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17_light2 - Light Color Color Addition &...

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Unformatted text preview: Light Color Color Addition & Subtraction Spectra UCSD: Physics 8; 2008 What do we see? Some objects are hot enough to emit in the visible range of wavelengths that our eyes can see Other devices such as laser pointers and LEDs emit light The light such light from the sun, from flames and from old style light bulbs They are not emitting because they are hot but rather because the atoms are given energy in a way that makes the atoms emit light unless the objects are hot enough ("red hot") to emit in the visible get bouncing of incident light Our eyes can't detect the near infrared that is the dominant emission from objects on the Earth's surface at 300 Kelvin Most objects we see reflected light Very occasionally we see light that has been absorbed, then reemitted at a different wavelength called fluorescence, phosphorescence, luminescence (e.g. clothes that glow in UV from black lamps) 2 UCSD: Physics 8; 2008 Light is characterized by frequency, or more commonly, by wavelength Visible light spans from 400 nm to 700 nm or 0.4 m to 0.7 m; 0.0004 mm to 0.0007 mm, etc. Colors 3 UCSD: Physics 8; 2008 White light White light is the combination of all wavelengths, with equal representation "red hot" poker has much more red than blue light red, green, and blue light bulbs sum to make white RGB monitor combines these colors to display white combined, white light called additive color combination--works with light sources blue light green light red light wavelength 4 UCSD: Physics 8; 2008 Additive Colors Red, Green, and Blue light sources can be used to synthesize almost any perceivable color Red + Green = Yellow Red + Blue = Magenta Green + Blue = Cyan These three dual-source colors become the primary colors for subtraction Why? White minus green is magenta White minus red is cyan Lack of blue = yellow 5 Questions on this coming later UCSD: Physics 8; 2008 Subtractive colors But most things we see are not light sources Reflection takes away some of the incident light thus the term subtractive If incident light is white, yellow is left when absorb blue incident white light reflected yellow light (blue gone) blue absorption (e.g., paint, dye) yellow light made of red and green 6 UCSD: Physics 8; 2008 What's responsible for absorption? Carotene makes carrots orange, tomatoes red, daffodils yellow, leaves turn carotene absorbs blue light Long, organic molecular chain most dyes, pigments are such resonances (for electron oscillations) in optical light Chlorophyll makes leaves green chlorophyll absorbs red and blue 7 UCSD: Physics 8; 2008 Questions Q: Why, when you mix all your paints together, do you just get dark brown or black? Why not white? Q: Why is the sky blue, and the low sun/moon orange? Are these related? Q: What colors does a ... Q: Magenta.. Q: Why are primary colors.. 8 UCSD: Physics 8; 2008 Our limited sensitivity to light In bright-light situations (called photopic, using cones), our sensitivity peaks around 550 nm, going from 400 to 700 In the dark, we switch to scotopic vision (rods), centered at 510 nm, going from 370 to 630 it's why astronomers like red flashlights: don't ruin night vision 9 UCSD: Physics 8; 2008 Spectra The spectrum represents the wavelength-bywavelength content of light can represent this in a color graphic as below or can plot intensity vs. wavelength previous plots of blackbody spectrum were of this form We can make a spectrum out of light, dissecting its constituent colors A prism is one way to do this A diffraction grating also does the job 10 Example Spectra white light spectrum hydrogen lamp spectrum helium lamp spectrum lithium lamp spectrum mercury lamp spectrum UCSD: Physics 8; 2008 Spectra provide "fingerprints" of atomic species, which can be used to identify atoms Anywhere in the universe! hydrogen absorption spectrum Solar Spectrum with (Fraunhofer) solar atmosphere absorption lines C: Hydrogen; D: Sodium; E: Iron; F: Hydrogen; G: Iron; H&K: Calcium 11 UCSD: Physics 8; 2008 Spectral Content of Light A spectrum is a plot representing light content on a wavelength-by-wavelength basis the myriad colors we can perceive are simply different spectral amalgams of light much like different instruments have different sound: it depends on its (harmonic) spectral content 12 UCSD: Physics 8; 2008 Light Sources Here are a variety of light sources. Included are: H-ITT IR LED* red LED* green laser pointer flourescence of orange H-ITT transmitter illuminated by green laser Note that light source has to be shorter wavelength to give fluorescence. * LED = Light Emitting Diode 13 UCSD: Physics 8; 2008 Colored Paper Reflected light (in this case, sunlight) off of paper appearing: blue green yellow orange red black aside from slight