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Physics Teacher Notes - Grothaus Chapter 27 – Light Title: Light ***Light is the ONLY thing you see! All visible objects either emit or reflect light.*** We can only see about 0.1% of the entire electromagnetic spectrum. We are “colorblind” to other waves Contrary to popular belief, dogs are not colorblind – they just can’t see all of the colors we can – only yellow, blue and violet Bulls are colorblind! They can’t even see red…. Insects can see in ultraviolet as well as all the colors we see. Using instruments, we can see lower – infrared detecting devices and radio and microwave receivers And higher – ultraviolet, X-ray, and gamma-ray detectors http://www.personal.psu.edu/afr3/blogs/siowfa13/2013/09/black-and-white-247-thetruth-about-animal-color-vision.html http://www.colormatters.com/color-matters-for-kids/how-animals-see-color 27.1 Early Concepts of Light Some early philosophers believed that light traveled from our eyes to the objects we look at. “That’s why we don’t see anything until we look at it”. Some thought light consisted of tiny particles, although some (Empedocles and Huygens) thought it traveled in waves. The particle theory – light seems to move in straight lines and doesn’t seem to spread out like most waves do The wave theory became the accepted theory in the 19th century. In 1905, Einstein published a theory called the “photoelectric effect”. The theory states that light consists of particles called photons. Photons are massless bundles of concentrated electromagnetic energy. Scientists now agree that light has a dual nature, part particle, part wave. Conceptual Physics Page 1 of 13 Chapter 27 – Light Physics Teacher Notes - Grothaus In this chapter, we will only discuss the wave nature of light. 27.2 The Speed of Light The actual speed of light is 299,792,458 m/s, which we round to 300,000 km/s = 300,000,000 m/s 300,000,000 m/s = 187,000 mi/s The astronauts traveling in the space shuttle go about 5 mi/s The speed of light in a vacuum is a universal constant. c = 300,000 km/s 7.5 times around the Earth in one second 1.28 seconds for light reflected off the moon to get to the Earth 8 minutes from the sun to the Earth 4 years from the nearest star – Alpha Centauri The distance light travels in one year is called one light-year Alpha Centauri is 4 light-years away! A light year is a distance measurement – not a time measurement!!! d = vt = (3.0 10 8 m / s)(1year)( 365days 24hours 3600sec )( )( )= 1year 1day 1hour 9.461015 m or 5.879 1012 miles “All that we see, even our own reflection, is from the past.” The diameter of our galaxy is 100,000 light-years Some galaxies are 10 billion light-years from Earth If one exploded 5 billion years ago….. Interesting facts: Aristotle thought light was instantaneous Galileo tried to measure by putting two people on hills about a mile apart – estimated that light was 10 times the speed of sound. Only for theorists: VSauce – Would headlights work at the speed of light? https://www.youtube.com/watch?v=ACUuFg9Y9dY watch on your own? 27.3 Conceptual Physics Electromagnetic Waves Page 2 of 13 Chapter 27 – Light Physics Teacher Notes - Grothaus Light is energy emitted by accelerating electric charge (often electrons in atoms). This is a wave that’s partly electric and partly magnetic – electromagnetic wave Light is only a very small portion of the entire electromagnetic spectrum. Lower frequencies than red is called infrared (longer wavelengths) Higher frequencies are called ultraviolet (shorter wavelengths) Rabbits Multiply In Very Unusual expensive Gardens Radio Microwaves Infra-Red Visible light Ultra-violet X-rays Gamma rays IMPORTANT: Sound waves are mechanical waves – a vibration of material particles. Light waves are electromagnetic waves and are not vibrations of material, but are vibrations of pure energy Sound is not part of the EM spectrum. It has its own spectrum, of which we only hear a small part. 27.4 Light and Transparent Materials Light is energy carried in an electromagnetic wave that is generated by vibration of electric charges. When light hits an object, electrons in that object are forced into vibration (think resonance). How matter receives light depends on the frequency of the light and the natural frequency of the electrons in that matter. 