Physics 132 - Exam I Sheet- Mechanical waves require a medium to propagate and do so by disturbing themedium. The physical disturbance is the wave. It cannot propagate through emptyspace. Example: water- Non-mechanical waves do not require a medium to propagate. An example is lightwhich propagates through air/empty space.- What are the molecules doing compared to the direction the wave is moving? For example, in class, when we did the wave, you physically moved up and down but the wave propagated from left to right. Information and energy can be transported through a wave.- Wave properties: amplitude is the maximum displacement from equilibrium, period is the time required to complete one cycle of motion, frequency is the number of cycles per second, wavelength is the distance from crest to crest or from the same point to the same point- When a wave hits a boundary, it will reflect back. How it reflects back is dependent on the boundary type. If it is a hard boundary,the reflected wave comes back opposite phase of incoming, it comes back inverted/out of phase. A hard boundary is anything that is fixed and cannot move. This happens when you are GOING UP an index of refraction. If it is a soft boundary, the reflected wave comes back in the same phase as the incoming phase. This happens when you are GOING DOWN an index of refraction- When two waves meet each other, they will add together and information is exchanged. Waves continue to propagate through each other. If there is a positive and a negative, interacting waves will be out of phase, an example of destructive interference. If interacting waves are in phase (same sign), they will add together, an example of constructive interference.- When the reflected periodic wave interacts with the incoming periodic wave, it leads to standing waves.- Light behaves as a particle and a wave. Wave properties of light are reflected by: frequency, wavelength and velocity, superposition (light waves add up), interference (constructive vs. destructive), diffraction (light bends around corners)- Wavelength and frequency are inversely related. The wavelength of visible light corresponds to its color.- Speed of light in materials is slower than empty space. The ratiobetween the speed in vacuum and the speed in the material is its index of refraction- Light slows down because wavelengths get smaller and frequencyCANNOT change. Wavelength changes but frequency stays the same. When moving from one medium to the next, it is physicallyimpossible for the frequency to change. One side can't be moving at500 cycles/s and another at 1 cycle/s. That means waves is notpropagating. - If light SLOWS DOWN, that means wavelength got smaller. If light SPEEDS UP, then the wavelength got bigger. Small wavelengths imply LARGE index of refraction. Big wavelengths imply a SMALL index of refraction. Larger index of refraction means it takes longer to get through the material.- Interference of Light: Double Slit Experiment - when laser is shone through two slits, it leads to symmetrical altering of bright and dark spots. This is because the two slits act as two light sources that create two types of interference patterns (major dark/bright horizontally & vertical dark/bright in between bands) and symmetrical because when light goes through one opening that is proportional to the size of the wavelength. Bright spots means constructive interference AND Dark spots means destructiveinterference. Central maxim is where there is the maximum brightness of light & largest constructive interference (m = 0) Destructive interference means no light, right below m = 0(the central maxim). If the two waves interfere perfectlyconstructively, bright spot. If two waves interfere perfectlydeconstructive, dark spot. Some will be partly in phase or outof phase that's why some bright spots dimmer!- extra distance is one wavelength farther = waves will interfereconstructively because the difference will be an integeramount; extra distance is half wavelength farther = interfere destructively because the difference will be a half-integeramount.- Equations for the double slit experiment: y = defined as thedistance from the central bright spot to whatever spot youcare about/distance to the bright spot; L = distance to thescreen; d = spacing between splits; m is the spot we careabout OR the number of gaps to the bright spot- Diffraction Grating: many evenly spaced slits, examples include CDs and DVDs- We use light to look at things that are really small. In order to do so, we analyze interference patterns and make measurements to understand structure.- Peacock feathers have no pigments - the light interference patterns are what give peacock feathers their color- If light goes from a lower index of refraction to a higher one it hits a hardboundary and SLOWS DOWN. Hitting a hard boundary: 180 degrees phasechange. ONLY the light that is reflected experiences a phase change!- If light goes from a high index of refraction to a lower one it hits a soft boundaryand SPEEDS UP. Hitting a soft boundary: there is no phase change- Most light (90%) goes through and 10% is reflected. The light that keeps on goingthrough does not experience a phase change, only the reflected light experiences aphase change. Two light rays out of phase, you see nothing..- A thin film is sandwiched in between two mediums and they do not necessarilyhave to be the same. Here are the thin film interference equationso For equation 1, to find minimum thickness, m = 0. For equation 2, to findminimum thickness, m = 1o For questions as to which colors are/are not reflected, test multiple m valuesand compare against the visible light spectrum- Anti-reflection coating, light that comes in from behind your lenses doesn't hit the lens. reflect back & blind you!- We can only see light when it is scattered towards our eyes (spraying fog allows you to see laser light demo)- Light travels in a parallel fashion when travelling long distances and it follows the reflection rule where the angle of incidence = angle of reflection- When you move an object farther away from a mirror, the image does NOT get smaller. A mirror needs to be 1/2 your total heightso you can see your whole self.- If going from low index of refraction (n = 1) to high index of refraction (n = 2), then thelight ray travelling through tends to bend TOWARD the normal- If going from high index of refraction (n = 2) to low index of refraction (n = 1), then thelight ray
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