ReflectionLight and Color for NonscientistsPHYS 1230• Geometrical optics - how does light change direction?– Reflections (mirrors, seeing your image)Pace of Class?A) Too slowB) Too fastC) Just rightExamples of Reflections• RADAR• SONAR• Radio wave (AM) propagation• Reflection from a mirror• Reflection from glass• Silvered sunglasses• Periscopes• Magic mirrors• Two-way mirrors• Sun pillarsDemonstrations and Simulations on Reflection• Rope tied to a wall - hard reflection• Rope tied to a string - soft reflection• Full reflection - you image in a vanitymirror, reflection from a mirror• Partial reflections - image in glasswindow, half-silvered mirror• Colored reflections - gold mirrors,aluminum mirrors, silver mirrorshttp://phet.colorado.edu/new/simulations/sims.php?sim=Wave_on_a_Stringhttp://micro.magnet.fsu.edu/primer/java/reflection/reflectionangles/index.htmlGeometrical optics - Reflections• Light travels in straight lines until it reflects off something• Reflections of any kind of wave occur whenever the mediumof propagation changes abruptly e.g. rope tied to a wall or astring• What counts is the change in the wave’s speed ofpropagation. If no speed change occurs, there is no reflection• If the speed changes dramatically, most of the wave isreflected. If there is little change in speed, little reflectionoccurshttp://www.glenbrook.k12.il.us/gbssci/phys/mmedia/waves/fix.htmlReflections• Hard reflections: wave goes from fast medium to a slow medium• Slow reflections: wave goes from slow medium to a fast mediumApplications of reflections - RAdio Detection And Ranging• Radar is based on reflected or echoed electromagnetic waves of abillion hertz (or what wavelength?)• If we know the speed of the wave, we can send out a pulsetowards an object, and measure how long it takes for thereflection to return to us. Then -Distance = speed x timeRAdio Detection And Ranging for Weather Forecasts• Doppler radar is a type of weather radar that determines whether atmospheric motionis toward or away from the radar. It uses the Doppler effect to measure the velocity ofparticles suspended in the atmosphere.• It employs the apparent shift in frequency of radio waves to perceive air motion andconsequently predict tornadoes and precipitation sooner than previous radars, as wellas measure the speed and direction of rain and ice.http://www.weather.com/maps/maptype/dopplerradarusnational/index_large_animated.htmlwww.crh.noaa.gov/lmk/glossary.htmRadar transmits a pulse, then measuresreflected echo (backscatter )by Tony Freeman, Jet Propulsion LaboratoryAn imaging radar works very like a flash camera in that it provides its own light to illuminatean area on the ground and take a snapshot picture, but at radio wavelengths. A flash camerasends out a pulse of light (the flash) and records on film the light that is reflected back at itthrough the camera lens. Instead of a camera lens and film, a radar uses an antenna anddigital computer tapes to record its images. In a radar image, one can see only the light thatwas reflected back towards the radar antenna.What is Imaging Radar ?Radar transmits a pulse, then measuresreflected echo (backscatter )A typical radar (RAdio Detection and Ranging) measures the strength and round-trip time ofthe microwave signals that are emitted by a radar antenna and reflected off a distant surfaceor object. The radar antenna alternately transmits and receives pulses at particular microwavewavelengths (in the range 1 cm to 1 m, which corresponds to a frequency range of about 300MHz to 30 GHz). For an imaging radar system, about 1500 high- power pulses per secondare transmitted toward the target or imaging area, with each pulse having a pulse duration(pulse width) of typically 10-50 microseconds (us). At the Earth's surface, the energy in theradar pulse is scattered in all directions, with some reflected back toward the antenna. Thisbackscatter returns to the radar as a weaker radar echo. These echoes are converted to digitaldata and passed to a data recorder for later processing and display as an image. Given that theradar pulse travels at the speed of light, it is relatively straightforward to use the measuredtime for the roundtrip of a particular pulse to calculate the distance or range to the reflectingobject.What is Imaging Radar ?Backscatter is also sensitive to the target's electrical properties, including watercontent. Wetter objects will appear bright, and drier targets will appear dark. Theexception to this is a smooth body of water, which will act as a flat surface andreflect incoming pulses away from a target; these bodies will appear dark.Imaging different types of surfaces with radarSonar• Bats use a system similar to radar - called sonar• The waves bats emit are not electromagnetic waves but ultrasonic soundwaves, at frequencies around 100,000 Hz (humans hear sound frequencies upto 20,000 Hz max)• Sonar is also used to measure water depths, or to detect submarinesSonarECHOLOCATIONBats send out sound waves using their mouth or nose. !When the sound hits an object anecho comes back. !The bat can identify an object by the sound of the echo. They can eventell the size, shape and texture of !a tiny insect from its echo . !Most bats use echolocationto!navigate in the dark!and find food.Concept QuestionRadar is based on -A) Reflection of radio waves by ice and moisture in the atmosphere?B) Absorption of radio waves by ice and moisture in the atmosphere?C) Reflection of sound waves by ice and moisture in the atmosphere?Concept QuestionRadar uses wavelengths in the range of -A) 300 kmB) 30 cmC) 500 nmMirrors and MetalsHow to mirrors work?• Visible light propagates though transparent media - obviously!• But visible light cannot propagate through metals. Why?Transparent Media• Consider light propagation from air to glass, or air to water• The EM field of the light causes the charges in the glass to oscillate• These oscillating charges generate the reflected reflected (2-4%) andtransmitted waves (96-98%)• The light also slows down in the glass or airTransparent mediumincident transmittedreflectedincident transmittedMetals• Metals have many free electrons that are not attached to individualatoms (this is why they are good conductors of electricity)• These free electrons move under the action of the light wave to cancelout any field generated by the light (because they are free to move!)• The