Conceptual Physics

# Conceptual Physics

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## Conceptual Physics

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1. Conceptual Physics Chapter Twenty Eight Notes: COLOR .

2. Introduction • As discussed in the last chapter, electromagnetic waves are waves which are capable of traveling through a vacuum. Unlike mechanical waves which require a medium in order to transport their energy, electromagnetic waves are capable of transporting energy through the vacuum of outer space. Electromagnetic waves are produced by a vibrating electric charge and as such, they consist of both an electric and a magnetic component. • Electromagnetic waves exist with an enormous range of frequencies. This continuous range of frequencies is known as the electromagnetic spectrum. The diagram below depicts the electromagnetic spectrum and its various regions. The longer wavelength, lower frequency regions are located on the far left of the spectrum and the shorter wavelength, higher frequency regions are on the far right. Two very narrow regions within the spectrum are the visible light region and the X-ray region. Within the visible range, the different frequencies/wavelength determine what color we see!

3. 28.1 The Color Specrum • The focus of chapter 28 will be upon the visible light region - the very narrow band of wavelengths located to the right of the infrared region and to the left of the ultraviolet region. Though electromagnetic waves exist in a vast range of wavelengths, our eyes are sensitive to only a very narrow band. Since this narrow band of wavelengths is the means by which humans see, we refer to it as the visible light spectrum. Normally when we use the term "light," we are referring to a type of electromagnetic wave which stimulates the retina of our eyes. In this sense, we are referring to visible light, a small spectrum from the enormous range of frequencies of electromagnetic radiation. This visible light region consists of a spectrum of wavelengths which range from approximately 700 nanometers (abbreviated nm) to approximately 400 nm. Expressed in more familiar units, the range of wavelengths extends from 7 x 10-7 meter to 4 x 10-7 meter. This narrow band of visible light is affectionately known as ROYGBIV. • Each individual wavelength within the spectrum of visible light wavelengths is representative of a particular color. That is, when light

4. of that particular wavelength strikes the retina of our eye, we perceive that specific color sensation. Isaac Newton showed that light shining through a prism will be separated into its different wavelengths and will thus show the various colors that visible light is comprised of. The separation of visible light into its different colors is known as dispersion. • Each color is characteristic of a distinct wavelength; and different wavelengths of light waves will bend varying amounts upon passage through a prism. For these reasons, visible light is dispersed upon passage through a prism. Dispersion of visible light produces the red (R), orange (O), yellow (Y), green (G), blue (B), and violet (V)colors. It is because of this that visible light is sometimes referred to as ROY G. BIV. (Incidentally, the indigo is not actually observed in the spectrum but is traditionally added to the list so that there is a vowel in Roy's last name.) The red wavelengths of light are the longer wavelengths and the violet wavelengths of light are the shorter wavelengths. Between red and violet, there is a continuous range or spectrum of wavelengths. The visible light spectrum is shown in the diagram below.

5. Spectrum of Visible Light

6. When all the wavelengths of the visible light spectrum strike your eye at the same time, white is perceived. The sensation of white is not the result of a single color of light. Rather, the sensation of white is the result of a mixture of two or more colors of light. Thus, visible light - the mix of ROYGBIV - is sometimes referred to as white light. Technically speaking, white is not a color at all - at least not in the sense that there is a light wave with a wavelength which is characteristic of white. Rather, white is the combination of all the colors of the visible light spectrum. If all the wavelengths of the visible light spectrum give the appearance of white, then none of the wavelengths would lead to the appearance of black. Once more, black is not actually a color. Technically speaking, black is merely the absence of the wavelengths of the visible light spectrum. So when you are in a room with no lights and everything around you appears black, it means that there are no wavelengths of visible light striking your eye as you sight at the surroundings. • How many colors are there in this swatch? How many were you taught in elementary school? The simple named colors are mostly monosyllabic in English — red, green, blue, brown, black, white, gray. (Yellow is the one exception to this rule, but it's still pretty simple.) Brevity indicates a pre-English, Anglo-Saxon origin. Monosyllabic words are generally the oldest words in the English language — head, eye, nose, foot, cat, dog, cow, eat, drink, man, wife, house, sleep, rain, snow, sword, sheath, God, and the "four letter words" — words that go back a thousand years. Some of the names for colors are loan words from French — orange and beige, since the "zh" sound doesn't exist in pure English (garage is a very french word) and violet and purple, since they just sound too fancy to be anglo-saxon.

