calculate d i = 1 f - 1 d o 1/d i = 1/25cm 1/32cm 1/d i = d i = cm

83 b. What is the magnification? f = 25 cm d o = 32 cm d i = cm M = -d i d 0 = cm 32 cm M = Negative means upside down, enlarged if greater than one

84 Ex 3 A smooth reflective disco ball hangs from a ceiling. It has a diameter of 50 cm. An object (16 cm tall) is placed 3 meters from the mirror. diameter = 50 cm What is C and f? Why is f negative? d o = 3 meters (is this unit a problem?) What kind of image? Why? Virtual, all images are upright in convex mirror f= cm d o = 300 cm h o = 16 cm 1 d i Calculate d i = 1 f - 1 d o 1/d i = 1/-12.5cm -1/300cm 1/d i = d i = -12 cm Negative because behind mirror

85 Ex 3 A smooth reflective disco ball hangs from a ceiling. It has a diameter of 50 cm. An object (16 cm tall) is placed 3 meters from the mirror. diameter = 50 cm What is C and f? d o = 3 meters (is this unit a problem?) What is the height of the image? Determine M, then use that to determine image height Calculate M f= cm d o = 300 cm h o = 16 cm d i = -12 cm M = - d i d o M = - (-12cm)/300cm M = 0.04 h i = 0.04 (16 cm) = 0.64cm

86 Example 4: Suppose you are 19 cm to the side of the bell of your friend s trumpet and you see your image at 14 cm. Treating this as a concave mirror, what would be its focal length and radius of curvature? d o = 19 cm d i = 14 cm do di f cm 1 = 1 14cm f f = 8.06 cm R = 2f = 2(8.06cm) = cm

87 A CHALLENGE Problem Example 5: Suppose you place a 5.0 cm tall pencil in front of a concave mirror. The mirror has a focal length of 24 cm. The pencil forms an image that appears to be at the same position as the pencil, but the image is inverted. a. Where did you place the pencil? h o = 5 cm f = 24 cm d o = d i So d d f o i d d f o o 2 d o = 1 f d o = 2f = 2(24cm) = 48 cm

88 b. How tall is the pencil s image? h o = 5 cm h h i o d d i o h i h 0 = -1 h i = -h o = - 5 cm