Optics: Reflection and Refraction

1. Optics:Reflection and Refraction

3. Shadows • Umbra – total shadow, light is blocked • Penumbra – only part of the light is blocked

4. Shadows • Solar eclipse • The Moon passes between the Sun and the Earth and casts a shadow During what phase of the moon are eclipses possible?

5. Solar eclipse from space

6. Solar Eclipses • If a solar eclipse is caused by the Moon passing in between the Sun and the Earth, why don’t we see an eclipse every ~28 days?

8. Solar Eclipses • Total solar eclipse visible from the US: August 21st 2017

11. Lunar Eclipses • Lunar eclipses occur when the Moon passes through Earth’s shadow.

12. Blood Moon • Indirect sunlight passes through Earth’s atmosphere, where blue light is scattered, this filters out the blue light and illuminates the Moon in a red glow

14. Reflection and Refraction • Reflection: light is returned to the medium from which it came • Refractions: light crosses from one transparent material to another

15. Principle of Least Time

16. Principle of Least Time • Principle of Least Time: Out of all the possible paths light might take to get from one point to another, light always takes the path that results in arriving in the least amount of time.

17. Ants follow this principle too

18. Reflection • Law of Reflection • Angle of Incidence EQUALS the Angle of Reflection

19. Reflection • Straight/Plane Mirror • Image behind the mirror is called a VIRTUAL IMAGE • Examples • (Billiards and Bank Shots…)

20. Straight/plane mirror • Virtual image is… • As far behind the mirror and the real object is in front of the mirror. • Same size as object

21. Straight plane mirror

22. Bankshot in Billiards

23. Warm-up • If you stand 3 ft in front of a mirror, how far away is your image from you? 2. If you walk toward a mirror at 4m/s, how fast is your image approaching you? 3. If you are 4 ft tall, how tall does your mirror have to be to see your whole body?

24. Reflection (cont’d) To see more of her head in the mirror, shea) should hold the mirror closer b) should hold the mirror father away c) needs a bigger mirror

25. Reflection (cont’d) • Concave Mirror • When up-close, images are magnified and upright • Example: Vanity mirrors • When the object is far away, images are minified and inverted • Convex Mirror • Images are always minified and upright • Example: “7-Eleven” mirror

27. Object behind focus

28. Object in front of focus

30. Rules for Concave Mirrors • Any incident ray traveling parallel to the principal axis on the way to the mirror will pass through the focal point upon reflection. • Any incident ray passing through the focal point on the way to the mirror will travel parallel to the principal axis upon reflection.

31. Rules for Convex Mirrors • Any incident ray traveling parallel to the principal axis on the way to a convex mirror will reflect in such a manner that its extension will pass through the focal point. • Any incident ray traveling towards a convex mirror such that its extension passes through the focal point will reflect and travel parallel to the principal axis.

32. Refraction • Cause of Refraction • The change in speed of a light wave

33. Refraction

34. Refraction • Snell’s Law of Refraction • When a light ray slows down as it enters a new medium, the ray will be bent towards the perpendicular. • When a light ray speeds up as it enters a new medium, the ray will be bent away from the perpendicular. • n1 sin 1 = n 2 sin 2 • n = c / v , n is called the index of refraction, v is the speed of light in the medium • n = 0 / 

35. Snell’s Law n1 * sin θ1 = n2 * sin θ2 Snell's law gives the relationship between angles of incidence and refraction for a wave impinging on an interface between two media with different indices of refraction. 

36. Snell’s law practice • Light travels from air into an optical fiber with an index of refraction of 1.44.  (a)  In which direction does the light bend?  (b)  If the angle of incidence is 22o, what is the angle of refraction inside the fiber?  (c)  Sketch the path of light as it changes media.

37. Snell’s law •  Using the information given in the following diagram, calculate the optical index of refraction for medium B

38. Index of refraction • The amount by which light slows in a given material is described by the index of refraction, n. The index of refraction of a material is defined by the speed of light in vacuum c divided by the speed of light through the material v: n = c/v

39. How would the fish see you?

40. A coin lies submerged at the bottom of a pan of water. Does refraction of light from the coin make it appear deeper, or make it appear shallower than it really is? Bruno wishes to "spear" a fish with a laser. Should he aim the laser beam above, below, or directly at the observed fish to make a direct hit? Refraction (con’t)

41. Total Internal Reflection When light goes from a more dense medium to a less dense medium at a great enough angle it will be reflected back into the dense medium

42. Total Internal Reflection Requirements: • the light is in the more dense medium and approaching the less dense medium. • the angle of incidence is greater than the so-called critical angle.

43. Diamonds Why does the cut of a diamond matter?

44. Diamonds

45. Penny activity (per table) • Grab a beaker from the sink and fill almost to the top with water. Grab a penny and a paper plate.

46. Refraction • Mirages • Inferior Mirages • When the air is warmer on the ground light will travel faster through it and bend upward • Image appears lower than it is and upside down • Example: “puddle” on the road

47. Refraction • Mirages • Superior Mirages • When the air near surface is cooler than air above (temperature inversion) light is bent downward • Image appears higher than it is and right side up • Example: Flying Dutchman and ghost ships

48. Rainbows • Rainbows • Blue wavelengths of light bend more than red and get dispersed in a raindrop

49. Double Rainbow

50. The Flying Dutchman