7. 28.2 Color by Reflection • We have previously learned that visible light waves consists of a continuous range of wavelengths or frequencies. When a light wave with a single frequency strikes an object, a number of things could happen. The light wave could be absorbed by the object, in which case its energy is converted to heat. The light wave could be reflected by the object. And the light wave could be transmitted by the object. Rarely however does just a single frequency of light strike an object. While it does happen, it is more usual that visible light of many frequencies or even all frequencies are incident towards the surface of objects. When this occurs, objects have a tendency to selectively absorb, reflect or transmit light certain frequencies. That is, one object might reflect green light while absorbing all other frequencies of visible light. Another object might selectively transmit blue light while absorbing all other frequencies of visible light. The manner in which visible light interacts with an object is dependent upon the frequency of the light and the nature of the atoms of the object. In this section we will discuss how and why light of certain frequencies can be selectively absorbed, reflected or transmitted.

8. Atoms and molecules contain electrons. It is often useful to think of these electrons as being attached to the atoms by springs. The electrons and their attached springs have a tendency to vibrate at specific frequencies. Similar to a tuning fork or even a musical instrument, the electrons of atoms have a natural frequency at which they tend to vibrate. When a light wave with that same natural frequency impinges upon an atom, then the electrons of that atom will be set into vibrational motion. (This is merely another example of the resonance principle introduced earlier.) If a light wave of a given frequency strikes a material with electrons having the same vibrational frequencies, then those electrons will absorb the energy of the light wave and transform it into vibrational motion. During its vibration, the electrons interacts with neighboring atoms in such a manner as to convert its vibrational energy into thermal energy. Subsequently, the light wave with that given frequency is absorbed by the object, never again to be released in the form of light. So the selective absorption of light by a particular material occurs because the selected frequency of the light wave matches the frequency at which electrons in the atoms of that material vibrate. Since different atoms and molecules have different natural frequencies of vibration, they will selectively absorb different frequencies of visible light.

9. Reflection of light waves occur because the frequencies of the light waves do not match the natural frequencies of vibration of the objects. When light waves of these frequencies strike an object, the electrons in the atoms of the object begin vibrating. But instead of vibrating in resonance at a large amplitude, the electrons vibrate for brief periods of time with small amplitudes of vibration; then the energy is reemitted as a light wave. If the object is opaque, then the vibrations of the electrons are not passed from atom to atom through the bulk of the material. Rather the electrons of atoms on the material's surface vibrate for short periods of time and then reemit the energy as a reflected light wave. Such frequencies of light are said to be reflected. • The color of the objects which we see are largely due to the way those objects interact with light and ultimately reflect it to our eyes. The color of an object is not actually within the object itself. Rather, the color is in the light which shines upon it and is ultimately reflected to our eyes. We know that the visible light spectrum consists of a range of frequencies, each of which corresponds to a specific color. When visible light strikes an object and a specific frequency becomes absorbed, that frequency of light will never make it to our eyes. Any visible light which strikes the object and becomes reflected to our eyes will contribute to the color appearance of that object. So the color is not in the object itself, but in the light which strikes the object and ultimately reaches our eye. The only role that the object plays

