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Incandescence and Luminescence

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Physics 1230: Light and Color Chuck Rogers, [email protected] Ryan Henley, Valyria McFarland, Peter Siegfried physicscourses.colorado.edu/phys1230 PLEASE COME PICK-UP YOUR EXAM 1 BOOKLETS from the front table. Congratulations on completing Exam 1. Great job! Exam 1 Weighted Average: 76 +/- 12 of 100 points Group Exam Average: 86 +/- 10 of 100 points. 1 Physics 1230 Exam 1 Weighted 9 Average: 76+/-12 of 100 points 8 7 6 5 4 Frequency 3 2 1 0 Bin 2 Physics 1230: Light and Color Chuck Rogers, [email protected] Ryan Henley, Valyria McFarland, Peter Siegfried physicscourses.colorado.edu/phys1230 Exam 1 Questions?? 1. Write your question on a separate page. 2. Attach to otherwise untouched exam. 3. Bring it to Prof. Rogers and I will have a look. 4. Will evaluate and get back to you. Please send your questions in the next week. 3 Physics 1230: Light and Color Chuck Rogers, [email protected] Ryan Henley, Valyria McFarland, Peter Siegfried physicscourses.colorado.edu/phys1230 Lecture 10: More EM Waves and making light by: Incandescence and luminescence. 4 How do you generate light (electromagnetic radiation)? a. Stationary charges b. Charges moving at a constant velocity c. Wiggling charges d. b and c e. a, b, and c Ans. is c. Accelerating charges create EM radiation. The Sun Surface of sun- very hot! + + Whole bunch of free electrons + whizzing around like crazy. Equal number of protons, but + heavier so moving slower, less EM waves generated. Go to radiowave sim EM radiation often represented by a sinusoidal curve. OR What is the wave showing? What does the curve tell you? a.The spatial extent of the E-field. At the peaks and troughs the E-field is covering a larger extent in space b.The E-field’s direction and strength along the center line of the curve c. The actual path of the light travels d.more than one of these e.none of these. EM radiation often represented by a sinusoidal curve. OR What is the wave showing? What does the curve tell you? a.The spatial extent of the E-field. At the peaks and troughs the E-field is covering a larger extent in space b.The E-field’s direction and strength along the center line of the curve c. The actual path of the light travels d.more than one of these e.none of these. Clicker question Would the electric field in this electromagnetic wave cause electrons in the wire to move? A. Yes, along the length of the wire B. Yes, to the right C. No Clicker question Wave moves up and down, travel to the right. Would the electric field in this electromagnetic wave cause electrons in the wire to move? A. Yes, along the length of the wire This is why radio B. Yes, to the right antenna are C. No oriented up and down Suppose you move the antenna to Here? Would the electric field in this electromagnetic wave cause electrons in the wire to move? A. Yes, along the length of the wire B. Yes, to the right C. No Suppose you move the antenna to Here? Would the electric field in this electromagnetic wave cause electrons in the wire to move? A. Yes, along the length of the wire The wave is B. Yes, to the right TRAVELING!! C. No Antenna will not be at zero long… You know: Electromagnetic wave • Charge a piece of tape. • Now, wiggle your tape up and down • You’ve made an electromagnetic wave! Why can’t you see it? Wiggle Clicker question Wiggling a charged piece of tape creates EM Waves, but we cannot see them. To make visible light I should wiggle the tape: A) slower B) faster C) same speed D) visible light is just different… You know: Electromagnetic wave • Charge a piece of tape. • Now, wiggle your tape up and down • Wiggle fast enough and you’ve made electromagnetic light waves! X Wiggle Electromagnetic radiation Wave fills all space Wave can travel outward in different directions A great place for QUESTIONS! Wavefront Take a cleansing breath… Goals for today • Identify the general mechanism through which an incandescent bulb creates light • Learn how to draw a spectral distribution curve • Be able to use the relationship between temperature and peak wavelength of emission in incandescent light emission to predict how the peak wavelength will change given a change in temperature • Understand the concept of color temperature in everyday lighting How is light (electromagnetic waves) created? There are two main ways to make light 1. Incandescence: Heat an object up, and it glows. (“Blackbody radiation”) 2. Luminescence: