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Interference and Storage

Interference and Storage. What limits how much we can store on CD-ROM. Interference of Waves. A max. wave 1. If crests match crests, then waves interfere constructively Crests will match if waves are one wavelength, two wavelengths, … apart: path difference = m l. A max. wave 2. 2A max.

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Interference and Storage

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  1. Interference and Storage What limits how much we can store on CD-ROM

  2. Interference of Waves Amax wave 1 • If crests match crests, then waves interfere constructively • Crests will match if waves are one wavelength, two wavelengths, … apart: path difference = ml Amax wave 2 2Amax sum

  3. Destructive Interference Amax • If crests match troughs (180° out of phase), then waves interfere destructively • Crests will match troughs if waves are one/half wavelength, three/half wavelengths, … apart: path difference = (m+½)l wave 1 Amax wave 2 sum

  4. What This Means for Light • Light is electromagnetic radiation • A light wave is oscillating electric and magnetic fields • The amplitude of the oscillation represents the maximum electric (or magnetic) field and determines the intensity of light • Intensity depends on the square of the maximum electric field: I = Emax2/(2cm0) • Constructive interference produces brighter light; destructive interference produces dimmer light.

  5. Comparing Interference 2Emax Emax Medium amplitude of electric field yields medium intensity light Double amplitude of electric field yields quadruple intensity (very bright) light Zero amplitude of electric field yields zero intensity (no) light

  6. Diffraction • Waves spread out, or diffract as they pass through a slit Direction of wave motion l Wave Crest Wave Trough a l = wavelength a = aperture width

  7. The Double Slit Experiment • Waves spreading out from two points, such as waves passing through two slits, will interfere l Wave crest Wave trough Spot of constructive interference Spot of destructive interference d

  8. Diffraction Patterns • Light traveling through a single slit also creates a pattern, due to interference between wavefronts passing through different regions of the slit l Wave crest Wave trough Spot of constructive interference Spot of destructive interference a

  9. y b q a tan q =y/D D b a/2 q Path length difference = a/2 sin q Single Slit Math

  10. Diffraction Math • The locations of successive minima are given by • tan q = y/D • for small angles, sin q ~ q ~ tan q = y/D

  11. Diffraction by a circular aperture • A circular aperture of diameter d • Single slit of width a

  12. Do the Activity, Continuing through it After finishing Diffraction Pattern of a Red Laser, first two or three groups should jump to Green Laser part, then give green lasers to other groups when done

  13. Resolvability • Two objects are just resolved when the central diffraction maximum of one object is at the first minimum of the other. (Rayleigh’s criterion) • As before, q approximately y/L

  14. Comments on Resolvability • If want to resolve objects closer to each other (smaller y), need smaller wavelength of light or larger aperature • This is called the diffraction limit

  15. Why Do We Care? • CD-ROMS and other optical storage devices

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