10. is that it might contain atoms capable of selectively absorbing one or more frequencies of the visible light which shine upon it. So if an object absorbs all of the frequencies of visible light except for the frequency associated with green light, then the object will appear green in the presence of ROYGBIV. And if an object absorbs all of the frequencies of visible light except for the frequency associated with blue light, then the object will appear blue in the presence of ROYGBIV. Consider the two diagrams on the following page. The diagrams depict a sheet of paper being illuminated with white light (ROYGBIV). The papers are impregnated with a chemical capable of absorbing one or more of the colors of white light. Such chemicals which are capable of selectively absorbing one or more frequency of white light are known as pigments. In Example A, the pigment in the sheet of paper is capable of absorbing red, orange, yellow, blue, indigo and violet. In Example B, the pigment in the sheet of paper is capable of absorbing orange, yellow, green, blue, indigo and violet. In each case, whatever color is not absorbed is reflected.

11. Example A: Green will be reflected and so the paper appears green to an observer. Example B: Red will be reflected and so the paper appears red to an observer. Example A:Green will be transmitted and so the object appears green to an observer. Example B:Both green and blue will be transmitted and so the object appears greenish-blue to an observer.

12. 28.3 Color by Transmission • Transmission of light waves occur because the frequencies of the light waves do not match the natural frequencies of vibration of the objects. When light waves of these frequencies strike an object, the electrons in the atoms of the object begin vibrating. But instead of vibrating in resonance at a large amplitude, the electrons vibrate for brief periods of time with small amplitudes of vibration; then the energy is reemitted as a light wave. If the object is transparent, then the vibrations of the electrons are passed on to neighboring atoms through the bulk of the material and reemitted on the opposite side of the object. Such frequencies of light waves are said to be transmitted. • The color of the objects which we see are largely due to the way those objects interact with light and ultimately transmit it to our eyes. The color of an object is not actually within the object itself. Rather, the color is in the light which shines upon it and is ultimately transmitted to our eyes. We know that the visible light spectrum consists of a range of frequencies, each of which corresponds to a

13. specific color. When visible light strikes an object and a specific frequency becomes absorbed, that frequency of light will never make it to our eyes. Any visible light which strikes the object and becomes transmitted to our eyes will contribute to the color appearance of that object. So the color is not in the object itself, but in the light which strikes the object and ultimately reaches our eye. The only role that the object plays is that it might contain atoms capable of selectively absorbing one or more frequencies of the visible light which shine upon it. So if an object absorbs all of the frequencies of visible light except for the frequency associated with green light, then the object will appear green in the presence of ROYGBIV. And if an object absorbs all of the frequencies of visible light except for the frequency associated with blue light, then the object will appear blue in the presence of ROYGBIV.

14. Subtractive Process e.g.R O Y G B I V    blue filter We See Mostly Blue

15. 28.4 Sunlight • The sun emits light waves with a range of frequencies. Some of these frequencies fall within the visible light spectrum and thus are detectable by the human eye. Since sunlight consists of light with the range of visible light frequencies, it appears white. This white light is incident towards Earth and illuminates both our outdoor world and the atmosphere which surrounds our planet. As discussed earlier, the interaction of visible light with matter will often result in the absorption of specific frequencies of light. The frequencies of visible light which are not absorbed are either transmitted (by transparent materials) or reflected (by opaque materials). As we sight at various objects in our surroundings, the color which we perceive is dependent upon the color(s) of light which are reflected or transmitted by those objects to our eyes. So if we consider a green leaf on a tree, the atoms of the chlorophyll molecules in the leaf are absorbing most of the frequencies of visible light (except for green) and reflecting the green light to our eyes. The leaf thus appears green. And as we view the black asphalt street, the atoms of the asphalt are absorbing all the frequencies of visible light and no light is reflected to our eyes. The asphalt street thus appears black

16. (the absence of color). In this manner, the interaction of sunlight with matter contributes to the color appearance of our surrounding world. In later sections, we will focus on the interaction of sunlight with atmospheric particles to produce blue skies and red sunsets. While sunlight consists of the entire range of frequencies of visible light, not all frequencies are equally intense. In fact, sunlight tends to be most rich with yellow light frequencies. For these reasons, the sun appears yellow during midday due to the direct passage of dominant amounts of yellow frequencies through our atmosphere and to our eyes. The appearance of the sun changes with the time of day. While it may be yellow during midday, it is often found to gradually turn color as it approaches sunset. This can be explained by light scattering. As the sun approaches the horizon line, sunlight must traverse a greater distance through our atmosphere; this is demonstrated in the diagram below.

