Chapter 16: Interference and Diffraction

Objectives Understand “Huygen’s Principle.” Understand how waves diffract. Explain how wavelength affects diffraction.

Huygen’s Principle Christian Huygens: wavelets produce a wavefront.

Huygen’s Principle and Diffraction diffraction: the bending of a wave as it passes around an obstruction or through an opening. Huygen’s principle helps to show how waves diffract. Huygen’s Principle also shows reflection/refraction

Diffraction and Ocean Waves straight wavefront wavefront diffracts around the edge of a jetty

Diffraction and Wavelength shadows shorter waves diffract less, forming shadows longer waves diffract more, virtually no shadow

Diffraction and Wavelength Short light waves make shadows, but longer sound waves do not.

Electron Microscopes light diffracts around very small objects electron microscopes make smaller waves that reflect

Objectives Understand how interference fringes are formed by a double-slit or a diffraction grating. Be able to calculate the wavelength of light based on an interference pattern.

Interference Fringes Thomas Young “proved” that light is made of waves with his famous “double slit” experiment.

“Double Slit” Diffraction Math first order bright Interference can be used to calculate l : q = tan-1 (y / L) sinq = (l / d) l = d·sinq 90-q n screen n+1 d central bright q l d l q

Diffraction Grating Each successive slit in a diffraction grating diffracts light to form a fringe. n+1 n+2 n+3 l = d·sinq can be used to calculate wavelength

Calculating Wavelength What is the wavelength of a laser if fringes separated by 35.9 cm are made on a screen 98.2 cm from a diffraction grating with lines spaced 0.000200 cm apart?

Objectives Understand the importance of Thomas Young’s double-slit experiment. Understand how interference patterns can be used to determine crystalline and molecular structures. Understand how single-slit diffraction patterns form.

Thomas Young, Superstar He read by age 2 and by age 4 had read the Bible twice; he also played several instruments. He spoke eight languages by age 14. As a physicist, he helped define the concept of energy, he studied the elastic properties of materials, and he explained how waves constructively and destructively interfere. He was a practicing physician. Based on his studies of the eye, he determined how the eye focuses and he helped develop the idea of color addition. He was the first person to successfully use the Rosetta Stone to decipher Egyptian hieroglyphics! 1773 - 1829

Young’s Interference Experiment Thomas Young used a double-slit to “prove” that light is made of waves in 1802. Simon Poisson: monochromatic light should make a bright spot in the center of a shadow. ?

Diffraction and Crystal Structure Crystal lattice structures are determined by observing patterns formed by diffracting X-rays. Rosalind Franklin’s photo shows the X-ray diffraction pattern made by DNA. Watson and Crick stole the photo and determined the double-helix structure.

Single Slit Diffraction With a single slit, dark bands are observed where destructive interference occurs. ½ l / ½ d = ½ w / L / d = w / 2L w = 2L l /d dark n ½ w n+½ ½ d L ½ l dark

Resolving Power resolving power: the ability to see two images that are close together Airy disk

Objectives Understand how iridescence occurs due to thin-films and other microscopic structures. Understand how lasers work.

Iridescence iridescence: the interference of colors caused by reflection and refraction in thin films

Iridescence incident rays in-phase reflecting rays out-of-phase Rays of a single color reflect off two thin layers and cancel. Occurs if the thickness is an odd multiple of ¼ l (¼ , ¾, etc.). If red cancels, it appears cyan because W – R = C. Different thicknesses result in different colors.

Iridescence CDs and DVDs have pitted layers that produce iridescence. blue cancels, yellow observed ¼ l CDs and DVDs have pitted layers that produce iridescence. red cancels, cyan observed ¼ l

Laser Light laser: light amplification by the stimulated emission of radiation Typical Light incoherent (out-of-phase) polychromatic (many l) Laser Light coherent (all waves in-phase) monochromatic (single l) very intense narrow beam somewhat polarized

Stimulated Emission An excited atom usually emits a photon spontaneously. Einstein suggested that a passing photon of proper energy can stimulate an excited atom to emit a photon. The two photons will be coherent.

Population Inversion Einstein said that if a majority of atoms were excited (a population inversion), a group of in-phase photons would be produced through stimulated emissions He imagined the first laser.

How a Helium-Neon Laser Works High voltage excites He atoms. He atoms collide with Ne atoms, transfer energy, and produce a Ne population inversion. Ne atoms undergo stimulated emissions and produce laser light. The gases are in a long glass tube with slightly concave, mirrored ends. The laser light reflects back and forth, increasing in intensity. One end of the tube allows 1% of the light to escape—the laser beam.

First Ruby Laser: 1960