Diffraction and Interference

1. Diffraction and Interference Chapter 31

2. Huygen’s Principle • Light waves spreading from a point source can be thought of as overlapping secondary wavelets • And, every point on a wave front can be regarded as a new point source • As wavelet spreads, it appears less curved, join together to make a wave front

3. Huygen’s Principle in Water • Works for reflection and refraction • When waves go through small slit, something unexpected happens

4. Diffraction • Bending of a wave by means other than reflection/refraction • Light bends around corners (due to Huygen’s) • Shadows are sharp with large hole • Shadows are fuzzy with small hole

5. Diffraction by Wavelength • Amount of diffraction depends on object’s size vs. wavelength (longer wavelength vs. smaller object = more diffraction) • Am radio waves are larger so go around obstacles better than FM • For microscopes you need to use smaller wavelengths to see very small object (diffraction can blur the object if it is smaller than the wavelength of light) • Some use electron beams (smallest wavelength possible with current technologies)

6. Interference • Interference pattern: two overlapping waves will increase, decrease, or neutralize each other at certain points

7. Young’s Interference Experiment • Monochromatic (single color) light through two closely spaced pinholes (or slits)produces an interference pattern • Diffraction grating are many slits (not just two), produce even more complex patterns (for white light diffraction grating disperse the colors similar to a prism)

8. Single-Color Interference from Thin Films • Reflection of light from two surfaces that are very close together can create interference patterns • Waves travel different distances through each material (so can become out of phase as they reflect off different boundaries) • Interferometer uses this principle to measure very small distances

9. Iridescence from Thin Films • Rainbow of colors created by thin film reflections of white light • Different colors refract and reflect from the layers of different media and interfere on the way back to eye • Where green is canceled = magenta is seen • Where blue is canceled = yellow is seen • Etc… • Soap interferes based on thickness of bubble • Gasoline interferes based water/gas mix

10. Laser Light • Light is normally incoherent (all/many frequencies and phases) • Monochromatic light has a single frequency but the waves are not in phase • Lasers are coherent (one single frequency and all waves are in phase) • Light • Amplified by • Stimulated • Emission of • Radiation

11. The Hologram • 3-D photograph created through interference effects (limited parallax) • X-ray holograms could magnify images thousands of times due to short wavelength versus longer wavelength to see it