14 Visible light – 100 trillion times per second ( 10 Hz ) For something to move that fast, it has to have very little inertia – Electrons have such little mass, they have very little inertia and will vibrate See Figure 27.6 – electrons vibrate, but are “attached” to their nucleus Natural frequency – last chapter Conceptual Physics Page 3 of 13 Physics Teacher Notes - Grothaus Chapter 27 – Light Electrons have a natural frequency as well. Materials respond more to vibrations at some frequencies than others. These natural vibration frequencies depend on how strongly the electrons are attached to a nearby nucleus. “spring strengths” Glass is transparent (trans-): Natural frequency is in the ultraviolet range. Ultraviolet light – resonance occurs – amplitude of the waves increase – large vibration between electrons and their nuclei – more “bumping around” – energy gets lost as heat. Glass is not transparent to UV (no sunburn) Visible light – lower frequency – electrons are forced into vibrations with smaller amplitudes -- atoms hold the energy for much less time – less bumping around -- energy gets through and is reemitted as light on the other side (less loss as heat) When light goes through – slight delay – speed of light while in the material is less than c (always 3.0 10 8 m/s in vacuum) In atmosphere, very, very small delay, but still rounded to 3.0 10 8 m/s In water, light travels at 0.75c ( 2.25 10 8 m/s) In glass, light travels at 0.67c ( 2.0110 8 m/s) In a diamond, light travels at 0.40c ( 1.20 10 8 km/s) Infrared (even lower frequencies), vibrate the entire structure of the glass Increases the internal energy and makes it warmer See Figure 27.8, p.539 Conceptual Physics Page 4 of 13 Physics Teacher Notes - Grothaus Chapter 27 – Light Interesting fact: Black light is UV light. How can UV light get through the glass in a light bulb? Quartz is transparent to UV light – black light bulbs are made of quartz Translucent – light doesn’t go straight through – diffuse transmission (bathroom windows, shower doors, etc. (book doesn’t cover this, but you should know it) 27.5 Opaque Materials When light shines on an object: (1) bounce off (reflect), (2) transmitted through the material or (3) it can be absorbed by the material (usually some combination of these) Conceptual Physics Page 5 of 13 Physics Teacher Notes - Grothaus Chapter 27 – Light Materials that don’t let light through are opaque. – in this case, there is usually a combination of light reflecting off and being absorbed. In opaque materials, any vibrations given by light to the atoms and molecules are turned into random kinetic energy – they become warmer. Metals have an outer layer of “loose” electrons, not attached to any particular atom. why they conduct electricity and heat so well! When light shines on it, those electrons vibrate and reemit the light. This is a reflection – shiny Why do wet things look darker? Why is a black tar road hotter than a pane of window glass in the sunlight? Why do eyeglasses that turn to sunshades when exposed to the sun not work inside of a car? Vsauce – What is the speed of dark? (13.5 minutes..) https://www.youtube.com/watch?v=JTvcpdfGUtQ 27.5 Shadows A think beam of light can be called a ray. A beam of light can be thought of as made from a bundle of rays When light shines on an object, some of the rays are stopped (absorbed and/or reflected from the object) and some continue on in a straight line path. Interesting fact: The X-ray images of teeth, bones, and the contents of your carry-on bag are shadows, made by a kind of light that goes through most things, but is scattered more by dense materials such as metals. Shadow dancers: https://www.youtube.com/watch?v=a4Fv98jttYA Conceptual Physics Page 6 of 13 Chapter 27 – Light Physics Teacher Notes - Grothaus Eclipses: all about shadows – we’ll be doing that 27.7 Polarization Light travels in waves – transverse waves You can tell because of polarization – see p. 542 Waves travel in one plane – polarized Light sources usually aren’t polarized because they send light out in all directions. When you send it through a polarizer, you “filter” out all of the other components and are left with light waves that are only on one plane See p. 543 – filter examples Skipping rocks – only flat will skip. If it comes in vertically, it will just sink. Light that reflects at glancing angles from nonmetal surfaces, such as glass, water or roads, vibrates mainly in the plane of the reflecting surface. This means that you want sunglasses to be in what orientation to keep out light that is reflecting off of the ground? Demonstration with filters – two filters Conceptual Physics Page 7 of 13 Physics Teacher Notes - Grothaus 27.8 Chapter 27 – Light Polarized Light and 3-D Viewing Vision in three dimensions depends on both eyes giving impressions at the same time from a slightly different angle. Try this: Which eye do you use more? (“discover!” on p. 544) The combination of views in the eye-brain system gives depth. A pair of photographs or movie frames, taken a short distance apart (about average eye spacing), can be seen in 3-D when the left eye sees only the left view and the right eye sees only the right view See if you can see figure 27.18 in 3D. There are lots of different ways to show 3D, but all of them involve each eye seeing a different view – binocular vision. http://science.howstuffworks.com/3-d-glasses2.htm One of the methods involves showing two views through polarization filters onto a screen. One is shown