1. Physics 106 Lesson #27 Dr. Andrew Tomasch 2405 Randall Lab atomasch@umich.edu Interference and Diffraction: The Wave Nature of Light

2. What Do We Know About Light?  EM wave  Mostly travels in straight lines (Ray Tracing)  In vacuum light travels at  Reflects with  i =  r  Refracts when passing between media with different index of refraction: Bends toward the normal when passing into higher n → Snell’s Law

3. How do We Know Light is a Wave?  Distinguishing features of waves: Interference: two waves can add together and cancel Diffraction: waves can travel around corners

4. Interference: Thin Films  Light waves traveling from one medium to another undergo partial reflection and partial transmission at the interface of the two mediums

5. Soap Films

6. Diffraction  Diffraction: The bending of waves around small obstacles and the spreading of waves beyond small openings Small is defined in comparison to the wavelength  The fact that we can hear sounds around corners and behind barriers involves both the diffraction and the reflection of sound waves

7. Huygens’ Principle  Every point on a wave front acts as a source of tiny wavelets that move forward with the same speed as the wave  The wave front at a latter instant is the surface that is tangent to the wavelets

8. Diffraction  Waves passing through narrow single opening bend and spread out. The diffraction of light is only noticeable when the size of the opening is small compared to the wavelength of the light  The amount of bending increases as the size of the opening decreases

9. Diffraction of a Water Wave The extent of the diffraction increases as the size of the opening decreases.

10. Single-Slit Diffraction  This top view shows five sources of Huygens’ wavelets Demonstration

11. The Diffraction Grating We can replace the sine of the diffraction angle by the ratio of two sides of a triangle The slits of the grating are perpendicular to the plane of the page.

12. X-ray Diffraction  Complex diffraction patterns reveal the internal arrangements of crystals  Lower right panel shows the DNA diffraction pattern that lead to the discovery of the double-helix structure