Color and Light

Procedure

• Gather these items from your Optics Kit:
  • the prism
  • the three colored filters
  • the spectrometer built in a previous Activity

• Gather these items from your home or workplace:
  • three flashlights
  • a friend
  • a light bulb (40 watts or higher)

• Prism. Hold your prism in the sunlight and rotate it until you see a spectrum of colors. This shows the colors of light (thanks to dispersion -- the change in refractive index with wavelength) in a beam of white light. Next hold a pencil across one of the long faces of the prism. View the pencil through the prism so the ends of the pencil seem to be sticking out of the ends of the prism. Then sketch the paths of light through the prism using an end-on view. Compare with the drawing in the reading. If you have a second prism you can reconstitute white light by placing a second prism against the first. The exit beam is the same white color as the incident beam. No surprise because now the incident and exit prism faces are parallel to each other in the combined prism pair.

• Spectrometer. Look at a bright light bulb through your spectrometer with the slits toward the light bulb. (DON'T LOOK AT THE SUN). You will note the spectrum extends from the blue (~400 nm) to the red (~700 nm). Use the red, blue, and green plastic filters in your optics kit and place each in turn across the entrance slit. Note which colors are transmitted and which are blocked. If you hold the filters directly in front of the light bulb (no intervening spectrometer) you also note the same transmitted colors.

White light can be made from three colored lights

• Computer Monitor. Take the high power magnifier from the Optics Kit (lens B) and observe the colors of the patterns on a computer monitor or TV screen. The dot patterns (arrays of colored circles or rectangles used by different screen manufacturers) are colored red, blue, or green. When lit they produce the full range of colors seen on your monitor. Use the B lens on the Patterns in Nature header to see the transition in color patterns from the "green" Patterns to the "red" Nature.

• Projection TV. Just observe the light sources on a projection system (large screen TV, classroom computer projectors such as Proxima). You will find that the three light sources are red, green, and blue. If you are in a science museum (Arizona Science Center, for example) that features colored shadows, you will also note that the light sources are red, green, and blue.

• Flashlights and Filters. For this project you need to go beyond the items provided in the Optics Kit: 3 flashlights and a friend. Tape the 3 filters (red, blue, and green) on the lens cover of the flashlight and project the three sources (R, B, and G) on a white wall. The overlapping of the three sources produces white (R+G+B=W). If two sources overlap, you obtain other colors:

  		Red + Green = Yellow
  		Green + Blue = Cyan
  		Red + Blue = Magenta
  

• Color Printing. Printers use four plates (four-color printing) when making colored pages for a book or magazine. The four colors are yellow, cyan, magenta, and black. These colors very loosely are called yellow, blue or cyan blue, and red or magenta red. Use the "B" lens in your Optics Kit to look at the color prints in a book or magazine. The colors are produced by dots of yellow, cyan, and magenta.

• Two color white. This is a reminder that white can be produced by two colors as well as three colors and the full spectrum discussed above. Two colors that give white when added together are called complementary colors. Blue and yellow, cyan and red, as well as magenta and green are complementary colors. All 3 pairs can produce white.

Polarized Light

• Gather these items from your Optics Kit:
  • the polarizers (small dark rectangles)
  • the prism
  • a flashlight

• Look at a light source with both polarizers held together. Rotate one and notice the transmitted light intensity decrease. Maximum decrease is when the polarizers are at 90° -- crossed polarizers.

• Look through one polarizer at the image of the sun, reflected from a glass surface (a face from the prism in your Optics Kit is sufficient). Rotate the polarizer and note the intensity change.

• Look through one polarizer at a flashlight and note that the flashlight source is not polarized (no change in intensity as the polarizer is rotated). Then look at the flashlight beam reflected off a glass surface and rotate the polarizer. A portion of the reflected light is polarized.

• Most light sources are unpolarized except for
  • sunlight reflected off water (as above).

  • the blue sky (hold two polarizers side by side at 90° to each other. Rotate the pair and compare the variations in transmitted intensity.)

  • lasers. If you have a laser pointer, aim the laser at a wall or paper, place the polarizer over the laser head and rotate it. In one orientation the beam intensity almost vanishes.

  • Can you find any other examples?

• Look through two crossed polarizers with a stretchable piece of plastic between them. Pull the plastic holding one end. You will see an increase in transmission and even colors because the plane of polarization rotates as it passes through the plastic.

• You can make your own polarizers with 5 or so microscope slides and some heavy-weight, dark-colored (to minimize reflection) construction paper. Cut a strip of paper about 4" long and as wide as your microscope slide. Tape this at right angles to the sheet of construction paper as it will be the shelf for the slides. Put glue on the long bottom and one end of the slide and place it at a slant so that light incident will be at the Brewster angle.
  

  The Brewster angle is 56° for light in air incident on glass. We can approximate this by 60° and slant the slides so that the incident light is 60° from the normal. Put 5 or 6 slides in a slanted position and cut the top and side covers from the construction paper and you have made a polarizing filter. Use one of the plastic polarizers from the optics kit to test your Brewster angle polarizer.

  
  

To read about light sources go to the Sources of Light Readings

To read about color theory go to the Composition of Color Readings

To read more about polarized light and polarizers go to the Polarized Light Readings