 Download Presentation Light Ray, Light Ray

# Light Ray, Light Ray

Download Presentation ## Light Ray, Light Ray

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1. No Light, No Sight Light Ray, Light Ray A Cruise Through the Wonderful World of Reflection and Refraction (aka Geometric optics)

2. What is Light? • Electromagnetic Radiation with wavelength (visible) 400 – 700 nm (nanometers) • Travels in straight lines • Bounces (reflects) off certain materials • Refracts (bends) in transparent materials • Travels at c = 3 x 108 m/sec in vacuum

3. Index of Refraction n = c/v • Light slows down in transparent materials other than vacuum(like car slows in sand) • v = c/n velocity of light in medium • n is called index of refraction • n = about 1.5 for glass • n = 1.33 for water

4. Reflection • Light bounces off objects • Consider “rays” – light moving in straight line • Law of reflection: angle of incidence = angle of reflection

5. Law of Reflection

6. Law of Reflection Qi = Qr • I - incident ray • R - reflected ray • N – normal • Theta-I is angle of incidence • Theta –R is angle of reflection

7. Types of Reflection

8. How Plane Mirror Forms Image

9. Real vs. Virtual Image • Real image – light is present at the image position • Virtual image – no light is present at the image position

10. Image From Plane Mirror • Virtual • Behind mirror • Right side up(upright) • Left-right reversed • Located equal Distance behind mirror • Same size as object

11. How Curved Mirror Forms Real Images

12. How Law of Reflection Leads to Images from Spherical Mirror • C is center of curvature • F is focal point • Real image will be located on same side of mirror as object • C = 2f

13. Rules of Reflection – Two “Special” Rays • An incident ray parallel to principal axis will pass through the focal point after reflection. • An incident ray passing through the focal point will leave mirror parallel to principal axis

14. What’s So “Special?” About Special Rays? • Its easy to predict where they will go • Use Law of Reflection

15. Ray Diagram – Object Beyond C = 2f • Real image • Inverted • Smaller than object • Note use of two “special” rays • What is another?

16. Image inverted Located at 2f Same size as object Image inverted Further than 2f Larger than object Ray Diagram – Object at C = 2f or between f and 2f

17. Object at 2f

18. Object Between f and 2f

19. Object Closer to Mirror Than f“Shaving Mirror” Case • Image virtual • Behind mirror • Upright • Larger than object

20. Object Closer to Mirror Than F

21. Object at f • Must use different ray(goes to intersection of principal axis and mirror) • No image formed

22. Mirror Equation Derivation diagram • f is positive for concave mirror Question: Where will image formed by a lens with 20 cm focal length be if object is placed 60 cm from mirror? di = 30 cm

23. How Big is The Image? • M is magnification • - sign means image is inverted Question: If the object in the question on previous slide is 2 cm high, how high is image? 1 cm

24. Convex (Diverging) Mirrors • f is negative • Light rays spread, not focused; images virtual • 7-11 mirror foils thieves

25. Ray Diagram for Convex Mirror • One ray parallel to p.a. • Second ray heads for focus behind mirror • Diverging rays must be extended behind mirror (dotted lines) • Image virtual, upright, smaller

26. Ray Diagram for Convex Mirror • One ray parallel to p.a. • Second ray heads for focus behind mirror • Diverging rays must be extended behind mirror (dotted lines) • Image virtual, upright, smaller

27. Why Are Some Rearview Mirrors Convex?

28. Problem • A 1.6 m tall thief stands 10m away from a 7-11 mirror with 2m (negative) focal length. Where is the image and how tall is it? • Use • Note: negative di means image behind mirror di = -1.67 m hi = 0.267m

29. solution • 1/di = -5/10 – 1/10 = -6/10 • di = -10/6 = -5/3 = -1.67 m • hi/ho = -di/d0 = 1.67/10 • hi =.167 x 1.6 = 0.267m

30. Index of Refraction • Ratio of speed of light in vacuum to speed of light in material • n = c/v = 3.0 x 108 m/s/v • n always greater than one

31. Refraction: How Much Does It Bend • Angle of incidence Qi • Angle of refraction Qr • Snell’s Law: ni sin Qi = nr sin Qr

32. Helpful Analogy:The Band of Sand • What happens when a car drives into the sand? sand highway Which way does the car turn?

33. Toward and Away from Normal • When light enters a more dense (greater n) medium, it bends toward the normal • When light enters a less dense (smaller n) medium, it bends away from the normal

34. Mysteries? • Why do a person’s legs appear shorter when they are standing in water? • Why Does A Glass Rod Disappear in Mineral Oil?

35. Mysteries? • Why do a person’s legs appear shorter when they are standing in water? • Why Does A Glass Rod Disappear in Mineral Oil?

36. Mysteries? • Why do a person’s legs appear shorter when they are standing in water? • Why Does A Glass Rod Disappear in Mineral Oil? • Both have same index of refraction

37. Practical Application • Fluorocarbon (semi invisible under water) fishing line

38. Problem • Light strikes a flat piece of glass(n = 1.5) at 60 degrees to the normal. What is the angle of the light in the glass? Qr Qi = 60o

39. Solution • ni sin Qi = nr sin Qr • ni = 1 nr = 1.5 sinQr =1.00x sinQi /1.5= 0.577 Qr = 35.2o

40. Total Internal Reflection

41. Total Internal Reflection • Light ray leaves more dense medium • Angle of refraction approaches 900 • Past critical angle there is no refracted ray

42. Critical AngleThe angle of incidence past which there is no refracted ray • ni sin Qi = nr sin Qr • sin Qc = nr/ni sin900 =nr/ni • If ray emerges into air • sin Qc = 1/ni Qc

43. Example • What is the critical angle for light rays leaving a swimming pool? What does the world look like to a swimmer at the bottom of the pool? Sin Qc = 1.00/1.33 = 0.750 Qc = 490 Swimmer sees outside world compressed into a circle whose edge makes a 49 degree angle to the vertical

44. Applications of TIR • Prisms in Binoculars

45. Applications of TIR • Prisms in Binoculars

46. Fiber Optics • Fiber Optic Amplifier Module Spy under door optics Photos courtesy JDS Uniphase Inc. endoscope

47. Lenses • Lenses can focus or diverge light • Act like tiny prisms

48. Thin Lens • Convex lenses have two focal points