What Color Would a White Sheet of Paper Appear to Be if You Shine a Green Light on the Paper?

Colour Subtraction

The previous lesson focused on the principles of color improver. These principles govern the perceived color resulting from the mixing of different colors of calorie-free. Principles of color addition have of import applications to color television, colour computer monitors and on-stage lighting at the theaters. Each of these applications involves the mixing or addition of colors of lite to produce a desired appearance. Our agreement of color perception would not be complete without an understanding of the principles of color subtraction. In this part of Lesson 2, we will acquire how materials that take been permeated past specific pigments will selectively absorb specific frequencies of calorie-free in lodge to produce a desired appearance.

We have already learned that materials contain atoms that are capable of selectively absorbing one or more frequencies of light. Consider a shirt made of a material that is capable of absorbing blue light. Such a textile will absorb blue light (if blue light shines 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 tin we account for its advent? To reply this question (and any other like question), we will rely on our understanding of the three chief colors of light (red, green and bluish) and the three secondary colors of light (magenta, yellowish and cyan).

The Process of Color Subtraction

To begin, consider white light to consist of the three primary colors of light - blood-red, green and bluish. If white low-cal is shining on a shirt, and so red, green and blueish lite is shining on the shirt. If the shirt absorbs blueish lite, and then only blood-red and light-green light will be reflected from the shirt. So while red, green and bluish light polish upon the shirt, only red and green low-cal will reflect from information technology. Blood-red and green light striking your centre always gives the appearance of xanthous; for this reason, the shirt will appear yellow. This word illustrates the procedure of color subtraction . In this process, the ultimate colour appearance of an object is adamant past first with a unmarried colour or mixture of colors and identifying which colour or colors of light are subtracted from the original fix. The process is depicted visually by diagram at the right. Furthermore, the procedure is depicted in terms of an equation in the space below.

Due west - B = (R + G + B) - B = R + M = Y

Now suppose that cyan light is shining on the same shirt - a shirt made of a material that is capable of absorbing blue low-cal. What advent volition 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 practical again. In this situation, we brainstorm with merely blue and green primary colors of lite (recall that cyan low-cal consists of blue and green light). From this mixture, we must subtract blue low-cal. After the subtractive process, but green lite remains. Thus, the shirt will appear green in the presence of cyan light. Discover the representation of this by the diagram at the right and the equation below.

C - B = (Thou + B) - B = G

From these two examples, nosotros can conclude that a shirt that looks yellow when white light shines upon it volition expect green when cyan light shines upon it. This confuses many students of physics, specially those who all the same believe that the colour of a shirt is in the shirt itself. This is the misconception that was targeted earlier in Lesson ii as we discussed how visible light interacts with thing to produce color. In that part of Lesson 2, information technology was emphasized that the color of an object does not reside in the object itself. Rather, the color is in the light that shines upon the object and that ultimately becomes reflected or transmitted to our eyes. Extending this conception of color to the above 2 scenarios, nosotros would reason that the shirt appears yellow if in that location is some red and green light shining upon it. Yellow light is a combination of ruddy and light-green calorie-free. A shirt appears yellow if it reflects red and green light to our eyes. In gild to reflect ruby-red and green lite, these two primary colors of light must be present in the incident low-cal.

Test your understanding of these principles of color subtraction past determining the color appearance of the same shirts if illuminated with other colors of low-cal. Be sure to begin by determining the main color(south) of light that are incident upon the object and then subtracting the captivated colour from the incident color(s).

Complementary Colors and Color Subtraction

In the above examples, the paper captivated blueish light. Paper that absorbs blue light is permeated past a pigment known as a yellow pigment. While most pigments absorb more than a single frequency (and are known as compound pigments), information technology becomes convenient for our discussion to proceed it simple by assuming that a yellowish pigment absorbs a single frequency. A pigment that absorbs a unmarried frequency is known every bit a pure pigment . The following rule will assist in agreement what colors of lite are absorbed by which pigments.

Pigments absorb light. Pure pigments blot a single frequency or color of light. The color of light absorbed past a pigment is merely the complementary colour of that pigment.

Thus, pure blue pigments absorb yellowish low-cal (which can be thought of as a combination of ruby and green calorie-free). Pure yellow pigments absorb blue light. Pure green pigments absorb magenta light (which tin can be thought of as a combination of red and blue light). Pure magenta pigments absorb green calorie-free. Pure red pigments absorb cyan light (which tin can exist thought of as a combination of blue and green low-cal). And finally, pure cyan pigments absorb scarlet lite.

