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Wave Phenomena

Wave Phenomena. DIFFRACTION OF LIGHT OFF A COMPACT DISC. INTERFERENCE PATTERN OF WATER WAVES. Constructive and Destructive Interference. • Constructive interference produces maxima , where crests meet crests and troughs meet troughs to produce larger waves by superposition.

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Wave Phenomena

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  1. Wave Phenomena DIFFRACTION OF LIGHT OFF A COMPACT DISC INTERFERENCE PATTERN OF WATER WAVES

  2. Constructive and Destructive Interference • Constructive interference produces maxima, where crests meet crests and troughs meet troughs to produce larger waves by superposition. • For example, light wave maxima are bright, sound waves maxima are loud, water waves maxima are tall, etc. • Destructive interference produces minima, where wave crests meet wave troughs cancel out by superposition • For example, light wave minima are dark, sound waves minima are quiet, water wave minima are small, etc. • Waves from two sources combine constructively and destructively in space to form an interference pattern. Compare this to waves that interfere in time to form a beat pattern. click for web page click for applet click for applet

  3. min m = 1 max m = 1 min m = 0 max m = 0 min m = 0 max m = 1 min m = 1 Equations for Interference Maxima and Minima Honors only x θ d L wave sources path lengths to a distant point on the interference pattern path length difference INTERFERENCE MAXIMA INTERFERENCE MINIMA If the path length difference to any point is an integer multiple of the wavelength, then constructive interference will create a maximum If the path length difference to any point is halfway between an integer multiple of the wavelength, then destructive interference will create a minimum click for applet click for applet

  4. Wave Reflection RAYS VERSUS WAVES LAW OF REFLECTION FOR WAVES click for applet WAVES MAKE REAL AND VIRTUAL IMAGES

  5. Wave Refraction click for applet click for applet

  6. Huygens’ Principle • Dutch scientist Christiaan Huygens, a contemporary of Sir Isaac Newton, develops a wave theory in 1670 that went unnoticed for over a century • The theory (Huygens’ Principle) explains wave reflection and refraction, but also explains a new wave behavior called diffraction. click for applet click for applet click for applet Every point on a wave-front is considered a source of secondary spherical “wavelets” which spread out in the forward direction. The new wave-front is the “boundary” to all these secondary wavelets.

  7. Diffraction of Waves • Diffraction is the bending of waves around a barrier or through an opening. • Waves with longer wavelengths (lower frequency) diffract more. click for applet • Diffraction is apparent when waves pass through a gap of width similar to the wavelength. click for applet • AM radio signals (λ about 300m) diffract more than FM signals (λ about 3m), yielding better reception for larger wavelengths. • Low frequency (large wavelength) sound waves diffract more than high frequency around corners and through small spaces. • Diffraction is a wave behavior. Particles don’t diffract (although quantum physics predicts particle diffraction).

  8. Single Slit Diffraction • Light waves diffract when emitted through a tiny opening (slit) and create a diffraction pattern. The slit must be close to 10-7 meters! • Light waves from different portions of a single slit will constructively and destructively interfere and cause maxima and minima (light/dark fringes). • Single slits produce a bright, wide central maximum and other dimmer, more narrow (half as wide as central) maxima. click for applet click for applet INTERFERENCE MINIMA If the path length difference to any point is halfway between an integer multiple of the wavelength, then destructive interference will create a minimum LIGHT ENTERING SINGLE SLIT SINGLE SLIT DIFFRATION PATTERN

  9. Double Slit Diffraction • In 1801, Thomas Young proved light is a wave by creating an interference pattern for light passing through two small slits. click for applet • In the experiment, light passes through a single slit first to keep waves in phase. click for applet • Double slits produce a bright central maximum and other bright, equal width maxima. click for applet • The equations for maximum and minima are the same for all interference patterns. LIGHT ENTERING DOUBLE SLIT Example: The distance between the two small openings (slits) that allow light through an opaque surface and onto a viewing screen is 0.03 mm. The second-order bright fringe (maximum) is measuring at an angle of 2.15˚ What is the wavelength of the light? DOUBLE SLIT DIFFRATION

  10. Diffraction Grating • When more slits are added the interference pattern gets clearer, with brighter, and more distinct maxima click for applet • Gratings often have 1000s of slits per centimeter. Slit spacing, d, is the reciprocal. • Compact discs (6250 lines/cm) are diffraction gratings because small pits are etched into the surface that have depth and spacing near the size of the wavelength of light. 5 SLIT VS. DOUBLE SLIT • A diffraction grating is also used in a CD player to create a three-beam laser tracking system • Crystalline solids are natural diffraction gratings with atoms spaced about 10-10 meters, so that X-rays have wavelengths to show diffraction patterns. • Watson and Crick (1953) used X-ray diffraction to discover the double helix structure of DNA CD TRACKING LASER SODIUM CHLORIDE

  11. Resolution Resolving power is the ability of an optical instrument to separate two images that are close together. CAR HEADLIGHTS’ RESOLUTION Diffraction limits resolving power. Resolution is evidence that light is a wave. Shorter wavelength increases resolving power, so blue light is better for microscope illumination. 305 meters Blu-ray DVD players have more capacity (better picture) because the violet laser is 405 nanometers. Long wavelength radio waves need large resolving instruments. BLU-RAY DISC RADIO TELESCOPE

  12. Polarization of Light Light is a transverse wave composed of alternating electric and magnetic fields . Electric “field” Magnetic “field” Polarization is the oscillation of a transverse wave in only one plane. Polarized filters Reflection Double Refraction click for applet click for applet click for applet

  13. Thin Film Interference Most thin films like soap bubbles and oil slicks will produce colorful arrays caused by constructive interference of light waves. The conditions for interference depends on: 1. The index of refraction of the film and the surrounding media. 2. The thickness of the film. Film surround by media of lower index of refraction Film thickness must be λ/4, 3λ/4, 5λ/4, 7λ/4… for constructive interference. Film surrounded by media of lower index on one side, higher index on the other Film thickness must be λ/2, λ, 3λ/2, 2λ … for constructive interference. click for applet click for applet click for applet

  14. Lasers • Ordinary light is noncoherent (out of phase) varies in intensity, wavelength, and direction. • Laser light is monochromatic (one color), coherent (in phase) and unidirectional. • Einstein predicted “stimulated emission” - a photon hits an atom already excited by a photon, two identical photons are emitted. • Electrical or chemical energy is pumped into atoms to create an “active medium” such that atoms are already “excited”. • Mirrors are used to cause “amplification” - a domino effect of cascading “radiation” • “Light Amplification by Stimulated Emission Radiation” exits through a partially transparent mirror. • Lasers are widely used in science: • surgery, dermatology, ophthalmology • precision measurements • manufacturing • audio/video technology click for web page

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