Reflection and Refraction of Light Physics 102: Lecture 17 Reflection and Refraction of Light Today’s Lecture will cover textbook sections 23.3, 8 Physics 102: Lecture 17, Slide 1 1

Overview Reflection: qi = qr Refraction: n1 sin(q1)= n2 sin(q2) Last Time qi qr qi = qr Flat Mirror: image equidistant behind Spherical Mirrors: Concave or Convex Today q1 q2 n2 n1 Next time Refraction: n1 sin(q1)= n2 sin(q2) Flat Lens: Window Spherical Lenses: Concave or Convex Absorption Physics 102: Lecture 17, Slide 2

Concave Mirror Principal Rays 1) Parallel to principal axis reflects through f. 2) Through f, reflects parallel to principal axis. 3) Through center. #3 #2 O #1 f c Image is (in this case): Real or Imaginary Inverted or Upright Reduced or Enlarged Comment that this is “weird” to have a real image. **Every other ray from object tip which hits mirror will reflect through image tip Physics 102: Lecture 17, Slide 3

Preflight 17.1 Which ray is NOT correct? 1) 2) 3) p.a. 1) R f 2) 3) Physics 102: Lecture 17, Slide 4

Preflight 17.1 Which ray is NOT correct? Ray through center should reflect back on self. p.a. 1) R f 2) 3) Physics 102: Lecture 17, Slide 5

Mirror Equation do O f c do = distance object is from mirror: I di Positive: object _______ of mirror Negative: object _______ mirror di di = distance image is from mirror: Positive: _______ image (__________ of mirror) Negative: _______ image (__________ mirror) f = focal length mirror: Positive: _________ mirror Negative: _________ mirror (coming soon) Physics 102: Lecture 17, Slide 6

Mirror Equation do O f c do = distance object is from mirror: I di Positive: object in front of mirror Negative: object behind mirror di di = distance image is from mirror: Positive: inverted image (in front of mirror) Negative: upright image (behind mirror) f = focal length mirror: Positive: concave mirror Negative: convex mirror (coming soon) Physics 102: Lecture 17, Slide 7

Preflight 17.3 ACT: Concave Mirror The image produced by a concave mirror of a real object is: Always Real Always Virtual Sometimes Real, Sometimes Virtual ACT: Concave Mirror Where in front of a concave mirror should you place an object so that the image is virtual? 1) Close to mirror 3) Either close or far 2) Far from mirror 4) Not Possible Physics 102: Lecture 17, Slide 8

Preflight 17.3 The image produced by a concave mirror of a real object is: Always Real Always Virtual Sometimes Real, Sometimes Virtual Concave mirror: f > 0 Real Object means in front of mirror: do > 0 Mirror Equation: di can be negative or positive!

ACT: Concave Mirror Where in front of a concave mirror should you place an object so that the image is virtual? Mirror Equation: Close to mirror Far from mirror Either close or far Not Possible Concave mirror: f > 0 Object in front of mirror: do > 0 Virtual image means behind mirror: di < 0 When do < f then di <0 : virtual image. Physics 102: Lecture 17, Slide 10

Magnification Equation do O do ho Angle of incidence ho = height of object: Positive: di hi Angle of reflection I hi = height of image: Positive: Negative: m = magnification: Positive / Negative: same as for hi < 1: > 1: di Physics 102: Lecture 17, Slide 11

Magnification Equation do O do ho Angle of incidence ho = height of object: Positive: always q di hi Angle of reflection I hi = height of image: Positive: image is upright Negative: image is inverted m = magnification: Positive / Negative: same as for hi < 1: image is reduced > 1: image is enlarged di Physics 102: Lecture 17, Slide 12

Solving Equations Preflight 17.2 Example A candle is placed 6 cm in front of a concave mirror with focal length f=2 cm. Determine the image location. Preflight 17.2 Compared to the candle, the image will be: Larger Smaller Same Size p.a. R f Physics 102: Lecture 17, Slide 13

Solving Equations Preflight 17.2 Example A candle is placed 6 cm in front of a concave mirror with focal length f=2 cm. Determine the image location. di = + 3 cm (in front of mirror) Real Image! Preflight 17.2 Compared to the candle, the image will be: Larger Smaller Same Size p.a. R f Physics 102: Lecture 17, Slide 14

ACT: Magnification A 4 inch arrow pointing down is placed in front of a mirror that creates an image with a magnification of –2. What is the size of the image? 2 inches 4 inches 8 inches 4 inches What direction will the image arrow point? Up 2) Down Physics 102: Lecture 17, Slide 15

ACT: Magnification A 4 inch arrow pointing down is placed in front of a mirror that creates an image with a magnification of –2. What is the size of the image? 2 inches 4 inches 8 inches 4 inches Magnitude gives us size. What direction will the image arrow point? Up 2) Down (-) sign tells us it’s inverted from object Physics 102: Lecture 17, Slide 16

