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Chapter 34

Chapter 34. Geometric Optics. What is Geometric Optics. It is the study of light as particles. Geometric optics treats light as particles (or rays) that travels in straight lines.

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Chapter 34

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  1. Chapter 34 Geometric Optics

  2. What is Geometric Optics It is the study of light as particles. Geometric optics treats light as particles (or rays) that travels in straight lines. Physical optics (wave optics) deals with the wave nature of light, such as the spreading of waves (diffraction) and the interference of waves.

  3. Two types of lens

  4. Converging and Diverging Lens

  5. Symbols Diverging Lens = Concave Lens = Negative Lens Converging Lens = Convex Lens = Positive Lens

  6. Some notations f: focal length i: image distance p: object distance F: focal point O: object I: image Principle axis Your textbook: f: focal length s': image distance s: object distance

  7. Converging Lens

  8. Converging Lens (thin) Rules: 1. Rays parallel to axis Pass through focal point 2. Rays through focal point Pass parallel to axis 3. Rays through the center Pass through unaffected

  9. Where is the image? Trace at least two light rays. The image is at where the rays meet.

  10. How to find the image Trace two rays of light. Where they meet is where the image is.

  11. Example Complete the light rays below to find the image. Image

  12. What if you move the object beyond 2F?

  13. Solution

  14. In fact all light rays from the object pass through the image You only need two rays to find the image

  15. What if you now move it between F and 2F?

  16. At F?

  17. Between 0 and F?

  18. Demo Exploration of Physics

  19. Summary

  20. Real and Virtual Image Real image can be projected directly on a screen. Virtual image cannot be projected directly on a screen without extra lenses or mirrors. Real image forms when rays actually converge and meet. Virtual image forms when rays diverge and do not meet. Position of a virtual image is found by tracing the rays backward.

  21. Real and virtual image Real Virtual

  22. Lens Equation

  23. Lateral Magnification

  24. Magnification is related to i and p

  25. Magnification is related to i and p

  26. The sign of magnification m Since the image is inverted, we use minus sign in front.

  27. The sign of i i is positive if the image is on the right i is negative if the image is on the left i >0 i<0

  28. The sign of m i >0, m<0, inverted i <0, m>0, upright

  29. Example Assume f = 1m, complete the table below (in meters).

  30. Solution Assume f = 1m, complete the table below.

  31. Diverging Lens (thin) Rules: 1. Rays parallel to axis Pass through focal point 2. Rays through focal point Pass parallel to axis 3. Rays through the center Pass through unaffected

  32. 2cm Example: f =-2cm Negative f Diverging lens obeys the lens equation too. Except that fis now negative.

  33. Complete the light rays and find the image

  34. Solution

  35. Different p

  36. The typical case for concave lens For concave lens, the image is always virtual and upright, no matter where the object is.

  37. Can you see why i is always negative?

  38. Example: Diverging Lens You are given a diverging lens of focal length 20cm. You want to form an virtual image that is 1/3 the height of the object. Where should the object be placed?

  39. Ray Diagram for the Example

  40. Converging or diverging lens? Converging

  41. Converging or diverging lens? Diverging

  42. Converging or diverging lens? Converging

  43. Confusing signs

  44. Lensmaker’s Equation Proof not required. No need to memorize, will be given in the exam. One question in Mastering Physics. R>0 if convex (bulging) toward the object.

  45. Curved mirrors

  46. Curved Mirrors Concave mirror Converging mirror Positive mirror Convex mirror Diverging mirror Negative mirror

  47. Terminology

  48. Symbols Converging Mirror = Concave Mirror Diverging Mirror = Convex Mirror

  49. Only one focal point for curved mirrors A lens has two focal points (one on each side of the lens), but a curved mirror only has one focal point. It is important to remember where they are. Concave lens: F in front of the mirror Convex lens: F behind the mirror

  50. Concave Mirror Rules Rules: 1. Rays parallel to axis Reflect through focal point 2. Rays through focal point Reflect parallel to axis 3. Rays through the center Reflect with equal angle F Really the same rules as converging lens

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