Physics lab assignments
macaulay.cuny.edu
Kent State University
Act III Lab 10
Exploring color addition and subtraction
The idea: You know from the companion course that color is truly in our own brains. Objects don’t have color, and neither does light. Light has only different frequencies or wavelengths, and objects have only traps that catch one frequency or another and scatter the rest. Our eyes and brains are tickled differently by each frequency, and in response our brains paint a particular color in that speck of our visual field. The whole world is a giant paint-by-number kit, and instead of little color codes printed on the canvas, we receive little frequency codes in the colorless light we see.
Only three frequencies of light are necessary to generate the illusion of all others – those that tickle our eyes in ways that we have learned to call particular shades of red, green, and blue. That’s all that televisions and computer monitors can ever generate, but they can make more than 16 million different colors from them!
What you’ll learn: By the end of this lab you will understand 1) how additive and subtractive primary colors are related, 2) how televisions, computer monitors, and stage lights create colors by combining primary colors, and 3) the colors needed to generate any particular composite color.
What you’ll need: Computer with internet connection
What you’ll do: 1) With any browser of your choice, go to
www.michaelbach.de/ot/col_mix
and you will see an interesting and well-designed color simulator.
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Click fullscreen and then at the lower left, click preview, pull down the menu, and click white, giving you total control of the program.
2) First, an easy exercise. Turn any two sliders up all the way, while the third is off. Name on the Report Sheet the color that you see. Repeat the process until you have seen all three combinations of two colors each. Remember that the colors you see are illusions, painted by your brain as your eyes are tickled by two frequencies at once. All you ever see on the screen is red, green, or blue in various intensities.
Use this color wheel for the next part of the lab.
3) Let’s number the color swatches, as the hours on a clock. So, yellow is number 1, violet is number 6, and yellow-green is number 12. For each swatch, adjust the color sliders on the computer so that the window on the screen matches each color as well as possible. You should see an easy pattern! Write on the Report Sheet the slider settings for each color that give the best match. The sliders are not numbered, but you can number then in your imagination from zero at the bottom of each slider up to 5.
4) Next, make “your color,” based on your birth date, as described below.
www.urlnextdoor.com
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Red – take the month number of your birth (for example, 3 for March or 9 for September) and multiply it by 0.41, to scale the month numbers out along the range of intensities. Record that number on the Report Sheet and set the red slider to that number. You will of course have to estimate the slider setting, since we are supposing that it indicates whole numbers starting from zero.
Green – take the date of your birth during the month (for example, the 15 or the 22 ) and multiply it by 0.16. Record that number and set theth nd
green slider to its value, again estimating as well as you can the slider setting that seems closest to the number you calculated.
Blue – take the day of the week on which you were born, with Sunday equal to 1 and Saturday, 7. Multiply that number by 0.71, record it, and set the blue slider to it.
(What? You don’t know what day of the week you were born? Go to
http://www.timeanddate.com/calendar/generate.html
Enter the year and the country in which you were born, and then scroll down the calendar to see which day of the week matches your birth date.)
You will see a color swatch that is unique to that day, one of more than 2500 possibilities. Think of it as your own personal color! Save it and post it on the Report Sheet. If you are using a PC, press PrintScreen to capture the screen to the Clipboard, and use any photo editing program to crop out everything but the color swatch in the black circle. Are you a Mac user? Then press Command-Shift-3 to automatically save a PNG file on your desktop. Use any photo program to crop out everything but the color swatch in the black circle.
Describe the color as well as you can on the Report Sheet.
Please remember that your color, as all of the colors produced in this lab, is an illusion from your eyes and brains. The color that you think you saw never really existed at all; it was simply the averaged signal from your eyes to your brain. Wow!
5) Now experiment a bit with color subtraction, as well. Set the spotlights to produce each of the colors indicated in the table on the
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Report Sheet, and then insert the color filters by moving each slider all the way up. Record each result on the Report Sheet. Yes, there are many entries, but they go quickly, and the resulting table will be a great help for the homework and quiz in the lecture course!
6) Want more practice? Find a Post-it note, or an index card and a piece of tape, and use it to cover the right side of the screen, flush with the yellow filter, as shown on the next page, so that it hides the final color.
Post-it note or index card here to cover the final color swatch.
7) Click auto run and watch as the spotlights and filters change randomly. Whenever you feel like it, click stop, then consider the color of the light coming in to the set of filters and what each filter is removing. Make your best guess of the final color, then lift the Post-it note or index card to peek and see if you were correct. You don’t need to record any of these results; this is just a suggestion for more practice.
I hope that you will never think of color in quite the same way again!
Web extension
All television sets and computer monitors can only produce red, green, and blue – until now. Why did Sharp add a fourth color, yellow, on their Quattro line of flat-screen televisions?
Be sure to include the basic web address where you found your answer.
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