vertically, the other horizontally. Each eye on the glasses is differently polarized. Each eye sees a different image. They used to do it with different colors: red/cyan Problems for this unit: Conceptual Physics Page 8 of 13 Chapter 27 – Light Physics Teacher Notes - Grothaus Remember the formulas from the wave unit: Symbols and units: 1 sec Frequency f Hz Period T Velocity Wavelength sec v (time unit is in the numerator) meters/sec meters (time unit is in the denominator) Formulas: 1 T 1 T f f frequency and period are reciprocals of each other vf (v 1 ) T How do you put scientific notation numbers into a calculator? Example 1: 6 If you are listening to a radio station broadcast at 101 MHz ( 10110 Hz ) , what is the wavelength of the radio wave? 8 Remember that v = c 3.0 10 m / s When we are talking about wavelengths of visible light, we often talk about it in nanometers 1nm 109 m for example, violet light has a wavelength of 450 nm 9 this means that the wavelength is 450 10 m Example 2: Some laser pointers emit light waves with a wavelength of 533 nm. What is the frequency of this light? Optical Illusion website links Strong optical illusion simulation: really cool: https://www.youtube.com/watch?v=tVgOLWVYytM Conceptual Physics Page 9 of 13 Physics Teacher Notes - Grothaus Chapter 27 – Light Perfect explanation: https://www.youtube.com/watch?v=-IWk5NkxQF8 Optic illusions: a montage will be shown and discussed http://www.michaelbach.de/ot/ Vsauce blow your mind: https://www.youtube.com/watch?v=I31pi-6NgqM Peripheral vision effect: Scroll down to the “turn stars into monsters” part. This blows them away! http://gizmodo.com/5981603/20-optical-illusions-that-might-break-your-mind optical illusion montage – some classic, some not http://www.illusions.org/ **Google optical illusions, images – find some good ones here **Google specifically, “optical illusion vehicles” and “optical illusion chalk” Vsauce moving illusions: https://www.youtube.com/watch?v=Iw8idyw_N6Q If time? Einstein http://blog.newhorizons123.com/optical-illusion/ Veritasium: shadow illusions (not great) https://www.youtube.com/watch?v=liqF6EamiE4 OKGo – The Writing’s on the Wall https://www.youtube.com/watch?v=m86ae_e_ptU 28.11 The Atomic Color Code – Atomic Spectra When made to emit light, every element has its own characteristic color. Each one is a blend of different frequencies of light. Atoms have specific energy levels – rings around the nucleus Conceptual Physics Page 10 of 13 Physics Teacher Notes - Grothaus Chapter 27 – Light The closer to the nucleus, the less energy Picture: Analogy: Book above a table (release sound energy), screen door Remember that when energy hits an electron, it gets excited – goes up a level Think of a rubber band – If you stretch it, it wants to go back to its original shape – electrons do too. After an excited atom emits light in the form of a photon of a particular frequency, it returns to its normal state. Light has a spectrum of different frequencies. The energy transition of the electron is what determines the frequency of the photon that is released. Each photon carries an amount of energy that corresponds to its frequency. Red light (from neon) carries a certain amount of energy. A photon of twice the frequency has twice as much energy and is found in the ultraviolet part of the spectrum. When many atoms in a material are excited, many photons with many different frequencies are emitted. Measuring the frequencies of light in a spectrum is also measuring the relative energy levels in the atom emitting that light. The frequencies, or colors, of light emitted by elements are the “fingerprints” of the elements. We can do this by using an instrument called a spectroscope. It’s an arrangement of a thin slit through which the light is viewed and a prism that takes that light and separates it into its individual components and magnified so that you can see it. Or another type of spectroscope uses a diffraction grating. Conceptual Physics Page 11 of 13 Physics Teacher Notes - Grothaus Chapter 27 – Light Gas vs liquid or solid When you look at the light from a glowing element, you don’t see a continuous spectrum, you see a series of lines – a line spectrum Each line corresponds to a frequency of light – Each color line appears in the same position as that color in the continuous spectrum http://www.yourgemologist.com/Spectroscope/spectroscope.html Conceptual Physics Page 12 of 13 Physics Teacher Notes - Grothaus Chapter 27 – Light Lab Why look at the sun: http://www.nasa.gov/mission_pages/sunearth/news/light-wavelengths.html Auroras? Sun eruption: https://www.youtube.com/watch?v=GrnGi-q6iWc Sun documentary: https://www.youtube.com/watch?v=C2FETG7tCF0 Superhero physics – Superman laser beam https://www.youtube.com/watch?v=Un8xRUU173c Calculating the speed of light with a microwave: http://www.bbc.co.uk/bang/handson/lightspeed-maths.shtml Conceptual Physics Page 13 of 13