Energy What makes the color pink? Black and white TV summary Picture made from a grid of dots (pixels) Dots illuminated when electron beam hits phosphor Beam scanned across entire screen ~ 50 times a second Electron beam created and steered inside a cathode ray tube Created using electrostatic forces Steered using magnetic forces Deflection coils. Lecture 12 Color TV Light and the sun Reminders HW 6 available, Due Monday No labs this week Use the time to review..midterm 1 week from today Reading quiz now 1 -- -- Color TV Color TV- blend 3 color pixels. 3 electron beams, 3 control grids (control intensity of each beam) 3 sets of phosphor pixels. Essentially black & white TV times 3 with colored phosphors. Different color pixels - Color of emitted light determined by its wavelength (l) - Wavelength determined by energy of photon: E = hc / l - Photon energy determined by electron jump in phosphor atoms - Electron jump determined by separation of energy levels in phosphor atoms - Different phosphors produce different color light Black & white 8 pixels Color Standard TV screen: 525 lines high 400 dots across. Electron energy levels in Phosphor atom 3 See this next week: 4 Different color phosphors Why does color TV work? How does the eye see color? Electromagnetic Spectrum: Visible Light Black and white TV: -just one phosphor -bluish white light. Oscilloscope: -one type of phosphor -green light. Visible Light: electromagnetic radiation with wavelengths between 390 and 690 nm. A Photon: single packet of light energy of a particular wavelength Color TVs: -3 phosphors -blue, red, green. See this next week: 5 The Sun: emits photons at all of these different wavelengths Color TV emits: Red photons: Green photons: Blue photons: 6 1
How does the eye see color? The eye does not simply measure the wavelength of light to determine color black and white sensors for night vision (RODS) 3 types of sensors for color detection (CONES) Each cone responds differently to different color photons: Cone cell sensitivity: sensitivity: Level of stimulation of Cone 1: Red photons (650 nm): small to moderate stimulation Orange photons (600 nm): high stimulation Green photons (530 nm): small to moderate stimulation 7 Yellow photons will: a. Stimulate cone 1 much more than cone 2 or cone 3. b. Stimulate cone 1 and 2 about equally. Cone 3 no stimulation. c. Stimulate cone 2 much more than cone 1 or cone 3. d. Stimulate cone 3 only. e. Cannot see anything because Cone 1 only detects red and cone 2 only detects green and cone 3 only detects blue. 8 Each cone responds differently to different color photons: Color TV - making yellow Cone cell sensitivity: Color TV- blend 3 color pixels to make eye see full spectrum 3 electron beams, grids control intensity of each beam - (Negative charge on grid beam off) For yellow light: - Green electron beam on - Red electron beam on - Blue electron beam off 8 pixels Back to color TV. We have red, green, and blue photons coming out. How can we make our BRAIN see yellow? a. Red, green, and blue in equal amounts. b. Red and blue in equal amounts. No green. c. Red and green in equal amounts. No blue. d. Green and blue in equal amounts. No red. e. Some other combination of red, green, and blue. -- -- 9 10 Color demo Two Useful Sims What will happen if we spin this colored disk very fast? a) We will see blurry green and red b) We will see black c) We will see white d) We will see blue e) We will see yellow Color Vision http://www.colorado.edu/physics/phet/simulations/colorvision3/colorvision3.jnlp 12 2
What is white? The Spectrum of the Sun Seeing white light Emits slightly more blue/green Emits slightly less red Cone cell sensitivity: Cones of eye see combination of strength of colors given off by sun as white high and equal stimulation of all three cones Sensor level White light If less light, but still equal stimulation darker shade grey instead of white. 13 Cone 1 Cone 2 Cone 3 14 What about pink? Cone cell sensitivity: a. pink photon has more energy than a yellow photon b. pink photon has less energy than a yellow photon c. pink photons do not exist. Red 656 nm Blue 486nm Violet 434 nm 15 16 Finish TV s some recent buzzwords: Digital TV: New way of transmitting information to the TV. (CRT inside TV is unchanged) Send signals about brightness of each pixel digitally. (pixel 218 x 111 red-brightness 12, pixel 218 x 111 green-brightness 2, etc ) HDTV (high definition TV): More pixels per inch. Already do both (digital and more pixels) in computer monitors. Flat screen monitors-- completely different technology! see physics 2000 Light comes from back. Liquid crystal light valves at each pixel open and shut to allow desired amount of red, blue, green through. Light Electromagnetic spectrum Where does light come from Light interacting with matter Colors of objects absorption and reflection Scattering of light Refraction and reflection Plasma TV Yet another different technology Each pixel made of tiny R, G and B florescent lights Applied voltage accelerates electrons collide with noble gas atoms emit UV photons excites phosphor emits colored visible light 17 18 3