At present lets combine the procedure of colour subtraction with an understanding of complementary colors to determine the colour appearance of various sheets of newspaper when illuminated by diverse lights. We volition investigate three examples.

Example 1

Magenta lite shines on a sail of newspaper containing a yellow pigment. Decide the appearance of the paper.

Magenta lite can exist idea of as consisting of reddish light and blueish calorie-free. A yellowish paint is capable of arresting blueish low-cal. Thus, blue is subtracted from the calorie-free that shines on the paper. This leaves scarlet calorie-free. If the newspaper reflects the red calorie-free, then the paper volition look red.

M - B = (R + B) - B = R

Example 2

Xanthous light shines on a sheet of paper containing a ruddy pigment. Determine the appearance of the paper.

Yellow light can be thought of as consisting of red lite and green light. A red paint is capable of absorbing cyan light. That is, red paper can absorb both green and blue chief colors of calorie-free (call up that cyan light is a mixture of greenish and blueish light). And so reddish and green light smoothen on the paper; and dark-green light is subtracted. (There is no need to subtract blue lite since bluish light is not shining on the paper.) This leaves red light to be reflected. If the paper reflects the red low-cal, then the paper volition look scarlet.

Y - K = (R + Thousand) - Thou = R

Example iii

Yellowish low-cal shines on a canvas of paper containing a blueish pigment. Make up one's mind the advent of the paper.

Yellow light tin be thought of as consisting of crimson light and dark-green lite. A bluish pigment is capable of absorbing yellow light. That is, blue paper tin absorb both red and green primary colors of low-cal (remember that yellow light is a mixture of red and green lite). And then red and green light shine on the newspaper; and both the red and the green calorie-free are subtracted. In that location is no colour left to be reflected to the eye. Subsequently, the paper appears black.

Y - Y = (R + G) - (R + G) = No reflected lite = Black

Flickr Physics Photo

Three transparent protractors are overlaid on top of each other. The protractors are colored cyan, magenta, and yellowish. The iii protractors are illuminated with white light, sometimes referred to equally RGB light. Each protractor absorbs a single primary colour of calorie-free. The cyan protractor absorbs red low-cal. The magenta protractor absorbs green light. The yellow protractor absorbs bluish light. Where two protractors overlap, a single master color of lite shows through. For example, where the cyan and the yellow protractor overlap, the crimson and blue light are captivated and the green light is seem shining through. And where the cyan and the magenta protractor overlap, the red and green light are absorbed and the blue light is seem shining through. Finally, where the magenta and the xanthous protractor overlap, the greenish and blue light are absorbed and the ruby light is seem shining through. This photo illustrates the principles of colour subtraction.

Filters and Color Subtraction

The in a higher place discussion applies to the appearance of opaque materials. The distinction between opaque and transparent materials was made earlier in this lesson. Opaque materials selectively absorb one or more frequencies of calorie-free and reflect what is non absorbed. In contrast to opaque materials, transparent materials selectively blot one or more frequencies of light and transmit what is not captivated. Like opaque materials, transparent materials are permeated by pigments that incorporate atoms that are capable of absorbing lite with a single frequency or even a range of frequencies. Knowing the color(s) of the incident light and the color of light absorbed by the pigment or filter, the process of colour subtraction can be applied to make up one's mind the color appearance of a transparent material. Nosotros will consider iii examples in the space below; the examples are visually depicted in the diagrams below.

In Example A, white light (i.e., a mixture of red, green and blue) shines upon a magenta filter. Magenta absorbs its complementary colour - dark-green. Thus, greenish is subtracted from white lite. That leaves reddish and bluish low-cal to exist transmitted past the filter. For this reason, the filter will appear magenta (recollect that magenta light is a mixture of blood-red and blueish light) when illuminated with white low-cal. This process of color subtraction can be represented by the post-obit equation.

W - 1000 = (R + Thou + B) - G = R + B = Thousand

In Case B, xanthous low-cal (i.east., a mixture of red and green) shines upon the same magenta filter. Magenta absorbs its complementary color - green. Thus, light-green is subtracted from yellow light. That leaves ruby light to be transmitted past the filter. For this reason, the filter will announced red when illuminated with yellow light. This procedure of color subtraction tin can exist represented by the post-obit equation.

Y - G = (R + One thousand) - G = R

In Example C, cyan light (i.eastward., a mixture of blue and greenish) shines upon the same magenta filter. Magenta absorbs its complementary color - green. Thus, green is subtracted from cyan calorie-free. That leaves blue lite to exist transmitted by the filter. For this reason, the filter volition appear blue when illuminated with cyan low-cal. This procedure of color subtraction can be represented past the following equation.