fluorescence in yellow paper, paper colors operate by reflection only: never peeks above 100% white paper would be a flat line at 100% 14 UCSD: Physics 8; 2008 Fluorescent Paper Bright fluorescent paper follows different rules: absorbs blue or UV light and re-emits at some characteristic wavelength. These examples are of lime green paper and bright orange fluorescent paper. Note especially in the orange case, the light exceeds the amount that would be passively reflected off of white paper (100% level) 15 UCSD: Physics 8; 2008 Fluorescent Markers (hi-lighters) Likewise, fluorescent markers (hi-lighters) absorb and re-emit light. In this case, we see yellow, green, and pink fluorescent markers The pink actually has a bit of blue/violet in it, surprisingly All three have emission above the 100% that one gets from straight reflection 16 UCSD: Physics 8; 2008 Fluorescent lights Fluorescent lights stimulate emission among atoms like argon, mercury, neon they do this by ionizing the gas with high voltage as electrons recombine with ions, they emit light at discrete wavelengths, or lines Mercury puts out a strong line at 254 nm (UV) this and other lines hit the phosphor coating on the inside of the tube and stimulate emission in the visible part of the spectrum 17 UCSD: Physics 8; 2008 Fluorescent Spectrum http://mo-www.harvard.edu/Java/MiniSpectroscopy.html http://en.wikipedia.org/wiki/Image:Fluorescent_lighting_spect 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. Hg: mercury; Tb: Terbium; Eu: Europium Hg Hg Tb Tb Hg Hg Eu Tb Eu Eu Eu Eu 18 UCSD: Physics 8; 2008 Black Lights http://en.wikipedia.org/wiki/Black_light standard Hg-driven flourescence single europium-based phosphor (SrB4O7F:Eu2+) uses glass that blocks visible light > 400 nm many objects absorb the UV light and themselves fluoresce 1. Eu 2. Hg 19 UCSD: Physics 8; 2008 LCD Monitor Green gets all of this line Red gets all of this line LCD monitors use fluorescent lights to illuminate the pixels (from behind). The black curve shows what my LCD laptop monitor looks like in a section of the screen that's white. Blue, green, and red curves show sections of the screen with these colors Note that the colors are achieved simply by Absorption from white 20 Blue gets all of this line Thus LCDs just filter the background light UCSD: Physics 8; 2008 Transmission of Glass, Sunglasses By obtaining a spectrum of sunlight reflected off of a piece of white paper (using a spectrograph), then doing the same thing through the fiber and also through sunglasses, the transmission properties of each can be elucidated. The fiber is about 82% transmission for most wavelengths, but has significant UV absorption. This is why you can't get sunburn through glass 21 The sunglasses block UV almost totally! UCSD: Physics 8; 2008 Sunlight and The Blue Sky sodium hydrogen calcium oxygen in earth atmos. These plots show the spectrograph's response to sunlight on white paper and to the blue sky. The spectrograph is not very efficient in UV or IR, and its sensitivity curve is shown in black. You can notice the violet hump in the blue sky (brighter than white paper here). Also, can see the solar atmosphere absorption lines in both sun and sky 22 hydrogen UCSD: Physics 8; 2008 Blackbody corrected The spectrograph software lets you claim a source to be a blackbody of specified temperature, so it can correct for its efficiency curve (black curve on prev.). Here we see the result of this process, which has made the sun curve look like a perfect blackbody peaking at 500 nm. But it also assumed that Fraunhoffer lines were artifacts to be removed Note the dramatic rise of the sky toward the blue/UV end. The lighter blue is without the UV-absorbing fiber in place 23 UCSD: Physics 8; 2008 More realistic spectrum Correcting the raw spectra from two slides back with the response curve, we arrive at a more realistic sun and sky spectrum. The black line is a blackbody at 5900 K, which fits the sun reasonably well. This time, the absorption lines survive. The blue sky now also looks smoother, and on top of this is plotted a theoretical 1/4 model for molecular scattering 24 Though not in words, this explains why the sky is blue! UCSD: Physics 8; 2008 How do diffraction gratings work? A diffraction grating is a regular array of optical scattering points spherical wave emerges from each scattering point constructively or destructively interfere at different angles depending on wavelength 25 UCSD: Physics 8; 2008 Another look at diffraction gratings For a given wavelength, a special angle will result in constructive interference: d sin = this angle is different for different wavelengths 26 ...
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This note was uploaded on 04/10/2008 for the course PHYS 8 taught by Professor Tytler during the Spring '08 term at UCSD.

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