1. Today 2/3  Total Internal Reflection  No HW due Today  Lab: Refractive index with Pins and Blocks  Start reading chapter 26.6-8 on Lenses  HW:2/3 “Refraction” Due Thursday, 2/6  Exam IThursday, Feb 13

2. This Week’s Lab Pins “Images” will not line up when the “objects” do

3. This Week’s Lab Pins “Objects” will not line up when the “Images” do

4. Refraction-Bending Light Water - index of refraction, n W = 1.33 Wave crests are closer together in higher index materials because the waves travel slower. Air - index of refraction, n A = 1.0 Think of rows in a marching band that slow down in a mud puddle.

5. Refraction-Bending Light Water - index of refraction, n W = 1.33 When light enters at an angle, it “bends.” Air - index of refraction, n A = 1.0 Think of rows in a marching band that slow down in mud puddle. Ray direction always perpendicular to the waves.

6. Ray diagram example Water - index of refraction n W = 1.33 Air - index of refraction n A = 1.0 Refraction makes it seem as though its closer Image Object

7. Ray diagram example Water - index of refraction n W = 1.33 Air - index of refraction n A = 1.0 Refraction makes it seem as though its farther Image Object

8. Example: 4cm 7cm tall  2 When looking into a glass of water, where (at what angle,  2 ) should I place my eye so that I can just barely see the red dot? n = 1.33 n = 1.0  1 n 1 sin  1 = n 2 sin  2 tan  1 = 4/7  1 = 30° 1.33 sin 30° = 1.0 sin  2  2 = 41.7  How deep does the water appear to be? tan  2 = 4/d d = 4/tan41.7  = 4.5cm Note notation: light from n 1 to n 2 d

9. Total internal reflection Water - index of refraction n W = 1.33 n A sin  A = n W sin  W Air - index of refraction n A = 1.0 AA WW As  W increases, so does  A until  A becomes 90°.  C is called the “critical angle”. If  W is greater that  C no light will escape the water and all will be “internally reflected.” Total internal reflection only happens when light goes from high n to low n and it depends on both n’s! n A sin 90  = n W sin  C or n A = n W sin  C CC

10. Example: What is the critical angle for light traveling from water into air? n 2 = n 1 sin  C n 1 = 1.33 CC n 2 = 1.0 sin  C = n 2 /n 1 = 0.752  C = 48.8° Note that there is no critical angle for light from low index to higher index.

11. Example: What happens if the angle of incidence is greater than 48.8°? n 2 = n 1 sin  C n 1 = 1.33  1 = 50° n 2 = 1.0 50°

12. Example: What happens if I place a sheet of glass on the water, n g = 1.5? n 2 sin  2 = n 1 sin  1 n 1 = 1.33  1 = 50  n 2 = 1.0 n g = 1.5 No internal reflection possible-low to high n! n 2 = n 1 sin  C 1.5sin  g = 1.33sin50°  g = 42.8° Now what happens?  C = Arcsin1/1.5 = 41.8° The light reflected back into the glass.  g = 42.8 