Discharge Lamps Discharge Lamps They come in a variety of colors, - PDF document

Discharge Lamps 1 Discharge Lamps 2 Observations about Observations about Discharge Lamps Discharge Lamps � They often take a few moments to turn on They often take a few moments to turn on Discharge Lamps Discharge Lamps � They come in a variety of colors, including white They come in a variety of colors, including white � They are often whiter than incandescent bulbs They are often whiter than incandescent bulbs � They last longer than incandescent bulbs They last longer than incandescent bulbs � They sometimes hum loudly They sometimes hum loudly � They flicker before they fail completely They flicker before they fail completely Discharge Lamps 3 Discharge Lamps 4 4 Questions about 4 Questions about Question 1 Question 1 Discharge Lamps Discharge Lamps � Why phase out incandescent lamps? Why phase out incandescent lamps? � Why phase out incandescent lamps? Why phase out incandescent lamps? � How can colored lights mix so we see white? How can colored lights mix so we see white? � How can white light be produced without heat? How can white light be produced without heat? � How do gas discharge lamps produce their light? How do gas discharge lamps produce their light? Discharge Lamps 5 Discharge Lamps 6 Shortcomings of Thermal Light Shortcomings of Thermal Light Question 2 Question 2 � Incandescent lamps are reddish and inefficient Incandescent lamps are reddish and inefficient � How can colored lights mix so we see white? How can colored lights mix so we see white? � � Filament temperature is too low, thus too red Filament temperature is too low, thus too red � � The temperature of sunlight is 5800 K The temperature of sunlight is 5800 K � The temperature of an incandescent lamp is 2700 K Th The temperature of an incandescent lamp is 2700 K Th f f i i d d l l i 2700 K i 2700 K � An incandescent lamp An incandescent lamp � emits mostly invisible infrared light, emits mostly invisible infrared light, � so less than 10% of its thermal power is visible light. so less than 10% of its thermal power is visible light. 1

Discharge Lamps 7 Discharge Lamps 8 Seeing in Color Seeing in Color Question 3 Question 3 � We have three groups of light We have three groups of light- -sensing cone cells sensing cone cells � How can white light be produced without heat? How can white light be produced without heat? � Their peak responses are to red, green, and blue light Their peak responses are to red, green, and blue light � Those are therefore the primary colors of light Those are therefore the primary colors of light � When the primaries mix, we see others colors Wh Wh When the primaries mix, we see others colors h h i i i i i i h h l l � When the primaries mix evenly, we see white When the primaries mix evenly, we see white Discharge Lamps 9 Discharge Lamps 10 Fluorescent Lamps Fluorescent Lamps (Part 1) Fluorescent Lamps Fluorescent Lamps (Part 2) (Part 1) (Part 2) � Fluorescent tubes Fluorescent tubes � Collisions in the plasma cause Collisions in the plasma cause � contain low density gas and metal electrodes, contain low density gas and metal electrodes, � electronic excitation in the gas atoms electronic excitation in the gas atoms � which inject free electric charges into the gas which inject free electric charges into the gas � and occasionally ionize the gas atoms, and occasionally ionize the gas atoms, � to form a plasma � t f rm to form a plasma—a gas of charged particles t f rm pl m pl m a gas of charged particles f h r f h r d p rti l d p rti l � which helps to sustain the plasma. which helps to sustain the plasma. hi h h lp t hi h h lp t t in th pl m t in th pl m � � and electric fields cause current to flow in the plasma. and electric fields cause current to flow in the plasma. � Excited atoms emit light through fluorescence Excited atoms emit light through fluorescence � Fluorescence is part of quantum physics Fluorescence is part of quantum physics Discharge Lamps 11 Discharge Lamps 12 Quantum Physics of Atoms Quantum Physics of Atoms Atoms and Light Atoms and Light � In an atom, In an atom, � Electron Electron orbitals orbitals are standing waves: are standing waves: � the negative electrons “orbit” the positive nucleus the negative electrons “orbit” the positive nucleus � they do not change with time they do not change with time � and form standing waves known as and form standing waves known as orbitals orbitals. . � they involve no charge motion they involve no charge motion � Each orbital can have at most two electrons in it Each orbital can have at most two electrons in it � th � they do not emit (or absorb) light. th they do not emit (or absorb) light. d n t mit ( r b d n t mit ( r b rb) li ht rb) li ht � An electron in a specific orbital has a total energy, An electron in a specific orbital has a total energy, � While an electron is changing While an electron is changing orbitals orbitals, , � that is the sum of its kinetic and potential energies. that is the sum of its kinetic and potential energies. � there is charge motion and acceleration, there is charge motion and acceleration, � An atom’s electrons An atom’s electrons � so the electron can emit (or absorb) light. so the electron can emit (or absorb) light. � are normally in lowest energy are normally in lowest energy orbitals orbitals – – the ground state the ground state � but can shift to higher energy but can shift to higher energy orbitals orbitals – – excited states. excited states. � Such orbital changes are call Such orbital changes are call radiative radiative transitions transitions 2