Interference & diffraction

1. Chapter 19 Interference & diffraction

2. Incoherent Light Coherent Light • Light waves are out of phase with each other. • Example: Sunlight • Waves of same wavelength are in phase with each other. • Example: Lasers

3. Fun Fact… • Did you know that the term “laser” is actually an acronym for • Light Amplification of Stimulated Emission of Radiation? • Technically the term should be Light Oscillation of Stimulated Emission of Radiation but then then the acronym would be L.O.S.E.R. 

4. Thin Films • Soap bubble demo • Note direction and color of lines. • What’s going on?

5. Film gets pulled to bottom so thickness increases as you move down the film. • Incident ray hits the front of the film. • Some rays reflect inverted. • Some rays refract. • Refracted rays hit “back” of the film and are reflected upright. Incident Ray Reflected from Front Reflected from Back

6. Thin Films - continued • If the film thickness is 1/4λ, the ray reflected from the back of the film is ½ λ behind the ray reflected from the front. Get destructive interference. Incident Ray Reflected from Front Reflected from Back Waves are “Out of Phase” by ½ λ

7. Thin Films - continued Incident Ray • But…remember that the wave is inverted when reflected (because it’s coming off a more dense medium) so you get constructive interference. Reflected from Front Reflected from Back Waves are “In Phase”

8. Thin Films - continued • Other wavelengths are partially or completely cancelled. • Light most strongly reflected is that with 1/4λ equal to the film thickness. • As film thickness changes, different colors are reflected (see the spectrum). • Spectrum repeats itself when thickness of film = ¾, 5/4, 7/4λ, etc. • Black (or blank) area at the top of the film isn’t thick enough to cause constructive interference. • Film usually breaks at the area where no color is reflected.

9. Examples of Thin Films Mother of Pearl Soap Bubbles Oil on Water