THE ELECTROMAGNETIC SPECTRUM. (We will go into more detail later but we need to establish some basic understanding here)

Transcription

1 What is color?

2 THE ELECTROMAGNETIC SPECTRUM. (We will go into more detail later but we need to establish some basic understanding here) Light isn t just white: colors is direct evidence that light has a range of energies.

3 THE ELECTROMAGNETIC SPECTRUM. (We will go into more detail later but we need to establish some basic understanding here) Light isn t just white: colors is direct evidence that light has a range of energies. Visible light shown through a prism is just like a rainbow in the sky (water droplets are the prism)

4 Four primary modes of LIGHT interacting with THINGS Transmission: light passes through object (refraction is type of transmission) Emission: light originates from object (it glows and sends out energy!) Reflection/Scattering: light hits object and bounces off (it s shiny if smooth, or a certain color**) Absorption: light hits object and is absorbed (energy from light goes into object, heating it)

5 In a cartoon world where color is dramatically simplified Let s pretend there is only light of three different energies: High Energy = Blue Medium Energy = Green Low Energy = Red (This is actually what computers do) All together they make WHITE LIGHT.

6 In a cartoon world where color is dramatically simplified Computer screens EMIT light (LCD), so to pop a picture of a red sweater on the screen, the computer uses RED light only:

7 In a cartoon world where color is dramatically simplified Computer screens EMIT light (LCD), so to pop a picture of a red sweater on the screen, the computer uses RED light only:

8 In a cartoon world where color is dramatically simplified Computer screens EMIT light (LCD), so to pop a picture of a red sweater on the screen, the computer uses RED light only: But what about a red sweater in real life? What makes a red sweater RED?

9 In a cartoon world where color is dramatically simplified What type of interactions with light do you think are happening when we perceive a sweater to be red?

10 In a cartoon world where color is dramatically simplified But what about a red sweater in real life? What makes a red sweater RED? (a dark room)

11 In a cartoon world where color is dramatically simplified But what about a red sweater in real life? What makes a red sweater RED? (a dark room)

12 In a cartoon world where color is dramatically simplified But what about a red sweater in real life? What makes a red sweater RED? (a flashlight) (a dark room)

13 In a cartoon world where color is dramatically simplified But what about a red sweater in real life? What makes a red sweater RED? (a flashlight) (shining white light) (which is actually red + green + blue light) (a dark room)

14 In a cartoon world where color is dramatically simplified But what about a red sweater in real life? What makes a red sweater RED? (a flashlight) (shining white light) (which is actually red + green + blue light) (a dark room)

15 In a cartoon world where color is dramatically simplified But what about a red sweater in real life? What makes a red sweater RED? (a flashlight) (shining white light) (which is actually red + green + blue light) (a dark room)

16 In a cartoon world where color is dramatically simplified But what about a red sweater in real life? What makes a red sweater RED? (a flashlight) (shining white light) (a dark room) (which is actually red + green + blue light) (blue & green: absorbed by the sweater)

17 In a cartoon world where color is dramatically simplified But what about a red sweater in real life? What makes a red sweater RED? (a flashlight) (shining white light) (which is actually red + (a dark room) green + blue light) (blue & green: absorbed by the sweater) (red light reflected & scattered off surface of sweater)

18 In a cartoon world where color is dramatically simplified But what about a red sweater in real life? What makes a red sweater RED? (a dark room)

19 In a cartoon world where color is dramatically simplified But what about a red sweater in real life? What makes a red sweater RED? (a flashlight) (a dark room)

20 In a cartoon world where color is dramatically simplified But what about a red sweater in real life? What makes a red sweater RED? (a flashlight) (shining white light) (which is actually red + green + blue light) (a dark room)

21 In a cartoon world where color is dramatically simplified But what about a red sweater in real life? What makes a red sweater RED? (a flashlight) (shining white light) (which is actually red + green + blue light) (a dark room)

22 In a cartoon world where color is dramatically simplified But what about a red sweater in real life? What makes a red sweater RED? (a flashlight) (shining white light) (which is actually red + green + blue light) (a dark room)