17. 28.5 Mixing Colored Light Color Addition • Color perception, like sound perception, is a complex subject involving the disciplines of psychology, physiology, biology, chemistry and physics. When you look at an object and perceive a distinct color, you are not necessarily seeing a single frequency of light. Consider for instance that you are looking at a shirt and it appears purple to your eye. In such an instance, there my be several frequencies of light striking your eye with varying degrees of intensity. Yet your eye-brain system interprets the frequencies which strike your eye and the shirt is decoded by your brain as being purple. • The subject of color perception can be simplified if we think in terms of primary colors of light. We have already learned that white is not a color at all, but rather the presence of all the frequencies of visible light. When we speak of white light, we are referring to ROYGBIV - the presence of the entire spectrum of visible light. But combining the range of frequencies in the visible light spectrum is not the only means of producing white light. White light can also be produced by combining only three distinct frequencies of light, provided that they

18. are widely separated on the visible light spectrum. Any three colors (or frequencies) of light which produce white light when combined with the correct intensity are called primary colors of light. There are a variety of sets of primary colors. The most common set of primary colors is red (R), green (G) and blue (B). When red, green and blue light are mixed or added together with the proper intensity, white (W) light is obtained. This is often represented by the equation below: R + G + B = W • In fact, the mixing together (or addition) of two or three of these three primary colors of light with varying degrees of intensity can produce a wide range of other colors. For this reason, many television sets and computer monitors produce the range of colors on the monitor by the use of of red, green and blue light-emitting phosphors. • The addition of the primary colors of light can be demonstrated using a light box. The light box illuminates a screen with the three primary colors - red (R), green (G) and blue (B). The lights are often the shape of circles. The result of adding two primary colors of light is easily seen by viewing the overlap of the two or more circles of

19. primary light. The different combinations of colors produced by red, green and blue are shown in the graphic below. (CAUTION: Because of the way that different monitors and different web browsers render the colors on the computer monitor, there may be slight variations from the intended colors.) • These demonstrations with the color box illustrate that red light and green light add together to produce yellow (Y) light. Red light and blue light add together to produce magenta (M) light. Green light and blue light add together to produce cyan (C) light. And finally, red light and green light and blue light add together to produce white light. This is sometimes demonstrated by the following color equations and graphic: R + G = Y R + B = M G + B = C

20. Yellow (Y), magenta (M) and cyan (C) are sometimes referred to as secondary colors of light since they are produced by the addition of equal intensities of two primary colors of light. The addition of these three primary colors of light with varying degrees of intensity will result in the countless other colors which we are familiar (or unfamiliar) with. Newton’s Color Wheel Prism spectrum is a straight line, so why did Isaac Newton describe color using a circular wheel?

21. Adding Color Lights Stream of red & green photons looks same as yellow photons (metamerism) YELLOW Eye to Brain or Notice overlap of red, green, & blue is seen as white light Theatrical lighting

22. 28.6 Complementary Colors • Any two colors of light which when mixed together in equal intensities produce white are said to be complementary colors of each other. The complementary color of red light is cyan light. This is reasonable since cyan light is the combination of blue and green light; and blue and green light when added to red light will produce white light. Thus, red light and cyan light (blue + green) represent a pair of complementary colors; they add together to produce white light. This is illustrated in the equation below: R + C = R + (B + G) = White • Each primary color of light has a secondary color of light as its complement. The three pairs of complementary colors are listed below. The graphic at the right is extremely helpful in identifying complementary colors. Complementary colors are always located directly across from each other on the graphic. Note that cyan is located across from red, magenta across from green, and yellow across from blue.

