Lect17 handout
- 1. Physics 102: Lecture 17, Slide 1 Physics 102: Lecture 17 Reflection and Refraction of Light
- 2. Physics 102: Lecture 17, Slide 2 Recall from last time…. Reflection: Refraction: Flat Mirror: image equidistant behind Spherical Mirrors: Concave or Convex TodayLastTime θi θr θ1 θ2 n2 n1 θi = θr n1 sin(θ1)= n2 sin(θ2)
- 3. Physics 102: Lecture 17, Slide 3 O Concave Mirror Principal Rays f c 1) Parallel to principal axis reflects through f. #1 3) Through center. #3 Image is (in this case): • Real (light rays actually cross) • Inverted (Arrow points opposite direction) • Reduced (smaller than object) **Every other ray from object tip which hits mirror will reflect through image tip 2) Through f, reflects parallel to principal axis. #2 I
- 4. Physics 102: Lecture 17, Slide 4 C f 1) 2) 3) p.a. Preflight 17.1 Ray through center should reflect back on self. 20% 45% 35% Which ray is NOT correct?
- 5. Physics 102: Lecture 17, Slide 5 O I Mirror Equation 1 do + 1 di = 1 f f c do di • do = distance object is from mirror: Positive: object in front of mirror Negative: object behind mirror • di = distance image is from mirror: • Positive: real image (in front of mirror) • Negative: virtual image (behind mirror) • f = focal length mirror: • Positive: concave mirror +R/2 • Negative: convex mirror –R/2 Works for concave, convex, or flat
- 6. Physics 102: Lecture 17, Slide 6 Preflight 17.3 The image produced by a concave mirror of a real object is: 1) Always Real 2) Always Virtual 3) Sometimes Real, Sometimes Virtual
- 7. Physics 102: Lecture 17, Slide 7 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 2) Far from mirror 3) Either close or far 4) Not Possible
- 8. Physics 102: Lecture 17, Slide 8 3 Cases for Concave Mirrors Inside F•C F •Object Image •C F •Object Image •C F •Object Image Between C&F Past C Real Virtual Real
- 9. Physics 102: Lecture 17, Slide 9 O I Magnification Equation do do ho Angle of incidence d i-hi Angle of reflection o i o i d d h h m −=≡ o o d h =)tan(θ i i d h− = di • ho = height of object: • Positive: always • 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 θ θ θ θ
- 10. Physics 102: Lecture 17, Slide 10 Solving Equations A candle is placed 6 cm in front of a concave mirror with focal length f=2 cm. Determine the image location. cm2 11 cm6 1 =+ id di = + 3 cm (in front of mirror) Real Image! C f p.a. Preflight 17.2 Compared to the candle, the image will be: • Larger • Smaller • Same Size
- 11. Physics 102: Lecture 17, Slide 11 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? 1) 2 inches 2) 4 inches 3) 8 inches What direction will the image arrow point? 1) Up 2) Down 4 inches
- 12. Physics 102: Lecture 17, Slide 12 3 Cases for Concave Mirrors Inside F•C F •Object Image •C F •Object Image •C F •Object Image Between C&F Past C Inverted Enlarged Real Upright Enlarged Virtual Inverted Reduced Real
- 13. Physics 102: Lecture 17, Slide 13 f image object Demo: • two identical spherical mirrors • each mirror is positioned at the focal point of the other Demo: optical illusion
- 14. Physics 102: Lecture 17, Slide 14 O Convex Mirror Rays c 1) Parallel to principal axis reflects through f. 2) Through f, reflects parallel to principal axis. #2 I 3) Through center. #3 Image is: Virtual (light rays don’t really cross) Upright (same direction as object) Reduced (smaller than object) (always true for convex mirrors!): f #1
- 15. Physics 102: Lecture 17, Slide 15 Solving Equations 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?
- 16. Physics 102: Lecture 17, Slide 16 Preflight 17.4 The image produced by a convex mirror of a real object is 1) always real 2) always virtual 3) sometimes real and sometimes virtual
- 17. Physics 102: Lecture 17, Slide 17 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 • • 1 do + 1 di = 1 f o i o i d d h h m −=≡
- 18. Physics 102: Lecture 17, Slide 18 Index of Refraction Frequency is the same, wavelength decreases Recall speed of light c = 3x108 m/s is in vacuum In a medium (air, water, glass...) light is slower n is a property of the medium: nvacuum = 1 nair = 1.0003 nwater = 1.33 nglass = 1.50 v = c/n n ≥ 1 c = λ/f v < ccλ1 λ2 Speed of light in vacuum Speed of light in medium “Index of refraction” vacuum glass
- 19. Physics 102: Lecture 17, Slide 19 Reflected wave Refracted wave Incident wave n1 n2 > n1 λ2 < λ1 λ1 θ1 θr θ2 Snell’s law of Refraction n1 sin(θ1)= n2 sin(θ2) When light travels from one medium to another, v (and λ) changes (v = c/n). So the light bends!
- 20. Physics 102: Lecture 17, Slide 20 n1 n2 Snell’s Law Practice n1 sin θ1 = n2 sin θ2 normal 2 θ 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 θr = 60. The other part of the beam is refracted. What is θ2? sin(60) = 1.33 sin(θ2) θ2 = 40.6 degrees θ1 = θr = 60 θ1 θr Usually, there is both reflection and refraction!
- 21. Physics 102: Lecture 17, Slide 21 n2 n1 d d′ ′d = d n2 n1 Apparent depth: Apparent Depth 50 actual fish apparent fish