C - G = (B + G) - M = B

The reasoning modeled in the above three examples can be used in any situation, regardless of the color of the incident light and the color of the filter. As yous approach such problems, whether they involve transparent or opaque materials, be sure to remember in terms of principal colors of lite and to apply the logical reasoning steps. Avoid memorizing and avoid shortcuts. If a filter is capable of absorbing a colour of lite that is not present in the mixture of incident light, and so only disregard that color. Since that colour of light is not incident upon the object, it cannot contribute to the colour appearance of the object.

We Would Like to Propose ...

Sometimes it isn't enough to just read near information technology. You lot have to collaborate with it! And that's exactly what you do when you use one of The Physics Classroom's Interactives. We would like to suggest that you combine the reading of this folio with the utilise of our Colored Filters Interactive. The Interactive is found in the Physics Interactives section of our website and allows a learner to explore the affect of filters of varying color upon white low-cal and its private R, G, and B components.

Primary Colors of Paint

A trip to the local paper or film developing company volition reveal these same principles of color subtraction at piece of work. The iii chief colors of pigment used past an artist, colour printer or movie developer are cyan (C), magenta (M), and xanthous (Y). Artists, printers, and film developers do not deal direct with light; rather, they must apply paints or dyes to a white sheet of paper. These paints and dyes must exist capable of absorbing the appropriate components of white light in order to produce the desired affect. Most artists start with a white canvass and apply paints. These paints accept to subtract colors and so that you might see the desired epitome. An artist tin create any color by using varying amounts of these three main colors of paint.

Each primary color of paint absorbs one chief color of low-cal. The color absorbed by a primary colour of paint is the complementary color of that paint. The three colors that are primary to an creative person (magenta, cyan, and xanthous) decrease red, green, and blue individually from an otherwise white canvass of paper. Thus,

Magenta paints absorb green low-cal.

Cyan paints absorb carmine light.

Yellow paints absorb blue light.

Allow's suppose that an artist wishes to use the three main colors of paint in society to produce a movie of the colorful bird shown at the correct. The bird will be painted onto white paper and viewed nether white light. It is hoped that the bird will have green tail feathers, a bluish lower body, a cyan upper body, a red head, a magenta heart patch, a yellow eye and middle feathers, and a black beak. How can the three primary colors of pigment be used to produce such a likeness? And how can we explain the answers in terms of color subtraction?

To produce a dark-green tail, paints must be applied to the tail region in social club to absorb red and blueish light and leave green to exist reflected. Thus, the green tail must exist painted using yellow paint (to blot the blueish) and cyan paint (to absorb the red).

To produce a blue lower body, paints must be practical to the lower torso region in order to absorb crimson and greenish light, leaving blueish calorie-free to be reflected. Thus, the blue lower body must be painted using magenta paint (to absorb the dark-green) and cyan pigment (to absorb the red).

To produce a red head, paints must be applied to the caput region in social club to absorb blue and dark-green light, leaving red calorie-free to be reflected. Thus, the crimson caput must exist painted using magenta pigment (to absorb the green) and yellow paint (to blot the blue).

To produce a cyan upper body, paints must be applied to the upper trunk region in guild to blot red, leaving green and blue calorie-free to be reflected. If green and blue light are reflected from the upper body region, it will announced cyan (recollect that blue and green light combine to class cyan light). Thus, the cyan upper body must exist painted using merely cyan paint (to blot the red).

To produce a magenta eye patch, paints must be practical to the eye patch region in order to absorb green, leaving reddish and blue lite to exist reflected. If cherry-red and blue light is reflected from the eye patch region, information technology volition appear magenta (retrieve that blue and red low-cal combine to form magenta light). Thus, the magenta heart patch must be painted using simply magenta paint (to absorb the green).

To produce a yellowish eye and middle feathers, paints must be applied to the eye and middle feather regions in order to blot blue, leaving red and dark-green light to be reflected. If cherry and dark-green low-cal is reflected from the eye and middle plumage regions, it will appear yellow (retrieve that ruby and light-green light combine to form yellowish light). Thus, the xanthous eye and middle feathers must exist painted using but yellow paint (to absorb the blue).

This information is summarized in the graphic below.