23. Complementary Colors of Light Red and Cyan Green and Magenta Blue and Yellow • The production of various colors of light by the mixing of the three primary colors of light is known as color addition. The color addition principles discussed on this page can be used to make predictions of the colors which would result when different colored lights are mixed.

24. Additive Complements LARRY After-image of red is cyan because Larry gets tired so when white light excites all three Stooges, Moe & Curly stronger than Larry. R CURLY MOE C Cyan = White - Red

25. 28.7 Mixing Colored Pigments Color Subtraction • The previous lesson focused on the principles of color addition. These principles govern the perceived color resulting from the mixing of different colors of light. Principles of color addition have important applications to color television, color computer monitors and on-stage lighting at the theaters. Each of these applications involve the mixing or addition of colors of light to produce a desired appearance. Our understanding of color perception would not be complete without an understanding of the principles of color subtraction. In this part of the chapter, we will learn how materials which have been permeated by specific pigments will selectively absorb specific frequencies of light in order to produce a desired appearance. • We have already learned that materials contain atoms which are capable of selectively absorbing one or more frequencies of light. Consider a shirt made of a material which is capable of absorbing blue light. Such a material will absorb blue light (if blue light shines

26. upon it) and reflect the other frequencies of the visible spectrum. What appearance will such a shirt have if illuminated with white light and how can we account for its appearance? To answer this question (and any other similar question), we will rely on our understanding of the three primary colors of light (red, green and blue) and the three secondary colors of light (magenta, yellow and cyan) • To begin, consider white light to consist of the three primary colors of light - red, green and blue. If white light is shining on a shirt, then red, green and blue light are shining on the shirt. If the shirt absorbs blue light, then only red and green light will be reflected from the shirt. So while red, green and blue light shine upon the shirt, only red and green light will reflect from it. Red and green light striking your eye always give the appearance of yellow; for this reason, the shirt will appear yellow. This discussion illustrates the process of color subtraction. In this process, the ultimate color appearance of an object is determined by beginning with a single color or mixture of colors and identifying which color or colors of light are subtracted

27. from the original set. The process is depicted visually by diagram at the right. Furthermore, the process is depicted in terms of an equation in the space below. W - B = (R + G + B) - B = R + G = Y • Now suppose that cyan light is shining on the same shirt - a shirt made of a material which is capable of absorbing blue light. What appearance will such a shirt have if illuminated with cyan light and how can we account for its appearance? To answer this question, the process of color subtraction will be applied once more. In this situation, we begin with only blue and green primary colors of light (recall that cyan light consists of blue and green light). From this mixture, we must subtract blue light. After the subtractive process, only green light remains. Thus, the shirt will appear green in the presence of cyan light. Observe the representation of this by the diagram at the right and the equation below. C - B = (G + B) - B = G

28. From these two examples, we can conclude that a shirt which looks yellow when white light shines upon it will look green when cyan light shines upon it. This confuses many students of physics, especially those who still believe that the color of a shirt is in the shirt itself. This is the misconception which was targeted earlier in the chapter as we discussed how visible light interacts with matter to produce color. In that part of the chapter, it was emphasized that the color of an object does not reside in the object itself. Rather, the color is in the light which shines upon the object and which ultimately becomes reflected or transmitted to our eyes. Extending this conception of color to the above two scenarios, we would reason that the shirt appears yellow if there is some red and green light shining upon it. Yellow light is a combination of red and green light. A shirt appears yellow if it reflects red and green light to our eyes. In order to reflect red and green light, these two primary colors of light must be present in the incident light.

