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Waves and Light

Waves and Light. A wave is a pattern that moves. As the pattern moves, the medium may “jiggle”, but on average it stays put. Example: Wave on a string, string bobs up and down but does not move along with wave. We usually think of periodic waves, but pulses are also waves. Periodic Waves.

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Waves and Light

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  1. Waves and Light

  2. A wave is a pattern that moves. • As the pattern moves, the medium may “jiggle”, but on average it stays put. • Example: Wave on a string, string bobs up and down but does not move along with wave. • We usually think of periodic waves, but pulses are also waves.

  3. Periodic Waves • Wavelength () = distance between peaks. • Frequency (f ) = number of peaks that pass by a point per second. • Amplitude = ½ of peak to trough “distance”

  4. Period and Frequency • We will often talk about the period of the wave T=1/f . The Period is the time interval between peaks. • Example: If the frequency of a wave is 20s-1 = 20 Hz, this means that 20 peaks pass by per second. Thus, the period of the wave is 1/20 s.

  5. Wavelength and Wave number • We sometimes refer to the waves number k=1/ = number of waves per unit length. • Very useful in some advanced methodologies, but we will not use it very much.

  6. Wave Speed (c) • A given type of wave (e.g. sound, light) moves at a constant velocity that is determined by the medium that supports the wave. (different speeds for different media) • Speed of sound in air is cs=340 m/s (on a typical day) • Speed of light in a vacuum is c=3108m/s. • Wavelength, frequency and speed are related by the equation c=f 

  7. Longitudinal and Transverse Waves • Transverse Wave: Wave Motion (disturbance) is perpendicular to direction of propagation of wave. • Example: water waves, light

  8. Longitudinal/Compressional waves: distrurbance is parallel to direction of propagation. • Example: sound waves in air and water

  9. Difference between compressional and transverse waves allows us to “see” into the earth

  10. Principle of Superposition • Waves obey the principle of superposition: When two or more waves are present in the same location, the net amplitude is just the sum of the individual amplitudes. Result is complex wave forms

  11. Electromagnetic Waves • All electromagnetic waves travel at in vacuum at the speed of light, c=3108m/s • Since c=f , we know that frequency is inversely proportional to wavelength. • “They” used to believe that light needed a medium to travel in.

  12. When Traveling through mater, different wavelength behave differently. • Examples, X-rays pass right through solid objects but visible light does not. • Infrared video. • We may use all wavelengths to study nature.

  13. Milky Way as Seen in Various Frequencies

  14. When discussing a wave, the term frequency refers to • The distance between two adjacent peaks • The number of peaks that pass a point per second • The time interval between two peaks passing a point.

  15. List the type of radiation from low frequency to high frequency • Infrared, visible, ultraviolet, x-ray, • Infrared, visible x-ray, ultraviolet • x-ray, ultraviolet, visible, infrared

  16. Infrared radiation can pass through some materials that block visible light and vice versa. • True • False

  17. Thermal Radiation • All objects with non zero temperature radiate energy.

  18. Two important points • Total radiated energy (area under curve) is much higher for higher temperatures. • Peak in radiation spectrum for higher temperatures is at shorter wavelengths

  19. Two Important Equations

  20. Example • Sun T=6000K • max=2900mK/6000K=0.483m (Visible) • Earth T=300K • max=2900mK/300K=9.66m (Infrared)

  21. Total Power Radiated by Sun • A=4Rs2 = 4(7108m)2 =6.161018m2 P=eAT4 =(5.6710-8W/m2K4)(1)(6.161018m2)(6000K)4 =4.51026W As a comparison, total electrical power generated on earth is 1013W. In one second, the sun generates as much energy as all of the power plants on earth do in 1,500,000 years!

  22. Line Radiation • All atoms/molecules also radiate at discrete frequencies that are determined by their structure. • Can use the emitted lines to determine what atoms/molecules are present. • Need to use quantum mechanics to calculate spectrum.

  23. Examples of line radiation for various elements

  24. Light spectrum from various sources

  25. Materials also can absorb light at the same frequencies that they emit it. • Solar spectrum showing absorption by gasses in outer layer

  26. Solar Spectrum observed on earth

  27. Wave Properties of light • Reflection: Waves bouncing off of objects. Important when objects are larger than the wavelength • Diffraction: Waves “bending” around objects Important when objects are about the same size as the wavelength • Refraction: light changing direction when it changes medium Important at all wavelengths

  28. Reflection

  29. Reflection:

  30. Refraction Total Internal Reflection

  31. Index of refraction • Index of refraction for a material, n

  32. Snell’s Law for Refraction If n1>n2 then 1< 2 (bends away from normal) If n1<n2 then 1> 2 (bends towards normal)

  33. Rainbows

  34. Diffraction

  35. Scattering by particulates • The amount of scattering depends on the particle size and the frequency of the light. (Rayleigh scattering ) • Small particles scatter blue light more than red light

  36. Why is the sky blue?

  37. Why are sunsets red

  38. Albedo:

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