23 In a cartoon world where color is dramatically simplified But what about a red sweater in real life? What makes a red sweater RED? (a flashlight) (shining white light) (a dark room) (which is actually red + green + blue light) (blue & green: absorbed by the sweater)

24 In a cartoon world where color is dramatically simplified But what about a red sweater in real life? What makes a red sweater RED? (a flashlight) (shining white light) (which is actually red + (a dark room) green + blue light) (blue & green: absorbed by the sweater) (red light reflected & scattered off surface of sweater)

25 Four primary modes of LIGHT interacting with THINGS Transmission: light passes through object (refraction is type of transmission) Emission: light originates from object (it glows and sends out energy!) Reflection/Scattering: light hits object and bounces off (it s shiny if smooth, or a certain color**) Absorption: light hits object and is absorbed (energy from light goes into object, heating it)

26 Shiny vs. Rough surfaces: Reflection vs. Scattering the object s apparent color is the wavelength of light it scatters or reflects most efficiently.

27 The visible light regime is only a small portion of the electromagnetic spectrum. Submillimeter! DISCUSSION: 1. Which waves travel with the fastest speed? 2. Which waves carry the most energy?

28 EARTH S ATMOSPHERE INTERACTS WITH LIGHT OF DIFFERENT WAVELENGTHS DIFFERENTLY. VISIBLE LIGHT RULES HERE. So it shouldn t be surprising that humans evolved to see visible light, since that s what available to see! But there s a MUCH wider spectrum of light emission out there (even on Earth) which we don t see.

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30 Color in the human eye is determined by our cones. We have 6-7 million of them! But they group in either blue, green, or red -sensitive wavelengths I m guessing ~3 of you in this class are color-blind That typically means you only have two of the cones groupings, not all three.

31 Color in the human eye is determined by our cones. We have 6-7 million of them! But they group in either blue, green, or red -sensitive wavelengths Fun fact: the Mantis Shrimp has 16 different color-receptive cones! Highly recommend reading about it: theoatmeal.com/comics/mantis_shrimp I m guessing ~3 of you in this class are color-blind That typically means you only have two of the cones groupings, not all three.

32 TOOLS FOR UNDERSTANDING THE E.-M. SPECTRUM. Light of different colors can be characterized by its energy, wavelength or frequency. we know this! the speed of light = 3x10 8 m/s c = frequency (units of inverse seconds, e.g. s -1, which is called a Hertz, 1 Hz) h = Planck s constant = 6.63x10-34 Js wavelength (units of distance, e.g. m) E = h = hc photon energy (in Joules)

33 TOOLS FOR UNDERSTANDING THE E.-M. SPECTRUM. Light of different colors can be characterized by its energy, wavelength or frequency. Wavelength Frequency Optical wavelengths run from ~4000Å to 10000Å 1Å = m 1µm = 10 6 m Frequency is measured in Hz, MHz, GHz, THz 1Hz = 1s 1 Energy E Energy is measured in Joules, or for photons, in units of electronvolts, or ev: 1eV = J

34 TOOLS FOR UNDERSTANDING THE E.-M. SPECTRUM. Light of different colors can be characterized by its energy, wavelength or frequency. Wavelength typically used for optical/near-ir regime Optical wavelengths run from ~4000Å to 10000Å 1Å = m 1µm = 10 6 m Frequency typically used for radio regime Energy E Frequency is measured in Hz, MHz, GHz, THz 1Hz = 1s 1 Energy is measured in Joules, or for photons, in units of electronvolts, or ev: typically used for X-ray regime 1eV = J

35 c = E = h = hc Practice: What kind of light is 900 GHz emission? A. Radio waves B. Submillimeter/ Microwaves C. Infrared D. Visible E. Ultraviolet F. X-ray G. Gamma Submillimeter!

36 Multiwavelength astronomical studies: a wealth of information! Can now measure brightness as a function of position and wavelength to learn about the physics of the Universe.