For example gas discharge lamp First: Incandescence How does a lightbulb glow? Hot things glow = incandescence: • Current runs through the filament • This makes the filament hot • Electrons jiggle in hot things emit light Heating objects PhET radio causes their waves electrons to jiggle and they emit light. Jiggle electrons at different frequencies, we get different kinds of electromagnetic radiation (light). cf Which part of this glowing piece of metal in a blacksmith shop is hotter? (Use your common sense for this one) A) redder part B) yellower part C) the whole thing must be the same temperature Color temperature The hotter the temperature of a thing, the faster the charges in it wiggle (higher wiggle frequency). So, the hotter the object, the: A. Shorter the wavelength of light it emits B. Longer the wavelength of light it emits C. Temperature and wavelength are not related at all. cf Temperature affects color emitted High temperature Short wavelength Hotter Cooler UV Infrared (IR camera) White hot glows over visible range, eye mixes to make white Temperature affects color emitted Wien’s Law: Brightest color wavelength is just proportional to the absolute temperature. 3 Tbrightest 3 10 meters Kelvin Tell me the brightest color of a hot object and I can tell you how hot it is! Temperature affects color emitted Example: What is the temperature of the Sun? 3 103 m*K T brightest 500nm brightest 3 103 m*K 3 103 m*K 3 1037 T 500 109 m 5 107 m 5 0.6 104 K 6000K 0 Celsius is the same as 273 Kelvin. Color temperature I have an incandescent light bulb at a temperature of 3000 K. What will be the peak wavelength of emission? A) 1000 nm B) 100 nm C) 3000 nm D) 300 nm E) None of these 3 Tbrightest 3 10 m*K Color temperature I have an incandescent light bulb at a temperature of 3000 K. What will be the peak wavelength of emission? A) 1000 nm B) 100 nm C) 3000 nm D) 300 nm E) None of these 3 Tbrightest 3 10 m*K Color temperature An incandescent light bulb at a temperature of 3000 K has a peak wavelength of 1000 nm. Do your eyes work well at 1000 nm? A) YES B) NO This is why incandescent light bulbs are so inefficient: Most of their light is invisible to us. Review clicker question The star Betelgeuse is reddish when you look at it in the night sky Is Betelgeuse hotter or colder than our yellow star (the sun): A) hotter B) colder C) same temperature Color temperature There are two main ways to make light 1. Incandescence: Heat an object up, and it glows. (“Blackbody radiation”) 2. Luminescence: For example discharge lamp More light sources To understand fluorescent light bulbs and light emitting diodes (LEDs) we need to understand mechanisms other than incandescence (often called blackbody radiation) Look at light tower with diffraction gratings Key points about atoms • Electrons are “bound” to atoms • There are only certain amounts of energy the electrons can have (energy levels) • The more energy the electron has, the further away it is from nucleus (higher energy level) 36 These are both simplified models of atom Atoms can absorb or emit energy (e.g., light) • Absorb: the electron moves up an energy level • Emit: the electron move down a level. + 37 This is (basic idea of) how LEDs, lasers, and fluorescent lights work One color (f) only Spacing between energy levels leads to the color emitted: E = hf Which type of light has the highest energy per photon? A. Red light B. Blue light C. Infrared E = hf D. Radio High frequency light has higher energy E = hf Low E High E Ionizing vs. non-ionizing radiation This is why we limit our exposure to uv and X-rays Fluorescence Absorbs in one wavelength and emits in another: Must absorb shorter wavelength (high energy) and emit higher wavelength (low energy) High E Low E (e.g, UV) (e.g., visible) Fluorescence Shine UV light on these; they emit light of another color Phosphorescence is just delayed fluorescence; it continues to glow. The lighting in this room • Tube is filled with argon and This process is called: mercury; spark in gas creates UV – A) Lasing B) Fluorescence but we can’t see UV C) Discharge • The UV light is absorbed by the D) Blackbody radiation coating and emits visible wavelength of a lower energy/frequency The latest in lighting: LED Another example of luminescence: Lasers: Light Amplification by Stimulated Emission You don’t have to know how they work. But very important part of modern technology. In the category of luminescence. Good time for a break!.
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