Today 1/27

1. Today 1/27 • HW: 1/27 Handout “Anti-Reflection Coating” due Thursday 1/30 • Today:Reflections at Boundaries 27.3 • Thin Film Interference 27.3 • Labs start Today • Peer Guidance Centerstill need Monday afternoon covered

2. Next Week’s Lab: Microwaves • A wave in the “electric field” • Reflection and transmission from and through wood • Reflection from metal (why metal is bad in your microwave) • Polarization (see section 24.6) • Interference (Young’s Double Slit)

3. Light to Heavy Both transmission and reflection Boundary feels like a fixed end to the light string Reflection just like fixed end, inverted Transmitted wavelength has the same shape except it’s shorter in length because it travels slower than the incoming wave. Slower, so not as far from boundary Inverted wave Shorter, “bunched up”

4. Heavy to Light Both transmission and reflection Boundary feels like a free end to the heavy string Reflection just like free end, not inverted Transmitted wavelength has the same shape except it’s longer in length because it travels faster than the incoming wave. Faster, farther from boundary Wave not inverted Longer, “spread out”

5. Light: Glass to Air Faster, farther from boundary Same as incoming wave Longer, “spread out” Light: Air to Glass Slower, so not as far from boundary Inverted wave Shorter, “bunched up”

6. Eyeball Thin Films Two reflections, front and back = two sources!

7. Thin Films A radio antenna broadcasts at a frequency of 93.7MHz ( = 3.2 m). A “mirror” 1.6m away reflects the signal. Where is the reception good along the dotted line? PLD = ? at the X (= ) PLD =  all along the dotted line so constructive everywhere. 1.6 m

8. Thin Films A radio antenna broadcasts at a frequency of 93.7MHz ( = 3.2 m). A “mirror” 1.6m away reflects the signal. Where is the reception good along the dotted line? Crest lines up with crest and the waves arrive in phase, constructive interference. Trough in Trough out 1.6 m Now move the mirror back

9. Thin Films Twin radio antennas broadcast in phase at a frequency of 93.7MHz ( = 3.2 m). Crest lines up with trough and the waves arrive out of phase, destructive interference. Trough in Trough out 2.4 m Now move the mirror back Now cause a “phase shift” at the boundary, turn crest into trough

10. Thin Films Twin radio antennas broadcast in phase at a frequency of 93.7MHz ( = 3.2 m). Crest lines up with crest and the waves arrive in phase, constructive interference. Phase shiftedTrough in Crest out 2.4 m Now cause a “phase shift” at the boundary, turn crest into trough

11. Eyeball Thin Films Wavelength ()Film thickness (PLD)Different  in filmPhase shifts at boundaries Interference

12. Index of refraction (n) Light slows down as it passes through glass, water, or any clear material. Index of refraction says how much. When the wave slows down it gets “bunched up” and has a shorter wavelength. When the wave exits the medium it returns to its original speed and wavelength.

13. Finding  in materials n1 = 1.0 v1 = c/n1 = c = 3 x 108 m/s Wavelength is shorter but frequency is the same in any material. If v changes by a factor 1/n and f remains constant then  must change by the factor 1/n also. Waves obey the equation:v = f v2 = c/n2 = 2.3 x 108 m/s 2 = 1/n2 Just a proportion n2 = 1.33

14. Example Light with  = 675 nm enters glass with n = 1.5. What is the wavelength in the glass? glass = /n glass = 675 nm/1.5 = 450 nm What is the wavelength after it exits the glass?  = 675 nm

15. Eyeball Thin Films The wavelength is different in the film. Wavelength ()Film thickness (PLD)Index of refraction (n)Phase shifts at boundaries

16. Eyeball Thin Films PLD = 2t, t = film thickness Wavelength ()Film thickness (PLD)Index of refraction (n)Phase shifts at boundaries t

17. Thin Films Sometimes reflections include phase shifts upon reflection Phase shifts - the tricky part Crest in crest out = no phase shift Crest in trough out = 1/2  phase shift Phase Shift? Yes, bouncing off higher n Phase Shift? No, bouncing off lower n Air Glass Air