29. 28.8 Why Sky is Blue A clear cloudless day-time sky is blue because molecules in the air scatter blue light from the sun more than they scatter red light.  When we look towards the sun at sunset, we see red and orange colors because the blue light has been scattered out and away from the line of sight.

30. The white light from the sun is a mixture of all colors of the rainbow.  This was demonstrated by Isaac Newton, who used a prism to separate the different colors and so form a spectrum.  The colors of light are distinguished by their different wavelengths.  The visible part of the spectrum ranges from red light with a wavelength of about 720 nm, to violet with a wavelength of about 380 nm, with orange, yellow, green, blue and indigo between.  The three different types of color receptors in the retina of the human eye respond most strongly to red, green and blue wavelengths, giving us our color vision. Tyndall Effect • The first steps towards correctly explaining the color of the sky were taken by John Tyndall in 1859.  He discovered that when light passes through a clear fluid holding small particles in suspension, the shorter blue wavelengths are scattered more strongly than the red.  This can be demonstrated by shining a beam of white light through a tank of water with a little milk or soap mixed in.  From the side, the beam can be seen by the blue light it scatters; but the light seen directly from the end is reddened after it has passed through the tank.  The scattered light

31. can also be shown to be polarised using a filter of polarised light, just as the sky appears a deeper blue through polaroid sun glasses. • This is most correctly called the Tyndall effect, but it is more commonly known to physicists as Rayleigh scattering--after Lord Rayleigh, who studied it in more detail a few years later.  He showed that the amount of light scattered is inversely proportional to the fourth power of wavelength for sufficiently small particles.  It follows that blue light is scattered more than red light by a factor of (700/400)4 ~= 10. Dust or Molecules? • Tyndall and Rayleigh thought that the blue color of the sky must be due to small particles of dust and droplets of water vapor in the atmosphere.  Even today, people sometimes incorrectly say that this is the case.  Later scientists realized that if this were true, there would be more variation of sky color with humidity or haze conditions than was actually observed, so they supposed correctly that the molecules of oxygen and nitrogen in the air are sufficient to account for the scattering.  The case was finally settled by Einstein in 1911, who calculated the detailed formula for the scattering of light from molecules; and this was found to be in agreement with

32. experiment.  He was even able to use the calculation as a further verification of Avogadro's number when compared with observation.  The molecules are able to scatter light because the electromagnetic field of the light waves induces electric dipole moments in the molecules. Note that the blue of the sky is more saturated when you look further from the sun. The almost white scattering near the sun can be attributed to Mie scattering, which is not very wavelength dependent. Clouds in contrast to the blue sky appear white to achromatic gray. The water droplets that make up the cloud are much larger than the molecules of the air and the scattering from them is almost independent of wavelength in the visible range.

33. 28.9 Why sunsets Are Red • As the path which sunlight takes through our atmosphere increases in length, ROYGBIV encounters more and more atmospheric particles. This results in the scattering of greater and greater amounts of yellow light. During sunset hours, the light passing through our atmosphere to our eyes tends to be most concentrated with red and orange frequencies of light. For this reason, the sunsets have a reddish-orange hue. The affect of a red sunset becomes more pronounced if the atmosphere contains more and more particles. The presence of sulfur aerosols (emitted as an industrial pollutant and by volcanic activity) in our atmosphere contributes to some magnificent sunsets (and some very serious environmental problems). One More Reason Why Physics is Better Than Drugs Photograph of Maui sunset by Becky Henderson

34.  Red, orange, yelloware transmitted more readily through the atmosphere Light of lower frequencies is transmitted while light of higher frequencies are scattered

35. 28.10 Why Water is Greenish-Blue • Why Water is Greenish Blue:  Red light subtracted from white light produces blue-green light (cyan). • When viewed from space, one of earth’s most commanding features is the blueness of its vast oceans and lakes. Small amounts of water do not portend the color of these large bodies of water; when pure drinking water is examined in a glass, it appears clear and colorless. A larger volume of water is required to reveal the azure color. • Water’s hue depends on a number of factors. On bright, clear days, the blue color associated with a lake or ocean may be attributed, in large part, to the reflection of the sky by the water. But even the water at the deep end of indoor swimming pools seems to have a blue-green color. Why? • When light penetrates water, it experiences both absorption and Rayleigh scattering. Water molecules are small enough to scatter shorter wavelengths, giving water its blue-green color.

