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Unformatted text preview: UCSD Physics 10 Light
Color Color Addition & Subtraction Spectra UCSD Physics 10 What do we see? Our eyes can't detect intrinsic light from objects (mostly infrared), unless they get "red hot" The light we see is from the sun or from artificial light (bulbs, etc.) When we see objects, we see reflected light immediate bouncing of incident light (zero delay) Very occasionally we see light that has been absorbed, then re-emitted at a different wavelength called fluorescence, phosphorescence, luminescence UCSD Physics 10 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 UCSD Physics 10 White light is the combination of all wavelengths, with equal representation "red hot" poker has much more red than blue light experiment: red, green, and blue light bulbs make white RGB monitor combines these colors to display white
combined, white light White light called additive color combination--works with light sources blue light green light red light wavelength UCSD Physics 10 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? because absence of green is magenta absence of red is cyan, etc. UCSD Physics 10 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 absence of blue
incident white light reflected yellow light (blue gone) blue absorption (e.g., paint, dye) yellow light made of red and green UCSD Physics 10 Questions Why, when you mix all your paints together, do you just get dark brown or black? Why not white? Why is the sky blue, and the low sun/moon orange? Are these related? UCSD Physics 10 Introduction to Spectra 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 The spectrum represents the wavelength-bywavelength content of light can represent this in a color graphic like that above or can plot intensity vs. wavelength UCSD Physics 10 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 UCSD Physics 10 For a given wavelength, a special angle will result in constructive interference: d sin = this angle is different for different wavelengths Another look at diffraction gratings UCSD Physics 10 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 UCSD Example Spectra
white light spectrum hydrogen lamp spectrum helium lamp spectrum lithium lamp spectrum mercury lamp spectrum hydrogen absorption spectrum Physics 10 Spectra provide "fingerprints" of atomic species, which can be used to identify atoms across the universe! Solar Spectrum with Fraunhofer solar atmosphere absorption lines C: Hydrogen; D: Sodium; E: Iron; F: Hydrogen; G: Iron; H&K: Calcium UCSD Physics 10 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 UCSD Physics 10 Our limited sensitivity to light In bright-light situations (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 UCSD Physics 10 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 has to be blue-ward (shorter wavelength) of the fluorescence for it to work. * LED: Light Emitting Diode UCSD Physics 10 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 chosen here, paper colors operate by reflection only: never peeks above 100% white paper would be a flat line at 100% UCSD Physics 10 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) UCSD Physics 10 Fluorescent Markers (hi-lighters)
Likewise, fluorescent markers (hi-lighters) absorb and re-emit light. In this case, we see yellow, green, and pink yellow 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 UCSD Physics 10 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 suppression Blue gets all of this line Thus LCDs just filter the background light UCSD Physics 10 Transmission of Glass, Sunglasses
By obtaining a spectrum of sunlight reflected off of a piece of white paper (using the spectrograph without the fiber feed), 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. The sunglasses block UV almost totally! This is why you can't get sunburn through glass UCSD Physics 10 Assignments and other stuff Assignments: Read Hewitt chapter 27 pp. 515526 Read Hewitt chapter 28 pp. 544547 Read Hewitt chapter 30 (just skim fluorescence onward) HW 7 due 5/30: 26.E.3, 26.E.4, 26.E.10, 26.E.14, 26.E.38, 26.P.4, 31.E.4, 31.E.9, plus additional problems on website Pick up a grating (one per person) in front of class You can build a groovy spectrometer using the diffraction grating used in class http://physics.ucsd.edu/~tmurphy/phys10/spectrometer.html ...
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