Chapter Twenty Eight Notes: COLOR . As discussed in the last chapter, electromag

Chapter Twenty Eight Notes: COLOR . As discussed in the last chapter, electromag www.phwiki.com

Chapter Twenty Eight Notes: COLOR . As discussed in the last chapter, electromag

Wolaner, Robin, Founder has reference to this Academic Journal, PHwiki organized this Journal Chapter Twenty Eight Notes: COLOR . 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 in addition to as such, they consist of both an electric in addition to 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 in addition to its various regions. The longer wavelength, lower frequency regions are located on the far left of the spectrum in addition to the shorter wavelength, higher frequency regions are on the far right. Two very narrow regions within the spectrum are the visible light region in addition to the X-ray region. Within the visible range, the different frequencies/wavelength determine what color we see! The focus of chapter 28 will be upon the visible light region – the very narrow b in addition to of wavelengths located to the right of the infrared region in addition to 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 b in addition to . Since this narrow b in addition to 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 b in addition to 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

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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 in addition to 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; in addition to 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), in addition to 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 in addition to the violet wavelengths of light are the shorter wavelengths. Between red in addition to violet, there is a continuous range or spectrum of wavelengths. The visible light spectrum is shown in the diagram below. 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 in addition to 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, in addition to the “four letter words” — words that go back a thous in addition to years. Some of the names as long as colors are loan words from French — orange in addition to beige, since the “zh” sound doesn’t exist in pure English (garage is a very french word) in addition to violet in addition to purple, since they just sound too fancy to be anglo-saxon.

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 in addition to the nature of the atoms of the object. In this section we will discuss how in addition to why light of certain frequencies can be selectively absorbed, reflected or transmitted. Atoms in addition to molecules contain electrons. It is often useful to think of these electrons as being attached to the atoms by springs. The electrons in addition to their attached springs have a tendency to vibrate at specific frequencies. Similar to a tuning as long as k 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 in addition to trans as long as m 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 as long as m 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 in addition to molecules have different natural frequencies of vibration, they will selectively absorb different frequencies of visible light. 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 as long as 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 as long as short periods of time in addition to 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 in addition to 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 in addition to 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 in addition to a specific frequency becomes absorbed, that frequency of light will never make it to our eyes. Any visible light which strikes the object in addition to 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 in addition to 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 as long as 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 as long as 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 in addition to violet. In Example B, the pigment in the sheet of paper is capable of absorbing orange, yellow, green, blue, indigo in addition to violet. In each case, whatever color is not absorbed is reflected. Example A: Green will be reflected in addition to so the paper appears green to an observer. Example B: Red will be reflected in addition to so the paper appears red to an observer. Example A: Green will be transmitted in addition to so the object appears green to an observer. Example B: Both green in addition to blue will be transmitted in addition to so the object appears greenish-blue to an observer. 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 as long as 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 in addition to 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 in addition to 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 in addition to 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

specific color. When visible light strikes an object in addition to a specific frequency becomes absorbed, that frequency of light will never make it to our eyes. Any visible light which strikes the object in addition to 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 in addition to 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 as long as 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 as long as the frequency associated with blue light, then the object will appear blue in the presence of ROYGBIV. e.g. R O Y G B I V blue filter We See Mostly Blue The sun emits light waves with a range of frequencies. Some of these frequencies fall within the visible light spectrum in addition to 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 in addition to illuminates both our outdoor world in addition to 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 as long as green) in addition to 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 in addition to no light is reflected to our eyes. The asphalt street thus appears black

(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 in addition to 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 in addition to 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. Color Addition Color perception, like sound perception, is a complex subject involving the disciplines of psychology, physiology, biology, chemistry in addition to physics. When you look at an object in addition to perceive a distinct color, you are not necessarily seeing a single frequency of light. Consider as long as instance that you are looking at a shirt in addition to 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 in addition to 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 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) in addition to blue (B). When red, green in addition to 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 in addition to computer monitors produce the range of colors on the monitor by the use of of red, green in addition to 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) in addition to 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

primary light. The different combinations of colors produced by red, green in addition to blue are shown in the graphic below. (CAUTION: Because of the way that different monitors in addition to 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 in addition to green light add together to produce yellow (Y) light. Red light in addition to blue light add together to produce magenta (M) light. Green light in addition to blue light add together to produce cyan (C) light. And finally, red light in addition to green light in addition to blue light add together to produce white light. This is sometimes demonstrated by the following color equations in addition to graphic: R + G = Y R + B = M G + B = C Yellow (Y), magenta (M) in addition to 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 Stream of red & green photons looks same as yellow photons (metamerism) Theatrical lighting or YELLOW Eye to Brain Notice overlap of red, green, & blue is seen as white light

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 in addition to green light; in addition to blue in addition to green light when added to red light will produce white light. Thus, red light in addition to 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, in addition to yellow across from blue. Complementary Colors of Light Red in addition to Cyan Green in addition to Magenta Blue in addition to 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. LARRY MOE CURLY After-image of red is cyan because Larry gets tired so when white light excites all three Stooges, Moe & Curly stronger than Larry. R C Cyan = White – Red

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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 in addition to 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 underst in addition to ing of color perception would not be complete without an underst in addition to ing 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 upon it) in addition to reflect the other frequencies of the visible spectrum. What appearance will such a shirt have if illuminated with white light in addition to how can we account as long as its appearance To answer this question ( in addition to any other similar question), we will rely on our underst in addition to ing of the three primary colors of light (red, green in addition to blue) in addition to the three secondary colors of light (magenta, yellow in addition to cyan) To begin, consider white light to consist of the three primary colors of light – red, green in addition to blue. If white light is shining on a shirt, then red, green in addition to blue light are shining on the shirt. If the shirt absorbs blue light, then only red in addition to green light will be reflected from the shirt. So while red, green in addition to blue light shine upon the shirt, only red in addition to green light will reflect from it. Red in addition to green light striking your eye always give the appearance of yellow; as long as 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 in addition to identifying which color or colors of light are subtracted 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 in addition to how can we account as long as its appearance To answer this question, the process of color subtraction will be applied once more. In this situation, we begin with only blue in addition to green primary colors of light (recall that cyan light consists of blue in addition to 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 in addition to the equation below. C – B = (G + B) – B = G

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 in addition to 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 in addition to green light shining upon it. Yellow light is a combination of red in addition to green light. A shirt appears yellow if it reflects red in addition to green light to our eyes. In order to reflect red in addition to green light, these two primary colors of light must be present in the incident light. 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 in addition to orange colors because the blue light has been scattered out in addition to away from the line of sight. 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 in addition to so as long as m 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 in addition to indigo between. The three different types of color receptors in the retina of the human eye respond most strongly to red, green in addition to 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

Cyan Stare, unfocused, at the flag as long as 10 seconds then look at white wall Cyan Magenta Yellow

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