Atreyi SahaPhysics 132, HatchJanuary 27, 2016Lecture 4- Index of refraction: a number that tells me how much the speed of light has changed in comparison to the speed of light in a vacuum o it is the ratio between the speed of light in a vacuum and the speed of light in the materialo we will make the assumption that the speed of light in air is the same as the speed of light in vacuum light is slower in air by a very small amount- When light enters a material, the wavelength changes but not the frequency. o Frequency refers to the number of times a particle oscillates per second In class wave - how many times you stood up and down and how quickly you went up and down This is different from the speed of the wave which is related to frequency times wavelengtho Wavelength changes from medium to medium but frequency does not changeo If frequency were to also change, then there would be a total mismatch as to what's going on You'd have side one that was bouncing up and down slowly On the other side of the boundary, you'd have a side two that was bouncing up and down rather quickly. In the case of light, the boundary is relatively small- just one molecule thick. That's sort of enormous mismatch just cannot occur and therefore the wave could not propagate Therefore, it is the wavelength that must change instead of the frequency If you want to go farther, the wavelength would get longer so you can cover a greater distance- The higher the frequency, the shorter the wavelength and vice versa.- Bigger wavelength means you are going faster because you travel a greater distance in the same unit of time (23:20?)o If you have a large wavelength, then your frequency is small.o If you slow down, wavelength got smaller. If I speed up, then the wavelength got bigger.- IMPORTANT (?)o Small wavelengths imply LARGE index of refractiono Big wavelengths imply SMALL index of refraction- Sample Problemo Larger index of refraction means it takes longer to get through the material so slower speed Slower speed implies smaller wavelengths N3 has the greatest index of refraction because it has the smallest wavelengthso Answer: C- Review of Superposition and interference:o When two or more waves are simultaneously present at a single point, the amplitude is the sum of the amplitudes of the individual waveso Since amplitudes may be positive or negative, waves may interfere constructively or destructively- When two waves are travelling along and these sound waves are in phase, then the sound will get louder. When the sound waves are out of phase, the sound will get quieter. - Practical examples : a performing centero The walls are triangle shaped so that there are no dead spots in the seatso For example, if you had two violins playing the same note and you happen to be sitting at the spot where there is destructive interference, then you would hear nothing.- Demo: ripple tanko Two waves are propagating out. Sometimes, they are in phase and therefore give you a large wave. Sometimes, they are out of phase giving you destructive interference and thus the wave goes down.o Ocean waves -superposition leads to more beacherosion & destructive interference leads to less beacherosiono Perspective: Bird's Eye View- Interference of Light: Young's double slit experimento Laser light shone through two slits leads tosymmetrical altering of dark/light spots on the screeno Bright spots: constructive interference; Dark Spots:destructive interferenceoPerspective:edge onshot, ifyou werejustlookingat theedge ofthe beachwith thewavescominginCentral maxim: wherethere is maximum brightness of light, largest amount of constructive interference; it is the middleof where there is no light sourceDestructive interference leads to no light, right below m =
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