Activities for Tuesday, November 20.
| Primary and Secondary Colors |
[1] Get in your mountain groups, and grab an optics kit.
[2] Separate out your Yellow, Magenta, and Blue-Green Filters. These three colors are called secondary colors.
[3] As you may know, white light can be thought of as consisting of three primary colors: Red, Blue, and Green. Each of your secondary color filters allows two of these primary colors to pass through, while blocking the third. Your first task is to determine which two primary colors each filter lets pass.
[4] To begin, look through both the blue-green and yellow filters simultaneously. What color do you see? Continue with the other combinations of secondary filters and fill in the table below.
Filter Combination
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Primary Color Passed
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Primary Colors Filtered Out
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Blue-Green & Yellow
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Blue-Green & Magenta
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Magenta & Yellow
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[5] Now use the information from the table above to fill in the table below
Filter
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Primary Colors Passed
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Primary Color Filtered Out
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Blue-Green
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Magenta
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Yellow
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[6] Use the information from your two tables to predict what you will see when you look through the Blue-Green, Yellow, and Magenta filters simultaneously. Check your prediction.
[7] Now suppose that a spot of red light and a spot of green light both shine on a piece of white paper. If the spots are moved so that they overlap, predict the color that you will see in the region of overlap. Use the information in your tables above to help you out.
[8] Take your Primary Red and Primary Green filters, and tape each over the top of one of your flashlights. Shine them on a piece of white paper, allow them to overlap, and check your prediciton.
Polarization of Light
[9] You have two polarizing discs in your kit. Take one of the discs and look at the room lights through it. Does the disc affect the intensity (brightness) of the light?
[10] Rotate the disc. Do you notice any difference in intensity as you rotate it?
[11] Now look at a room light through both polarizers simultaneously. Rotate one of them. Do you notice any changes in the intensity of the light as you do this? Is it possible to block out all of the light?
[12] Notice that there are little "tick" marks on the front of the polarizers. How are the tick marks of the two polarizers oriented when (a) the light passing through them is at its brightest, and (b) when there is no light passing through them.
The phenomenon that you are witnessing is called polarization. Simply put, light comes in two different orientations, which we may call vertical and horizontal. Your eyes are not sensitive to the polarization of light, so you don't notice any difference when you tilt your head. The polarizers (and your polarized sunglasses, if you own any) are sensitive to the different orientations of light.
[13] Calcite is a crystal that splits light up into its two different polarizations. Make a mark on a piece of white paper (a small dot or letter), and place your calcite crystal on top of it. What do you see? Rotate the crystal while observing the mark. Describe in detail what you see.
[14] Place the crystal on top of the mark, and look at it through one of your polarizers. Describe what you see as you rotate the polarizer.
[15] Materials under stress tend to polarize light. Because different wavelengths of light (and hence different colors) are affected differently by this process, colorful patterns can be produced. Place your mica crystal disc (the thicker one) between your two polarizers, and look through this arrangement at a light source. Observe what happens as you rotate one of the polarizers.
[16] Repeat Step [15] with the thinner benzoic acid crystal.
[17] If a member of your group has glasses, repeat this procedure with one lens of the glasses held between the polarizers. The color patterns will show the stresses and strains in the lens! This is actually a very useful technique in engineering for finding likely failure points.