Luminous Sources http://afonson311.files.wordpress.com/2011/03/sunset.jpg Light Production Luminous source Non-luminous source • Produces its own light • Example: sun, light bulb, lit match • Does not produce own light • Seen only by using reflected light Luminous Sources • • • • • • • • • Incandescence Electric discharge Phosphorescence Fluorescence Chemiluminescence Bioluminescence Triboluminescence Light-emitting diode (LED) Laser Incandescence • Production of light as a result of high temperature • Air from bulb removed and replaced with non-reactive gas • Prevents filament from reacting with oxygen and bursting into flame • 5-10% of energy converted to visible light • Most converted to infrared light (heat) • Inefficient Heated tungsten filament glows Electric Discharge light production by passing an electric current through a gas Electric Discharge • • • • Neon lights = Geissler tubes (1855) Vacuum pump removed most of the air from a closed tube Remaining air glowed when an electric current passed through Colour of glow depended on gas inside tube Phosphorescence • Light production by the absorption of UV light resulting in the emission of visible light over an extended period of time • Objects coated with phosphors that absorb UV light. Phosphors keep some of the energy and over time release some of the lower energy as visible light. • Example: glow-in-the-dark Fluorescence • Light production by the absorption of UV light resulting in immediate emission of visible light • Example: fluorescent dyes in detergent, highlighter pens, fluorescent lighting Fluorescent Light Structure of Fluorescent Lights • Light tube filled with low-pressure mercury vapour and an inert gas (e.g. Argon) • Inner surface of tube coated with fluorescent material known as phosphors How Fluorescent lights work (video) http://www.youtube.com/watch?v=ur5yPa4_j3c&NR=1 1. Electric Discharge: Electric current causes Hg atoms to emit UV light 2. Fluorescence: UV light strikes phosphors which convert the energy into visible light Fluorescent Light Pro • Same light output as incandescent bulb but less heat produced • Uses less electricity for same amount of light • Lasts longer Con • More expensive than incandescent bulbs • Contain mercury and should be treated like hazardous waste for disposal Chemiluminescence • Production of light as the result of a chemical reaction • “Cold light”: Little to no heat is produced Chemiluminescence How light sticks work: • One chemical in a narrow small glass vial • Other chemical in main body • Bending stick breaks glass vial • Chemical mix in the main body • Reaction produces visible light Chemiluminescence Application • • • • • • Camping Law enforcement Military personnel Entertainment venues Emergencies Underwater divers (source has no moving parts, completely sealed) • Hazardous environments where a spark could be dangerous (source does not require electric current) Bioluminescence • Production of light in living organisms as a result of a chemical reaction • Little to no heat produced • Function: protection from predators, lure prey, attract mates • Example: luciferase enzyme in fireflies catalyze oxidation of luciferin protein to produce light Angler Fish Bioluminescence • Green Fluorescent Protein (GFP): exhibits bright green fluorescence when exposed to blue light • Originally discovered in jelly fish • Now used extensively in research as a marker for gene expression Triboluminescence • Production of light from friction (rubbing), pressure (crushing) or mechanical shock (scratching) • Explained by the breaking of chemical bonds in the material • Most often seen in rubbing of certain crystals • Also works with Wintergreen hard candy, pulling apart 2 pieces of duct tape, peeling transparent Scotch tape Light-emitting diode (LED) • Production of light by an electric current flowing in a diode • Diode: a simple type of semiconductors • Semiconductor: a material that allows electric current to flow in only one direction • Applications: Christmas lights, traffic lights Light-Emitting Diode Comparison Compared to incandescent bulbs Pros • No filament (doesn’t burn out) • Not much heat production (less wasted energy) • More energy efficient (longer lifespan, lower power usage) Cons • Produces a cool ‘blue’ light instead of the warm ‘yellow’ light Compared to compact fluorescent bulbs Pros • No toxic mercury • More energy efficient (longer lifespan, lower power usage) Cons • Higher up-front costs LASER • • • • • Light Amplication by Stimulated Emission of Radiation Properties of Laser Light Lasers Incandescent • emit wavelengths of • emit wavelengths of the same energy level many different energy levels • results in a light beam of a single pure colour • results in white light (combination of all • monochromatic colours) Properties of Laser Light • Waves are directional (travel in the same direction) • Results in light that is concentrated into one narrow beam and can travel great distances without spreading out • Waves are coherent (wave fronts launch in unison) • Results in light that is very intense Application of Lasers - Manufacturing: cutting glass, burning through steel Astronomy: measure Earth-moon distance Research: surveyor to measure distance Entertainment: laser light shows Military: Boeing airborne laser, tactical high energy laser Media technology: CD, DVD Media Technology Application • CD & DVDs are read by lasers • Pits: bumps on discs that scatter laser light in all directions • Land: non-bumps that reflect laser light • Reflected light is converted into binary code (0’s and 1’s) DVD & Blu-Ray Construction DVD Laser Red Wavelength 650 nm Pit 0.4 µm Track pitch 0.74 µm Storage 4.7 GB Blu-Ray Blue 405 nm 0.15 µm 0.32 µm 25 GB http://electronics.howstuffworks.com/blu-ray1.htm http://electronics.howstuffworks.com/blu-ray2.htm