3 Cases for Concave Mirrors Object Image Upright Enlarged Virtual Inside F C F Object Image Inverted Enlarged Real Between C&F C F Object Image Inverted Reduced Real Past C Physics 102: Lecture 17, Slide 17

Convex Mirror Rays 1) Parallel to principal axis reflects ______________. 2) Through f, reflects ______________________. 3) Through center. Complete the rays! #1 O #2 #3 c Make this mirror almost flat so that it is like your rearview mirror f Image is: Virtual or Real Upright or Inverted Reduced or Enlarged (always true for convex mirrors!) Physics 102: Lecture 17, Slide 18

Convex Mirror Rays 1) Parallel to principal axis reflects through f. 2) Through f, reflects parallel to principal axis. 3) Through center. #1 O #2 #3 I f c Make this mirror almost flat so that it is like your rearview mirror Image is: Virtual (light rays don’t really cross) Upright (same direction as object) Reduced (smaller than object) (always true for convex mirrors!): Physics 102: Lecture 17, Slide 19

Solving Equations Example A candle is placed 6 cm in front of a convex mirror with focal length f=-3 cm. Determine the image location. Determine the magnification of the candle. If the candle is 9 cm tall, how tall does the image candle appear to be? Physics 102: Lecture 17, Slide 20

Solving Equations Example A candle is placed 6 cm in front of a convex mirror with focal length f=-3 cm. Determine the image location. Determine the magnification of the candle. If the candle is 9 cm tall, how tall does the image candle appear to be? di = - 2 cm (behind mirror) Virtual Image! m = + 1/3 hi = + 3 cm Image is Upright! Physics 102: Lecture 17, Slide 21

Preflight 17.4 Where should you place an object in front of a convex mirror to produce a real image? Object close to mirror Object far from mirror Either close or far You can’t Physics 102: Lecture 17, Slide 22

Preflight 17.4 Where should you place an object in front of a convex mirror to produce a real image? Mirror Equation: Object close to mirror Object far from mirror Either close or far You can’t di is negative! f is negative do is positive Convex mirror: f < 0 Object in front of mirror: do > 0 Real image means di > 0 Physics 102: Lecture 17, Slide 23

Mirror Summary Angle of incidence = Angle of Reflection Principal Rays Parallel to P.A.: Reflects through focus Through focus: Reflects parallel to P.A. Through center: Reflects back on self |f| = R/2 Physics 102: Lecture 17, Slide 24

Light Doesn’t Just Bounce It Also Refracts! Reflected: Bounces (Mirrors!) qi = qr qi qr Refracted: Bends (Lenses!) n1 sin(q1)= n2 sin(q2) q1 q2 n2 n1 Physics 102: Lecture 17, Slide 25

Index of Refraction 186,000 miles/second: it’s not just a good idea, it’s the law! Speed of light in vacuum Speed of light in medium Index of refraction so always! Physics 102: Lecture 17, Slide 26

Snell’s Law Preflight 17.6 n1 sin(q1)= n2 sin(q2) When light travels from one medium to another the speed changes v=c/n, but the frequency is constant. So the light bends: n1 sin(q1)= n2 sin(q2) Preflight 17.6 n1 q1 1) n1 > n2 2) n1 = n2 3) n1 < n2 q2 n2 Compare n1 to n2. Physics 102: Lecture 17, Slide 27

Snell’s Law Preflight 17.6 n1 sin(q1)= n2 sin(q2) When light travels from one medium to another the speed changes v=c/n, but the frequency is constant. So the light bends: n1 sin(q1)= n2 sin(q2) Preflight 17.6 n1 q1 1) n1 > n2 2) n1 = n2 3) n1 < n2 q1 < q2 q2 n2 sinq1 < sinq2 n1 > n2 Compare n1 to n2. Physics 102: Lecture 17, Slide 28

Snell’s Law Practice Example Usually, there is both reflection and refraction! A ray of light traveling through the air (n=1) is incident on water (n=1.33). Part of the beam is reflected at an angle qr = 60. The other part of the beam is refracted. What is q2? 1 r n1 n2 normal Physics 102: Lecture 17, Slide 29

Snell’s Law Practice Example Usually, there is both reflection and refraction! A ray of light traveling through the air (n=1) is incident on water (n=1.33). Part of the beam is reflected at an angle qr = 60. The other part of the beam is refracted. What is q2? q1 =qr =60 1 r sin(60) = 1.33 sin(q2) n1 q2 = 40.6 degrees n2 normal Physics 102: Lecture 17, Slide 30

Apparent Depth d d Apparent depth: n2 n1 apparent fish actual fish 50

See you later! Read Sections 23.3 - 23.5, 23.9 Physics 102: Lecture 17, Slide 32