The process of color subtraction is a useful means of predicting the ultimate colour appearance of an object if the colour of the incident light and the pigments are known. By using the complementary color scheme, the colors of light that will be absorbed by a given cloth can be determined. These colors are subtracted from the incident lite colors (if nowadays) and the colors of reflected calorie-free (or transmitted light) can be determined. So the color appearance of the object can be predicted.

Nosotros Would Like to Advise ...

Sometimes it isn't enough to merely read about it. Yous accept to interact with it! And that's exactly what you do when you utilise one of The Physics Classroom's Interactives. Nosotros would like to suggest that yous combine the reading of this folio with the use of our Painting With CMY Interactive. The Interactive is found in the Physics Interactives department of our website and allows a learner to explore the affect of varying colors of primary pigments.

Investigate!

Information technology's probably been a long time since you had a risk to play with those old Crayola crayons. It'southward time to get that box out at present! What color do yous get when you mix ii crayons from the Crayola box? Use the Phun With Crayola Crayons widget to find out. Enter the names of two crayons from the box. (Examples: tan, forest green, yellow, mauve, chocolate-brown, blood-red, periwinkle, and more. Then click the Mix 'Em push button to find out the result.

Flickr Physics Photo

Three colored spotlights - red, green and blue - illuminate a white screen. A hand is placed between the screen and the lights, thus casting overlapping shadows on the screen. From left to correct, you volition observe a yellow, magenta and cyan shadow. The xanthous shadow is produced by the blocking of the bluish low-cal; the red and dark-green lights converge to produce yellow. The magenta shadow is produced by the blocking of the green light; red and blueish lights converge to produce magenta. And finally, the cyan shadow is produced by the blocking of the red lite; blue and green lite converge to produce the cyan. The colors red and blueish are produced where the shadows overlap. The photo demonstrates principles of color subtraction and colour addition.

We Would Like to Advise ...

Sometimes information technology isn't plenty to just read about it. You accept to interact with it! And that's exactly what you do when you utilise one of The Physics Classroom's Interactives. We would similar to advise that yous combine the reading of this folio with the utilize of our Colored Shadows Interactive. The Interactive is found in the Physics Interactives department of our website and allows a learner to explore the arrangment of overlapping shadows produced when diverse combinations of scarlet, green and blue spotlights are projected onto a person.

Check Your Agreement

1. Blue jeans appear blue because the jeans are permeated past a chemic dye. Explain the role of the dye. That is, what does the dye do (blot or reflect) to the various frequencies of white light?

ii. A red shirt looks ruby when visible light ("ROYGBIV") shines upon it. Use your physics agreement to explain this phenomenon.

3. Limited your agreement of complementary colors and the rule of color subtraction past completing the following iii diagrams. White light (red-dark-green-blue) is shown incident on a canvass of paper that is painted with a paint that absorbs i of the principal colors of light. For each diagram, determine the colour of the two reflected rays and decide the color that the newspaper appears.

iv. In the diagrams below, several sheets of paper are illuminated by different chief colors of light (R for red, B for blueish, and G for green). Indicate what principal colors of light volition be reflected and the appearance of the canvas of paper. (Note the similarity betwixt this problem and the above trouble.)

v. Unlike colored light sources shine on different colored sheets of paper. The indicated paper color represents the advent of the paper when viewed in white light. Fill in the table below to evidence the colour of lite that reflects from the paper (i.e., the color observed).

	Color of Calorie-free	Colour of Paper	Color Observed
a.	Scarlet	Xanthous
b.	Red	Magenta
c.	Blue	Blue
d.	Bluish	Cyan
e.	Bluish	Ruddy
f.	Yellow	Red
k.	Yellow	Blue

half dozen. The following diagrams describe various main colors of lite (R for red, B for blue, and Thousand for green) incident upon a colored filter (C for cyan, M for magenta, and Y for yellowish). Determine which primary colors of light volition pass through the filters.

7. Suppose an object is permeated by a mixture of two or more paints and illuminated with white light. Make full in the table below to indicate the color appearance of the object.

Paint One	Paint 2	Color Observed
Cyan	Magenta
Magenta	Yellow
Cyan	Yellow
Cyan, Magenta &	Yellow

8. What primary paint colors (CMY) or combination of pigment colors would you use to paint the boy below? He has pink (magenta) skin, blue jeans, a yellowish sweater, a black baseball cap, red sneakers and aqua-colored socks. Bespeak the primary colors of paint to be used on the diagram below.

Skin: ______________ Jeans: ______________ Sweater: ______________ Cap: ______________ Sneakers: ______________ Socks: ______________

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Source: https://www.physicsclassroom.com/class/light/Lesson-2/Color-Subtraction