Diffraction and Interference Chapter 31

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

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

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

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)

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

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)

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

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

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

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