IV. Visible Light & Color
A. Visible light is the only electromagnetic energy that the human eye can see
1. White visible light is composed of a group of electromagnetic wavelengths that make up the colors we see 2. Each color within the visible light spectrum has it's own individual wavelength and frequency. 3. All of the color seen by the human eye is either transmitted or reflected light consisting of a mixture of the 3 primary colors: red, green, and blue. B. Primary Additive Colors of Light
1. The primary additive colors of light are red, green & blue 2. When red, green & blue light are mixed in equal proportions (added together), the 3 primary colors yield white light 3. When you mix any 2 primary additive colors, you make a primary subtractive color a. Green + Red = Yellow b. Green + Blue = Cyan c. Blue + Red = Magenta C. Primary Subtractive Colors of colorant (paint or pigment)
1. The primary subtractive colors are magenta, cyan and yellow 2. When mixed in equal proportions (added together), the 3 primary subtractive colors produce black 3. When you mix any 2 primary subtractive color paints, you make a primary additive color |
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2 ways to mix colors:
Mixing colored lights & mixing colored paints
The primary colors of light are also known as the additive colors, because, when you add these three colors of light (red, green and blue) your brain perceives white light. The primary colors of paints, however, are known as subtractive colors. When you mix the 3 subtractive color paints (cyan, magenta and yellow), you get black.
Mixing light (additive)
Mixing color light is called additive color mixing, because adding light from two or more sources produces the mixed color. The illumination from two or more colored light sources when added together will give brighter illumination than any of the lights by themselves. Click here for a good explanation.
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Mixing paints (subtractive)
Click on the picture to learn how the primary colors of paint combine to form black.
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Why do objects appear to have color?
D. Objects appear a certain color because they absorb (or subtract) all the visible light colors except the color that is reflected back to your eye
1. A red apple absorbs all wavelengths of light except red, which it reflects 2. A blue ball absorbs all wavelengths of light except blue, which it reflects 3. A yellow filter absorbs all wavelengths of light, except yellow, which it transmits. |
E. Explaining why a red apple looks black under green light
- In white light, a red apple absorbs all the colors (including green) of the spectrum except red, which it reflects.
- A green transparent filter absorbs (or subtracts) all the colors of the spectrum except green, which is transmitted through the filter (and comes through as green light).
- When the green light shines on a red apple, the red apple absorbs the green light.
- Since there is no red light shining on the apple, there is no red light to reflect, so the apple appears black.
How eyes see color (simple version)
At the back of your eye, there are rods and cones that sense the amount or intensity of light. You have 3 different types of cones in your eye which sense red, blue and green light. When red, blue and green light are mixed (added) together in different amounts, you will perceive different colors (see below). But it's a bit more complex than this....see the box on the right.
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How eyes see color (more complex version!)
Remember that I told you that you do not see me. Rather you see light that is reflecting off of me. Similarly, objects do not have color. Rather, the atoms or molecules in the object reflect, transmit or absorb light of different wavelengths that we then perceive as color. Our cones detect a range of wavelengths in the visible light spectrum. Each type of cone detects within a certain range - blue in the blue to cyan range, green in the cyan to green to yellow range and red in the yellow to orange to red range. Indeed, we can't tell if the yellow we see is of the yellow wavelength or it's because we are seeing red and green wavelengths of light added together. Either way, it appears yellow to us. How we perceive color results from a combination of which wavelengths our eyes are capable of detecting (physiological) AND how our brain processes this (psychological).
If you wish to see a much more detailed explanation, go to this PhysicsClassroom webpage. |
Why red and blue paint don't really make purple...
Go ahead and try it! You'll probably get a muddy purple. You can also play with the Phun with Crayola Crayons to see what color you would get by mixing two colors. This article on WikiAnswers explains, but perhaps you should try to figure it out from what you've learned so far.
There is one part of his answer that is slightly incorrect. Pigments are indeed "imperfect", in that they often absorb more than one wavelength of light. These are called compound pigments. But it is also possible to have a "perfect" or true pigment that absorbs only a single wavelength of light. That's why, with our light boxes, cards and filters, we had to distinguish what our eyes were perceiving from what we should expect if the filters and cards were "true" or perfect pigments.
There is one part of his answer that is slightly incorrect. Pigments are indeed "imperfect", in that they often absorb more than one wavelength of light. These are called compound pigments. But it is also possible to have a "perfect" or true pigment that absorbs only a single wavelength of light. That's why, with our light boxes, cards and filters, we had to distinguish what our eyes were perceiving from what we should expect if the filters and cards were "true" or perfect pigments.
Mix and SUBTRACT your own PIGMENTS!
In this interactive from the PhysicsClassroom, you can play with subtracting components of white light by mixing Crayola Crayon colors. "It's probably been a long time since you had a chance to play with those old Crayola crayons. It's time to get that box out now! What color do you get when you mix two 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, brown, crimson, periwinkle, and more. Then click the Mix 'Em button to find out the result."
Mix and ADD your own LIGHT!
In the interactive below (taken from the PhysicsClassroom), you can add varying amounts of red, green or blue light to produce the different colors you perceive. "On this page we've discussed adding red, green and blue light in equal intensities. What happens if they are added in unequal intensities? For instance, suppose you are on the stage lighting team for your school's theatre. Your task is to control the red, green and blue stage lights to produce various color effects for the upcoming show. Use the Color Addition widget below to adjust the strength of the red, green and blue lights relative to full strength. A 1.00 indicates that the light is on at full strength; a 0.00 means the light is off. (All numbers should range from 0.00 to 1.00.) Once adjusted, click the Mix 'Em Up button to find out the result of mixing red, green, and blue components at various strengths."