Diffraction and Interference Or Thomas Young was One Smart Cookie Standards: 4f: Students know how to identify the characteristic properties of waves: interference, diffraction.

Diffraction (Single Opening or Corner) Definition: Bending of wave by passing it through a single opening or corner Diffraction increases with wavelength Diffraction decreases with aperture size Light and sound exhibit diffraction Question: Define diffraction. How does it differ from reflection or refraction? Is the amount of diffraction directly or inversely related to the size of the hole? Is it directly or inversely related to the wavelength of the wave passing through the hole or around the corner. What type of waves exhibit diffraction? Activities: Show diffraction with ripple tank. Show that diffraction of water waves increases as hole is made smaller and when wavelengths are made longer. Show laser disk slides 1943-46 from Physical Science disk and again point out that more curvature occurs for smaller hole and longer wavelength. Give examples of sound diffraction including hearing a radio from around the corner, sound moving through an alley or around a wall. Demonstrate by ducking down and screaming. Show slide picture. Show slide picture of light being diffracted. Describe how as move down, hole is becoming smaller and smaller and we are seeing more and more images. This is consistent with what we saw for the water waves. Point out that diffraction occurs for a single opening or corner. Sound Examples: sound heard around corner Light Examples: Pin Hole camera, Shadow of scissors on page 484 of book

Huygens’ Explanation of Diffraction Every point on every wave can be regarded as a new point source for secondary waves Straight wave produce straight waves, curved wave produce curved waves Straight wave passing through small opening produces curved waves (diffraction) Could also explain refraction and reflection Question: State Huygen’s principle. Use this principle to explain the diffraction of a wave through a single small opening. Is this explanation more consistent with a wave or particle model? Activities: State or write Huygen’s principle as large waves are made of smaller waves or each point on a wave is a source of new waves. Show the transparency or opaque drawings of a straight wave producing a straight wave. Note that the new wave is drawn where constructive interference occurs between wavelets. Show transparency of opaque drawing of straight wave producing curved wave as moves through small opening. Note that opening reduces number of sources that produce wavelets and that result is a wave curved at edge. Discuss extreme case of only one source being in opening and large curvature that would result. This fits well with observations that the smaller the opening or longer the wavelength the greater the diffraction.

Consequences of Diffraction Radio wave propagation AM have longer wavelength, more diffraction and greater coverage than FM Small wavelength waves are line of site Resolution Resolution is how well near objects can be separated. Diffraction increases, resolution decreases Blue light for microscope Electron microscope Dolphin clicks and squeels Q=1.22 l/d Question: Describe how diffraction is beneficial or detrimental for the following: a) radio wave propagation b) resolution. Activities: Ask which type of radio station AM or FM can be heard from further distances (AM). Point out that AM waves are longer than FM and thus experience more diffraction given the same openings through buildings and mountain passes. Draw picture of mountain pass and show diffraction pattern of AM waves vs. line of site propagation of FM waves. Define resolution and discuss how diffraction would decrease it since you cannot tell where an individual wave came from. Refer to radio picture above and point out that can’t get a clear direction from the diffracted am waves. Thus diffraction harms resolution. Point out that blue or violet light is used to illuminate most microscope stages because it has the smallest wavelength. Point out that electron microscope uses electrons which have an even smaller wavelength to see smaller structure. Discuss how dolphins will click to “see” general area and then squeel as approach you to reduce diffraction and “see” detail.

Thomas Young’s Experiment (More than One Opening) Diagram of Experiment Monochromatic and coherent light Bright and Dark Fringes Young’s Explanation (interference of light) Testing Young’ explanation Equations D sin q = (n + 1/2)l where n= + and - 0,1,2,3… for dark fringes D sin q = nl where n=+ and - 0,1,2,3,… for light fringes Question: Draw a diagram of Thomas Young’s experiment that showed that light can experience interference. How do the light fringes differ for red and blue light? Is this phenomena more consistent with light being a particle or a wave? Activities: Discuss Thomas Young’s prodigous start: By age of 3 had read Bible 2 times, by age 14 fluent in 14 languages, graduated college at age 14. PHD in physics at 23, in medicine at 27. Figured out Egyptian hyroglyphics using rosetta stone, made major contributions to optics and mechanics. Died at 37 of tuberculosis. Demonstrate light producing multiple images or fringes using laser and double slit. 3) Show laser disk movie on interference pattern produced by in phase water waves. 4) Show transparency or slide picture of Young’s experiment and explain what caused light and dark lines. 5) Suggest that there is a test of whether Young has the right explanation. Red and Blue light should produce fringes at different distances. Use transparencies with colored waves to demonstrate this. Then demo with clear filimented light and slit films. Use diffraction grating to reinforce fringes in middle of films. 6) Point out that light interference is a wave phenomenon.

Is Light a Particle or Wave? Particle Theory Photon is massless particle of light Photo-electric effect Wave Theory Reflection Refraction Diffraction Interference Real answer is light acts like a particle when interacting with matter and like a wave when traveling through empty space.

Interference Images Monochromatic, coherent light Coherent white light

From Bubbles to Holograms

Thin Film Interference (Iridescence) Occurs when rays from top and bottom of film destructively interfere with one another. Color is complement of color that is destroyed through interference Color is determined by thickness of film Examples: soap bubbles, gasoline on water, opal, CD, seashells and pearls Check Question: What color corresponds to the thickest portion of a soap bubble? How about the thinnest? Laser Disk of colors in soap bubble

Diffraction Grating Many slits instead of 2 Reinforce bright and dark fringes in monochromatic light Produce spectra in white light Smaller spacing of lines produces wider spacing of bright and dark fringes Same equations as 2 slits; d=1/slit spacing

Laser Light Light from incandescent source Monochromatic Light Coherent Light Laser Light Properties Stays in straight line and does not spread out Follows conservation of energy All energy in one place rather than spread out

Hologram Third dimension is recorded in interference pattern which is reproduced by irregular diffraction grating. Set-up for making hologram Holographic properties Cutting results in multiple holograms Holographic magnification