36. The amount of long-wavelength absorption is a function of depth; the deeper the water, the more red light is absorbed. At a depth of 15 m, the intensity of red light drops to 25 percent of its incident value and fall to zero beyond a depth of 30 m. Any object viewed at this depth is seen in a blue-green ambient light. For this reason, red denizens of the sea, such as lobsters and crabs, appear black to divers not carrying a lamp. The City of San Diego Web site Green water in the Potomac River caused by an algae bloom A lost heron hitching a ride at sea.

37. 28.11 The Atomic Color Code Atomic Spectra • In physics, emission is the process by which energy in the form of a photon is released by a particle, for example, by an atom whose electrons make a transition between two electronic energy levels. The emitted energy is in the form of a photon with a specific frequency. The emittance of an object quantifies how much light is emitted by it. This may be related to other properties of the object through the Stefan–Boltzmann law. For most substances, the amount of emission varies with the temperature and the spectroscopic composition of the object, leading to the appearance of color temperature and emission lines. Precise measurements at many wavelengths allow the identification of a substance via emission spectroscopy. Emission spectrum of Hydrogen

38. Every element has its own specific glow • The light from the elements can be analyzed by a spectroscope • It is composed of thin slits, lenses, and a prism • It displays the spectrum of light • Line spectrum- images of the slit through which the light passes • Emission spectroscopyis a spectroscopic technique which examines the wavelengths of photons emitted by atoms or molecules during their transition from an excited state to a lower energy state. Each element emits a characteristic set of discrete wavelengths according to its electronic structure, by observing these wavelengths the elemental composition of the sample can be determined. Emission spectroscopy developed in the late 19th century and efforts in theoretical explanation of atomic emission spectra eventually led to quantum mechanics. Emission spectrum of Iron

39. 28.11The Atomic Color Code—Atomic Spectra A fairly pure spectrum is produced by passing white light through a thin slit, two lenses, and a prism.

40. Spectroscope

41. 28.11The Atomic Color Code—Atomic Spectra A spectroscope separates light into its constituent frequencies. Light illuminates the thin slit at the left, and then it is focused by lenses onto either a diffraction grating (shown) or a prism on the rotating table in the middle.

42. 28.11The Atomic Color Code—Atomic Spectra Light is emitted by excited atoms.

43. 28.11The Atomic Color Code—Atomic Spectra • The different electron orbits in an atom are like steps in energy levels.

44. 28.11The Atomic Color Code—Atomic Spectra • The different electron orbits in an atom are like steps in energy levels. • When an electron is raised to a higher level, the atom is excited.

45. 28.11The Atomic Color Code—Atomic Spectra • The different electron orbits in an atom are like steps in energy levels. • When an electron is raised to a higher level, the atom is excited. • When the electron returns to its original level, it releases energy in the form of light.

46. 28.11The Atomic Color Code—Atomic Spectra Relating Frequency and Energy The frequency of the emitted photon, or its color, is directly proportional to the energy transition of the electron. f ~ E A photon carries an amount of energy that corresponds to its frequency. Red light from neon gas, for example, carries a certain amount of energy. A photon of twice the frequency has twice as much energy and is found in the ultraviolet part of the spectrum.

47. Solar Spectrum

48. Extra

49. Mixing Colored Pigments Only four colors of ink are used to print color illustrations and photographs—magenta, yellow, cyan, and black.

50. Maxwell Color Disk Disk painted half red, half blue looks magenta when rapidly spinning.