420 ENERGY MAY BE PROPAGATED BY VIBRATIONS 15. Why do colors in nature harmonize? 16. Give some general rules for the use of colors. 17. How may sharp contrasts be obtained by the use of colors? 18. What are primary and secondary colors? 19. What colors will harmonize with yellow? With green? 20. Why does a mixture of paints generally produce a gray color? 21. Explain color blindness. 22. List all the methods for the production of color that you can think of. 23. Differentiate between additive and subtractive primary colors. 24. Differentiate between primary colors and complementary colors. 25. Explain the colors of a sunset. 26. Why does the ocean water seem to be so blue? 27. Why does the blue color of the ocean disappear on a cloudy day? 28. Explain the beautiful green water of the Hawaiian shore line. 29. Discuss the color harmonies in nature at each season of the year. 30. Explain why it is difficult to match colors in artificial light. 31. What color is produced when yellow and blue pigments are mixed? When yellow and blue lights are mixed? 32. Work out a hypothesis to explain the following data: Unsterilized milk placed in colored bottles and exposed to the sunlight remained sweet the longest when placed in red bottles and soured the quickest in bottles having colors toward the violet end of the spectrum.
UNIT VI SECTION 4 THE INVENTION OF POLARIZING-SCREENS PUT POLARIZED LIGHT TO WORK Introduction. The first recorded discovery of polarized light was made by Bartholinus of Denmark in 1670, who observed that an object viewed through Iceland spar appeared double. Bartholinus reasoned that the crystal separated the light into two beams. The later discovery of the physicist, Nicol, that Iceland spar crystals could be cut at certain angles and cemented together with Canada balsam to form prisms that would eliminate one of the above two beams of light made possible the polarimeter and polarizing microscope and thus harnessed polarized light for laboratory use. In 1852 W. D. Herapath discovered that quinine iodosulfate crystals would polarize a beam of light, but he did not succeed in mounting them in a satisfactory manner. It was the discovery of Edwin H. Land, publicly announced in 1934, that these crystals could be suspended in a plastic cellulose acetate film and aligned by stretching the film, which made possible the polarization of light economically on a large scale. Polarized Light Is Light Vibrating in One Plane Only. Light waves are transverse waves, that is, they vibrate at right angles to the direction that the light is traveling, just as waves set up in a pool of water by a stone thrown into it travel at right angles to the up-anddown direction in which the water surface moves. Ordinary light is a mixture of transverse vibrations in all possible directions. When light is vibrating in one plane only, it is said to be plane polarized, or just polarized. Light May Be Polarized by Double Refraction in Crystals. Light may be polarized by passing it through crystals of certain substances such as Iceland spar or tourmaline. When a piece of Iceland spar is placed over a dot, two dots are seen through the crystal because 421