Energy SHORT LONG What is light? Part of electromagnetic spectrum Wavelengths: 390-690nm Photon energies: 3-5 10-19 J Behaves as both electromagnetic wave and particle of energy (photon) What is waving? Wave consists of oscillating electric and magnetic fields Electromagnetic Spectrum Spectrum: All EM waves. Complete range of wavelengths. Wavelength (l) = distance ( x) until wave repeats Frequency (f) = # of oscillations per second at X l Blue light l Red light 19 Speed of light ( and all other EM waves) All EM waves (all colors of light) travel at same speed in vacuum Speed = speed of light (c) = 3 x 10 8 m/s. Green Blue Red Energy in light Light comes in little particles of energy, called photons Each wavelength has own size energy particle: Energy of one photon = h x speed of light = h f l h = Plank s constant = 6.6x10-34 J s c = speed of light =3.0x10 8 m/s speed = wavelength x frequency meters oscillation c = l f oscillations sec Total energy Photon picture: E total = number of photons x energy per photon Wave picture: More energy means bigger amplitude oscillations of E and B fields 22 How is light (and rest of EM spectrum) created? Accelerating (oscillating) electric charges create: a) An oscillating electric field b) An oscillating magnetic field But that is just an EM wave! Where do we find accelerating or oscillating charges? a) Electrons in an atom or solid moving from one allowed energy level to another - e.g. Discharge lamp, LED - Discreet wavelengths emitted b) Free electrons with lots of thermal energy that are whizzing around and bumping into stuff - e.g.hot light bulb filament, Surface of sun - BB radiation emitted e E B The Sun Almost all light on earth comes from sun. Ultimate source of nearly all energy on earth Hot ball of atomic/sub atomic soup - atoms (73% hydrogen, 25% helium) - free electrons, protons and neutrons Different layers at different temperatures: - Core hottest (14 million K) Powerhouse of Sun - Photosphere (5800K) Where visible light comes from Power output: 4*10 26 W (400 YottaWatts) - V. large power plant 1*10 9 W (1GW) 24 4
INTENSITY Sun s energy from nuclear fusion, like in a hydrogen bomb. Fusion - creation of larger nuclei from smaller ones - mass difference released as energy p p p p 4 hydrogen nuclei Where does the sun s energy come from? Various steps p n n p helium nucleus Lots of energy 2 neutrinos 2 positrons Hydrogen burning in core produces helium, and energy to power the sun Helium nuclei fuse to form larger elements (up to iron) more slowly Released energy goes into thermal energy means atoms and electrons in the sun are VERY HOT and have LOTS of kinetic energy they are moving and shaking a lot! n n 25 Light from the the Sun 1. Get electrons hot (5500 C) with nuclear fusion 2. Electrons have lots of kinetic energy. Shaking (oscillating) around with a range of frequencies 3. Emit electromagnetic radiation (light!) at those frequencies - Called Black body or thermal radiation. - Emitted light depends only on temperature Free electrons and protons 26 Blackbody spectrum of the sun What if sun was hotter? Sunlight would look: a. same as now, b. redder c. bluer A useful simulation Blackbody radiation WAVELENGTH 27 http://phet.colorado.edu/simulations/sims.php?sim=blackbody_spectrum 28 Power, Energy, Intensity What s the difference between 1 Joule of energy used in 1 second and 1 Joule of energy used in 1 year? Power = Energy / Time Watts = measure of power (Joules / second) What about how much area that Power goes into? Intensity = Power / Area = Energy / (time x area) Units: Watts/m 2 Light interacting with matter 1. Color of objects (absorption/reflection of different l) 2. Scattering Why the sky is blue Why sunsets are red 3. Refraction and reflection at interfaces Underwater objects Twinkling stars 29 5
Electron energy 1. Color of objects It s all about what the electrons in the atoms of the object are doing: What colors do they absorb? What colors do they reflect? 1. Leaves look green to us. What colors do they absorb? What colors do they reflect? Color of objects Types (colors) of photons absorbed depends on atomic energy levels in material. Level separation must match photon energy for absorption. Leaf contains several different molecules absorbing red, yellow and blue photons. a. Absorbs green. Reflects reds, yellows, blues b. Absorbs blue. Reflects reds, yellows, greens. c. Absorbs reds, yellows, blues. Reflects greens. d. Absorbs reds, yellows. Reflects greens and blues. 31 A The diagrams above show energy levels in 3 types of leaf molecule. One absorbs red, one yellow and one blue light. Which absorbs blue light? B C White objects Look at paper in bright sunlight, it looks white. This means that: a. paper is absorbing all colors of light from the sun equally b. scatters only green and yellow c. scatters all colors from sun equally d. absorbs all but red e. scatters white photons. 6