Sjuts' Science
HPS Waves Class Notes 2020Name _________________Objectives: Define diffraction and interference and justify how they illustrate the wave nature of light (Obj 5), Define the photoelectric effect and justify how it illustrates the particle nature of light (Obj 6).Wave Behavior: What causes a wave to change its behavior? If a wave encounters a new medium or another wave, its behavior will change.REFLECTION: when a wave bounces off a surface that it cannot pass throughWhat does this look like? Fixed End → Wave comes in as crest, reflects as trough (Newton’s 3rd Law)Loose End → Wave comes in as crest/trough, reflects as the sameExamples: sound (echos), light (glasses, contacts, mirrors, RADAR, fiber optics)2905125400050523875333375REFRACTION: bending of a wave as it enters a new medium at an angle, always accompanied by wavelength and speed change.What does this look like? Broken pencil, object isn’t where it appearsExamples: spearfishing, rainbows (atmosphere/prism), optics (contacts, glasses, microscopes, telescopes, binoculars)5238754857753067050485775DIFFRACTION: bending of a wave as it moves around an obstacle or passes through a slit, depends on size of obstacle or opening as well as wavelengthWhat does this look like? Longer wavelength = more diffraction, Smaller slit = more diffractionExamples: Tweets of songbirds vs. Hoots of owls → Tweets have shorter wavelengths so they don’t diffract as much, while owl hoots have longer wavelengths and diffract more.Diffraction allows you to hear conversations down the hallway and around the corner in the cafeteria. Spectrometers use diffraction (and interference) of light from slits or gratings to separate wavelengths. Faint peaks of E at specific wavelengths can be detected and recorded. A graph of these is called a spectral signature. Patterns in a spectral sig help scientists identify the physical condition and composition of stellar and interstellar matter (temperature and composition).-200024198120030670501962150ABSORPTION: transfer of E of a wave to a matter as a wave passes through itWhat does it look/feel like? Usually the matter’s temperature increases as the particles speed up.Examples: wearing black vs. white on a summer dayCONSTRUCTIVE INTERFERENCE: when 2 or more waves combine to produce a wave with a larger displacementWhat does it look like? Crest to crest, trough to trough at the same time (amplitudes of 2 waves added together)Examples: lasers, concert hall, stereo speakers, amplifiers, surfing 5619753048003324225309562DESTRUCTIVE INTERFERENCE: when 2 or more waves combine to produce a wave with a smaller displacementWhat does it look like? Crest to trough at the same time (partial or complete)Examples: car muffler, noise cancelling headphones10191753048003324225414338How are diffraction and interference related to each other? Shining a light through a slit!-952491752133676650257175Dual Nature of Light: Wave or Particle?!Chapter 18.1 (pg 536-7)Q: What wave behaviors did you read about? A: Reflection, refraction, diffraction, interference, absorptionQ: Does light do all these? A: YES!Q: So light is a wave, right? A: Well…. :) LIGHT AS A WAVE: Scientists saw light exhibit wave behaviors. Pictured below, diffraction, constructive and destructive interference. If light behaved like a stream of particles, what would we see?362902526575LIGHT AS A PARTICLE: Scientists discovered a phenomenon called the photoelectric effect, which is the emission of electrons from a metal caused by light striking the metal. Through experimentation, scientists found that the intensity of the light didn’t matter - only certain frequencies of light had enough energy to cause the electron emission. Einstein proposed that light, and all EM radiation, consists of packets of energy (now called photons). Each photon’s energy is proportional to the frequency of the light. Bright red or infrared light did not cause electrons to be emitted. Blue light, even dim blue light, did cause electrons to be emitted. 1143002921There is a frequency threshold, which is a minimum frequency of light that will cause the material to emit electrons. This varies by metal. -4857741247775Photoelectric Effect Examples: Solar Energy is produced by photovoltaic cells. These cells are made of semi-conducting metal which produces electrons when exposed to sunlight. (calculators, satellites, solar panels, etc)Photo-multiplier tubes - convert small intensities of light into electrical currents that can be analyzed. The electrons hit the CCD (charged coupled device) where they are stored, processed, and an image is read. (digital video and still cameras).Automatic garage door safety feature - as long as the beam of light strikes the photocell, the photoelectric effect generates electrons to produce current. If someone blocks it walking by, the current is interrupted and the door stops and opens up. ................
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