1. CHAPTER 22 Reflection and Refraction of Light A is A “Something cannot be itself and something else at the same time.” Aristotle Light exhibits wave-like properties when studied under certain conditions. Light exhibits particle-like properties when studied under certain conditions. Light does not exhibit wave-like and particle-like properties simultaneously.

2. Properties of Light Speed: In a vacuum (c): 2.997924574x10 8 m/s c = 3.00x10 8 m/s In other mediums:Speed of light is less than c. Nothing can travel faster than the speed of light. Direction: Direction:Light travels in a straight line path until it encounters a boundary between two different mediums.

3. Ray Model of Light Reflection: Reflection:Light rays reflect (bounce) off the surface of a new medium that it encounters in a very predictable fashion. The “Normal” is a line perpendicular to the surface of the second medium.  1 =  1 ’ Angle of Incidence (  1 ) = Angle of Reflection (  1 ’) Angles are measured from the Normal.

4.  I =  r Always!! This law is obviously true in some situations and not so obviously true in other situations. Light rays reflect from the smooth surface in only one direction because all the Normal lines are parallel to each other. All  i are the same  All  r are the same. Light rays reflect in many directions from a rough surface because each ray encounters the surface with a Normal not parallel to the other Normals. However, for each ray:  i =  r

5. Ray Model of Light Refraction: Refraction:The tendency for light ray to bend when traveling from one medium into another medium. Examples: Examples:Rays traveling from: air to water air to glass water to glass Law of Reflection  1 =  1 ’ v 1 = speed of light in air (still approximately 3.0x10 8 m/s) v 2 = speed of light in glass v 1  v 2  1 = Incident Angle  1 ’ = Reflected Angle  2 = Angle of Refraction

6. Light takes the quickest path between two points. Only 1 Medium  Straight Line Path 2 Mediums Encountered  Bent Line Path The bent line path allows light to travel relatively more distance in the medium in which it travels faster and less distance in the slower medium. Draw how you would travel if you were a lifeguard at point A trying to quickly reach a person at point B. Sand Shoreline Ocean A B

7. Light bends toward the Normal when passing from fast medium to a slow medium…just like the lifeguard. Light bends away from the Normal when passing from a slow to a fast medium…just like you in the ocean if you noticed someone stealing from your possessions on shore. Your possessions You Normal 11 22

8. Law of Refraction Index of Refraction (n) Index of Refraction (n) = speed of light in vacuum speed of light in medium n = cvcv Index of Refraction is another physical property of a substance (medium) n  1 because c  v Always! However n air  1.00 (to 3 sig.figs.) f 1 = f 2 Waves don’t “pile up” at the boundary. v 1  v 2 1  2 The wavelength changes at the boundary.

9. Law of Refraction (Snell’s Law) sin  1 sin  2 v1v2v1v2 = = constant (for two given mediums) sin  1 sin  2 c/n 1 c/n 2 =v 2 = cn2cn2 v 1 = cn1cn1 sin  1 sin  2 n2n1n2n1 = n 1 sin  1 = n 2 sin  2 Snell’s Law

10. Example Example (Snell’s Law) Find the angle (relative to the Normal) of the ray in the water. Strategy: Draw the Normal and measure  1 relative to the normal. Look up n 1 and n 2 n 1 = 1.00 (air) n 2 = 1.33 (water) Plug into Snell’s Law and Solve for  2 1.00 sin60 = 1.33 sin  2 sin  2 =.651  2 = sin -1 (.651)  2 = 41   1 =60  air water 30  41 

11. Example Example (Snell’s Law and “Critical Angles”) Find the “critical angle” where light travels parallel to the surface of the water upon leaving the water. Apply Snell’s Law 1.33 sin  C = 1.00 sin90 sin  C =.75  C = sin -1 (.75)  C = 49  Question: What happens to the light leaving the water if the incident angle is greater than  C ? air n=1.00 water n=1.33 Normal CC

12. Total Internal Reflection Apply Snell’s Law 1.33 sin60 = 1.00 sin  2 sin  2 = 1.15  2 = sin -1 (1.15) ERROR The red ray is reflected internally as if the air/water surface acted like a mirror. The law of reflection is followed:  1 =  2 Where will the red ray travel? 60  air n=1.00 water n=1.33  C =49   = 60  Normal

13. Fiber Optics (Total Internal Reflection Application) Core: Core: Transparent material about the diameter of a piece of spaghetti. Light travels through the core. High or Low value for n core ?? High Cladding: Cladding: Encases the core. Light is not supposed to travel through cladding. High or Low value for n cladding ?? Low Jacket: Jacket:Protective Coating End View Jacket Cladding Core