Atreyi SahaPhysics 132Prof. HatchFebruary 1, 2016Lecture #6: Thin FilmExample 1: a piece of plastic or glasswith a bunch of scratches/linesAs light goes through, each one of theslit spaces through which light passesbehaves like a wave. Propagates in acurvature pattern and interferesleading to certain patterns. Example 2: X-ray crystallographywas utilized by Watson to confirm thehelical shape of DNA; They had anidea of the helical structure, shinedlight through, and observed the shape.Other common examples ofdiffraction gratings: CD, DVDs - themicroscopic pits that store information on the CD act as a diffraction grating.We can understand molecule shape by shining light through. Example, boat in water disrupts the ocean waves. You can look at the different wave patterns resulting from disruption and understand the structure/size of the boat through the interference pattern.MAJOR CONCEPT: We use light to look at things that are really small. In order to do so, we analyze the interference patterns and make measurements to understand the structure.Moving on to focus on reflected light................- Peacock feathero There are no pigments that can code color in a peacock feather - the light interference patterns are what give peacock feathers their color- Index of refraction (n) = c (speed of light in a vacuum) / v (velocity inside the medium)o If light goes from a lower index of refraction to a higher index of refraction, then that is a hard boundary The speed of light as it enters the material SLOWS DOWN n1 = 1.001 --> n2 = 1.002 (light will hit a hard boundary in this case)o If light goes from a high index of refraction to a low index of refraction, then that is a soft boundary The speed of light as it enters the material SPEEDS UP (slow to fast)o The wavelength of light (lambdan) in a medium with index of refraction n is lambda n = lambda / n where plain lambda is the wavelength of light in vacuum- Phase changes due to reflected light:o Hitting a hard boundary: an electromagnetic wavetravelling from a medium of index refraction n1toward a medium of index of refraction n2 undergoesa 180 degrees phase change on reflection when n2 >n1 This is analogous to the reflected pulse on astring Shive machine: when it's a free boundary, the reflected wave is in phase and when it's a hard boundary, the reflected wave is out of phase Key: the light that is reflected experiences a phasechangeo Hitting a soft boundary: there is no phase change whenthe wave is reflected from a boundary leading to amedium or lower index of refraction This analogous to a pulse in a string reflectingfrom a free support (think Shive machine demo)- Interference in thin filmso Interference effects are commonly observed in thin films examples are soap bubbles and oil on water soap bubbles: when you shine light through the bubbles, you will see all colors of the rainbow example - glasses have anti-reflective coating (a thin film) to protect you from getting blinded by reflected lighto the interference is due to the interaction of the waves reflected from both surfaces of the film- Most light (like 90% ) goes on through,10% of light is reflected. o Light first hits a hard boundaryand undergoes a phase changeand is reflected 180 degrees.Then, light hits a soft boundaryand is reflected in the samephase as before. Secondaryreflections are very small. Thisis shown by the paralleldiagram.o The two light rays (original, reflected ones) are out of phase so you would see nothing.o Points of concern In double slit experiment, if one travelled one wavelength farther, then constructive interference. If one travelled 1/2 wavelength farther, then destructive interference. Here, you need to be concerned about the thickness of the hard boundary (in this case, thin film)(?) If thickness is 0.25 lambda, then it's 0.25 down the boundary and 0.25 up the boundary. The second ray will travel 1/2 wavelength more! If they are out of phase by 1/2 and the other wave travels 1/2 more, then they interfere constructively. The thickness of the material made the rays in phase.- Thin film interference videoo The thinness of the film is proportional to the wavelengtho Thin film example: Air-Soap-Air with soap being the thin film Basically, the thin film is sandwiched between two mediums and they do not necessarily have to be the sameo Changing index of refraction effects wavelength Bigger index of refraction means smaller wavelengthso IMPORTANT: reflected wave does not interfere with the incoming light; interference patterns we are seeing are due to reflected waveso You will see a brighter light when the reflected light is in phaseo Video time: 1:23 Light comes in, hits the first hard boundary and is reflected and the rest of the light travels on through We need to examine how many wavelengths thick the material is - count number of wavelengths inside the material The wavelength of light inside the material (thin film) will be different than the wavelength of light outside the material Light will also reflect from the second boundary. The reflected light from both boundaries will sum up- Quantitative analysis ofthin film interferenceWhat do you have to payattention to?o Reflected waves -whether they are in-phase or out of phaseo Thickness of thefilm/boundary 2t because theextra distancetravelled by thesecond light wenttwice the distanceo Lambda/n = gives youthe wavelength inside the film Check derivation in notebook!o In these problems, you will be given three indices of refraction: One in the film and Two on either side of the film The one that goes into the formula is the n of the filmo m = number of wavelengths the extra distance
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