37 What we have learned about light c = E = h = hc

38 Temperature, Blackbodies & Basic Spectral Characteristics.

39 How does temperature relate to energy? Cold things Hot things Which have more energy, or are they the same? DISCUSS.

40 Temperature is related to our perception of color because temperature is an indication of an emitting body s ENERGY. Temperature / Energy For example, temperature of a gas contained in this box is directly proportional to the sum of all of the kinetic energies of its particles (1/2 mv 2 )

41 Temperature is related to our perception of color because temperature is an indication of an emitting body s ENERGY. Temperature / Energy For example, temperature of a gas contained in this box is directly proportional to the sum of all of the kinetic energies of its particles (1/2 mv 2 )

42 Temperature is related to our perception of color because temperature is an indication of an emitting body s ENERGY. Temperature / Energy = h = hc For example, temperature of a gas contained in this box is directly proportional to the sum of all of the kinetic energies of its particles (1/2 mv 2 )

43 Temperature is related to our perception of color because temperature is an indication of an emitting body s ENERGY. Temperature / Energy = h = hc For example, temperature of a gas contained in this box is directly proportional to the sum of all of the kinetic energies of its particles (1/2 mv 2 )

44 Temperature is related to our perception of color because temperature is an indication of an emitting body s ENERGY. Temperature / Energy = h = hc For example, temperature of a gas contained in this box is directly proportional to the sum of all of the kinetic energies of its particles (1/2 mv 2 )

45 T / / 1

46 T / / 1 The exact form of this is called Wien s Displacement Law and was developed by Wilhelm Carl Wien in the shortest PhD thesis of all time:

47 T / / 1 The exact form of this is called Wien s Displacement Law and was developed by Wilhelm Carl Wien in the shortest PhD thesis of all time: wavelength in m = b T temperature in Kelvin (K) b = mk

48 What s the temperature of the human body?

49 What s the temperature of the human body? 98.6 o F

50 What s the temperature of the human body? Converting Temperatures Toolbox: T C = 5 9 (T F 32) T C = T K o F

51 What s the temperature of the human body? Converting Temperatures Toolbox: T C = 5 9 (T F 32) T C = T K o F = 37 o C = K 98.6 o F

52 What s the temperature of the human body? Converting Temperatures Toolbox: T C = 5 9 (T F 32) T C = T K o F = 37 o C = K 98.6 o F What wavelength of light is this?

53 Let s discuss: do you think the human body is exactly this temperature everywhere? Dr. Casey argues: Yes! We know something s not right when we measure a difference from this core temperature. We can measure that difference in our mouths, ears, and ehhem But Dr. Mann says: No! The human body has a range of temperatures all the time. That explains why our fingers, toes and ears get cold when we re outside. Who do you agree with? DISCUSS

54 Let s discuss: do you think the human body is exactly this temperature everywhere? Dr. Mann is right! The human body has a range of different temperatures, but is mostly at 98.6 o F

55 Let s discuss: do you think the human body is exactly this temperature everywhere? Dr. Mann is right! The human body has a range of different temperatures, but is mostly at 98.6 o F

56 Things that have one primary temperature but also exhibit a range of temperatures are known in physics as blackbodies. They radiate energy thermally.

57 Things that have one primary temperature but also exhibit a range of temperatures are known in physics as blackbodies. They radiate energy thermally. Humans are blackbodies, primarily glowing in the infrared.

58 Things that have one primary temperature but also exhibit a range of temperatures are known in physics as blackbodies. They radiate energy thermally. Humans are blackbodies, primarily glowing in the infrared. Candles (fire!) and Incandescent lightbulbs are blackbodies.

59 Things that have one primary temperature but also exhibit a range of temperatures are known in physics as blackbodies. They radiate energy thermally. Humans are blackbodies, primarily glowing in the infrared. Candles (fire!) and Incandescent lightbulbs are blackbodies. Stars are blackbodies. The sun glows primarily in the visible light.

