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Notes on Waves

Notes on Waves. Notes on Waves. Waves are ENERGY! Travel through medium (Electromagnetic waves can travel through vacuum.) Medium doesn’t move, only energy travels. Two Types of Waves. Transverse – oscillates perpendicular to the direction of travel

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Notes on Waves

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  1. Notes on Waves

  2. Notes on Waves • Waves are ENERGY! • Travel through medium (Electromagnetic waves can travel through vacuum.) • Medium doesn’t move, only energy travels

  3. Two Types of Waves • Transverse – oscillates perpendicular to the direction of travel • Longitudinal – oscillates parallel to the direction of travel (AKA Compressional)

  4. Notes on Waves Properties of Transverse Waves • Crest – the high point of the wave • Trough – the low point of the wave • Wavelength – the distance from crest to crest, or trough to trough. • Amplitude – the height of the wave, from midpoint (equillibrium) to crest, or midpoint to trough

  5. There are 2 ½ waves in the above wave train.

  6. Examples of Transverse Waves • Water waves; wave on a string; electromagnetic waves (e.g. radio waves, television signals, infrared waves, visible light, ultraviolet radiation, microwaves, x-rays); S-waves (earthquakes)

  7. Notes on Waves Properties of Longitudinal Waves • These waves must travel through a medium • Compression – regions where the molecules in the medium are bunched together • Rarefaction – regions where the molecules in the medium are spread apart • Wavelength – the distance from compression to compression • Amplitude – the distance that any one molecule is pushed away from equillibrium

  8. Examples of Longitudinal Waves • Waves through a slinky, sound waves, shock waves, P-waves (earthquakes)

  9. Earthquake Damage Vertical S-waves Lateral S-waves

  10. Notes on Waves Period & Frequency • Period – How long it takes for a single wave cycle; measured in seconds, minutes, hours, days, etc. (abbr.= T ) • Frequency – The number of wave cycles in one second; measured in Hertz 1 Hz = 1 cycle/s (abbr. = f ) • f= 1/T and T = 1/f

  11. Notes on Waves Period & Frequency Example problem : An ocean wave has a period of 8 seconds. What is the frequency of that wave? f = 1/T = 1/ 8 seconds = 1/8 Hz = 0.125 Hz

  12. Notes on Waves Wave Speed Waves travel at different speeds through different media.

  13. Notes on Waves Wave Speed • Wave speed = wavelength x frequency • Wave speed [v] is measured in m/s, cm/s, km/s, km/hr, etc. • Wavelength [λ] is measured in m, cm, km, etc. • Frequency [f ] is measured in Hz = 1/s • Sov = λ • f

  14. Notes on Waves Wave Speed Example problem A blue whale bellows in the deep ocean with a frequency of 15 Hz. If the wave has a length of 100.3 m, what is the speed of sound in the ocean? v = λf = (100.3 m)(15 Hz) = 1505 m/s

  15. Notes on Waves Waves Behavior • Reflection – when waves bounce off of a surface • Refraction – when waves change speed (and often direction) as they travel through different media. • Diffraction – when waves bend around corners • Interference – when waves interact with other waves • Constructive Interference – when two (or more) waves meet to make a bigger wave • Destructive Interference – when two (or more) waves meet to make a smaller wave

  16. Notes on Waves • Reflection – when waves bounce off of a surface • The angle of incidence equals the angle of reflection.

  17. Notes on Waves • Refraction – when waves change speed (and often direction) as they travel through different media.

  18. Refraction • In which medium does light travel faster? (glass rod appears bent)

  19. v is the speed of light in the new medium. c= 3.0 x 108m/s As the index increases the speed decrease. Draw a graph for index vs. speed. nis the absolute index of refraction. This is a measure of optical density. nis defined as the ratio of the speed of light in a vacuum to the speed of light in a new medium. Speed of Light

  20. n is the relative index of refraction. If air is not used, then remember nrel = n2/n1 What is the relative index when going from diamond intolucite? Relative Index of Refraction • If nrel < 1 ; speeds up • If nrel > 1 ; slows down

  21. Refraction n(water)=1.33; n(glass)=1.50; n(air)=1.00 Vw = 2.26 x 108m/s Vg = 2.00 x 108m/s Calculate the speed of light in water and glass.

  22. When a wave slows down it bends closer to the normal. {less to more – toward} n2>n1 When a wave speed up it bends away from the normal. {BLA – Big ―› Little – Away} n2<n1 Refraction n1- from n2 - into

  23. Refraction • If light rays bend closer to the normal when slowing down, why does the glass rod seem to bend away form the normal?

  24. Diverging rays enter your eyes. You “think” in Straight Lines. A virtual image appears to come from point y Apparent Depth

  25. Apparent Depth • If the chest is 20 m below the surface at what depth will the image appear? • Assume nsea water = 1.34

  26. Snell’s Law • n1sinθ1 = n2sin θ2 • v1/v2 = λ1 /λ2

  27. A monochromatic light ray f = 5.09 x 1014 Hz is incident on medium X at 55˚. The absolute index of refraction for material X is 1.66 Example What is material X? Determine the angle of refraction. Determine the speed of light in medium X.

  28. The index of 1.66 is Flint Glass Ex: Solution To find the angle of refraction use Snell’s Law.θ2= 30˚ To find the speed use n=c/v. v = 1.8 x 108 m/s

  29. Notes on Waves • Diffraction – when waves bend around corners

  30. Notes on Waves • Interference – when waves interact with other waves • Constructive Interference – when two (or more) waves meet to make a bigger wave • Destructive Interference – when two (or more) waves meet to make a smaller wave Destructive Interference Constructive Interference

  31. Constructive and Destructive Interference

  32. Notes on Waves • Doppler Effect . • The frequency (and wavelength) of a wave changes depending upon how the observer of the waves is moving relative to the source of the waves

  33. Notes on Waves: Doppler Effect If the source is traveling towards the observer, the wavelength is smallerand the frequency is higherthan if the observer and source had the same velocity. YouTube Video If the source is traveling away from the observer, the wavelength is largerand the frequency is lowerthan if the observer and source had the same velocity.

  34. Notes on Waves: Doppler Effect If the source is traveling towards the observer, the wavelength is smallerand the frequency is higherthan if the observer and source had the same velocity. Speed of Sound in air = 343 m/s If the source is traveling away from the observer, the wavelength is largerand the frequency is lowerthan if the observer and source had the same velocity.

  35. Notes on Waves: Doppler Effect Remember that the Doppler Effect applies to ALL waves, including electromagnetic waves (light!). When applied to light the Doppler effect is referred to as either a Red Shiftor a Blue Shift. It is by studying this data from surrounding stars and galaxies, that we know that the universe is expanding. Most stars and galaxies exhibit a Red Shift.

  36. Notes on Waves • Good Numbers to Know: Speed of sound indry air @ 20˚C = 343 m/s= 1,126 ft/s = 768 mph Speed of lightin a vacuum ~ 3.0 x 108 m/s= 186,282 mph

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