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UMass Amherst PHYSICS 132 - Lecture 3

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Atreyi Saha Physics 132, HatchJanuary 25, 2016Lecture 3: Waves/Waves and Optics- When a wave comes in and hits a boundary, thewave reflects back. All waves do this. o Hard Boundary - Anything that is fixed orcannot move Example: Going from concrete toair or air to concrete Going up an index of refraction In this case, the reflected wavecomes back inverted. Morespecifically, it is out of phase. In phase means same and out ofphase means oppositeo Soft Boundary Going down an index of reactiono Shive Demo Set up so we have two machines: one with short bars and another with long bars - this mimics wave moving from one medium to another Long bar: Up pulse sent, then up pulse comes back. We are going from one medium to another so the reflection that comes back is less pronounced as the original impulse. Most of the energy keeps propagating above. Going from slow to fast so soft boundary. Short bar: Up pulse sent, wave coming back was oriented downwards. This is because, when the wave tries to propagate onto the second machine it's going from a fast speed to a slow speed since waves through the long bars travel more slowly than waves through the short bar. This means that the wave hits a hard boundary. Wave speed faster for shorter bar machine- Interferenceo What happens when two waves hit each other? Energy and information are exchanged/hit each other Information doesn't exactly hit or collide - there is just an exchange in informationfrom particle to particle For example, one wave particle goes up two feet, then it tells the other wave particle to also go up two feet When information from two waves meets, they will add or subtract together. When they are separate, they will just keep on going.o When two waves meet each other, they will add together. If there is a positive and a negative, the sum will be zero. This means that the interacting waves were out of phase with each other and therefore this is an example of destructive interference. If the interacting waves are in phase (2 ups or 2 downs), they will add together constructively thereby generating a larger wave.o Wave continue to propagate even after there is constructive or deconstructive interference. They will continue to propagate through each other. They don't bounce off each other. The wave continues to propagate because it is the energy and information thatwas propagated. In class wave example - you as anindividual will move up and downBUT the wave continued topropagate from left to right.- String Demo: o Reflected waves and pulses (incomingwaves) adding together and slowly gettinglargero Standing Waves: (?)o Guitar: string is attached to boundaries atboth ends and if you pluck it just right,then that generates a standing wave whichwe can hear. By putting your had onthe guitar ends, you are putting yourhand on the dead spots or shorteningthe spring because it will have tohave a destructive interferencewhere your finger is- Types of deviceso Device 1: open on one end butclosed on another, closed end is ahard boundary like the tip of A1 Example: A bottle - you can set up a standing wave by blowing into the bottle; when you blow into a bigger bottle - the wave length will be bigger and sound will be lower; if you have a smaller bottle then you will have a smaller wavelength and higher soundo Device 2: open at both ends Example. When you twirl the pipe, it makes noise. Twirl faster then more high pitched noise. A pipe is open at both ends and therefore will set up different standing waves- Metal rod music demoo When Heath strokes the rod, you hear two distinct noises - this is because we have established multiple standing waveso This metal rod pipe is open at both ends. This is because both ends of the pipe can vibrate. The vibrations are different and therefore produce different sounds. Heath holds the rod in the middle so he is forcing a dead point (controlling the wavelengths that can pass?)`. Wave Optics- Electromagnetic waves transport energy and information. Supreme/Sole source of energy is coming from the sun in the form of light.- Light theories Light travels in a straight line (rayoptics) Light behaves as a wave Light behaves as a particle and wave- Light (an electromagnetic wave) has the fundamentalwave propertieso Such properties include: frequency, wavelength,velocity Visible light---Frequency is very high and wavelength is very small (? doublecheck)- Light also displays wave phenomenon o Such phenomenon include: superposition (light waves add up), interference (constructive vs. destructive) and diffraction (bends around corners)o Changes directions- Light travels at a slower speed when going through a medium (example - glass or diamonds)- Example problem: What is the frequency of orange light with wavelength 600 nm? o Use c = (lambda) (f)o Frequency = c/lambdao Solve for frequency, Answer is B) 5 x 10^14 Hz- Electromagnetic Spectrumo The wavelength of visible light corresponds to its color (ie. blue green light is around 500nm)o Towards the left, we have large wavelengths but small frequencies. Towards the right, wehave small wavelengths but large frequency. This highlights that wavelength and frequency are inversely related.o You can detect certain waves: infrared (heat), UV (sunburn), microwaves (freq and wavelength happens to match the vibrations of a water molecule; so microwaves get water molecules vibrating and we heat up substances this way because the individual particles have more energy) Dishes heat up because they have structures similar to water molecules- Speed of light in materials: the speed of light in materials is slower than empty space. The ratio between the speed in vacuum and the speed in the material is its INDEX OF REFRACTION (n)o n = ratio comparing the speed of light to the velocity of light in the material The closer the n value is to one, the closer the speed of light is to its speed in a vacuum. The speed of light in air, for example, is very close to the speed of light in a vacuum. Speed of light in a diamond is about 1/2 the speed of vacuum.o For example, light from air to glass causes light to slow down.- Why does light slow down?o This is because the wavelength gets smaller. The frequency does not change!o Frequency - how quickly a cycle occurs so when you are doing the wave, it is how quickly you went up and downo When moving from one medium to the next, it is physically impossible for the frequency to


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UMass Amherst PHYSICS 132 - Lecture 3

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