60 Basic spectral form of a blackbody. energy output wavelength

61 Basic spectral form of a blackbody. energy output peaks at a given wavelength that wavelength indicates the overall temperature of the blackbody wavelength

62 Basic spectral form of a blackbody. energy output peaks at a given wavelength that wavelength indicates the overall temperature of the blackbody trails off at long wavelengths this is called the Rayleigh-Jeans tail (i.e. cooler temperatures, lower energies than the peak) wavelength

63 Basic spectral form of a blackbody. energy output peaks at a given wavelength that wavelength indicates the overall temperature of the blackbody trails off at long wavelengths this is called the Rayleigh-Jeans tail (i.e. cooler temperatures, lower energies than the peak) dives to near nothing at shorter wavelengths (higher energies, higher temperatures): Wien s approximation. wavelength

64 Why these funny names for different sides of the peak? Basic spectral form of a blackbody. energy output peaks at a given wavelength that wavelength indicates the overall temperature of the blackbody trails off at long wavelengths this is called the Rayleigh-Jeans tail (i.e. cooler temperatures, lower energies than the peak) dives to near nothing at shorter wavelengths (higher energies, higher temperatures): Wien s approximation. wavelength

65 Why these funny names for different sides of the peak? Basic spectral form of a blackbody. energy output peaks at a given wavelength Lord Rayleigh & Sir Jeans that wavelength indicates the overall temperature of the blackbody trails off at long wavelengths this is called the Rayleigh-Jeans tail (i.e. cooler temperatures, lower energies than the peak) dives to near nothing at shorter wavelengths (higher energies, higher temperatures): Wien s approximation. wavelength

66 Why these funny names for different sides of the peak? Basic spectral form of a blackbody. energy output Wilhelm Wien peaks at a given wavelength Lord Rayleigh & Sir Jeans that wavelength indicates the overall temperature of the blackbody trails off at long wavelengths this is called the Rayleigh-Jeans tail (i.e. cooler temperatures, lower energies than the peak) dives to near nothing at shorter wavelengths (higher energies, higher temperatures): Wien s approximation. wavelength

67 Why these funny names for different sides of the peak? Basic spectral form of a blackbody. energy output Wilhelm Wien peaks at a given wavelength Lord Rayleigh & Sir Jeans that wavelength indicates the overall temperature of the blackbody trails off at long wavelengths this is called the Rayleigh-Jeans tail (i.e. cooler temperatures, lower energies than the peak) dives to near nothing at shorter wavelengths (higher energies, higher temperatures): Wien s approximation. wavelength

68 Why these funny names for different sides of the peak? Basic spectral form of a blackbody. Wilhelm Wien energy output peaks at a given wavelength Lord Rayleigh & Sir Jeans that wavelength indicates the overall temperature of the blackbody trails off at long wavelengths this is called the Rayleigh-Jeans tail (i.e. cooler temperatures, lower energies than the peak) None of these gentlemen were quite right. Enter Max Planck dives to near nothing at shorter wavelengths (higher energies, higher temperatures): Wien s approximation. wavelength

69 The Planck Function describes the thermal emission of blackbodies perfectly: very difficult to derive (took ages!), and we won t derive it or memorize it here, but it has the following form: B (,T)= 2h 3 c 2 1 e h kt 1 in terms of frequency B (,T)= 2hc2 5 1 e hc kt 1 in terms of wavelength Max Planck Who the players are in this game: B B frequency the energy output as a fn of frequency/wavelength and temperature wavelength c speed of light h Planck s constant T temperature k Boltzmann s constant

70 The Planck Function describes the thermal emission of blackbodies perfectly: very difficult to derive (took ages!), and we won t derive it or memorize it here, but it has the following form: B (,T)= 2h 3 c 2 1 e h kt 1 in terms of frequency B (,T)= 2hc2 5 1 e hc kt 1 in terms of wavelength I repeat: PLEASE DO NOT MEMORIZE THIS OR BE INTIMIDATED BY IT, IT IS ONLY A FUNCTION OF SOME VARIABLES :) Max Planck Who the players are in this game: B B frequency the energy output as a fn of frequency/wavelength and temperature wavelength T temperature c speed of light h Planck s constant k Boltzmann s constant

71 Property of a blackbody: If it s the same size but hotter then it s giving off more energy at all wavelengths Hotter energy output Cooler wavelength