Friday, April 12, 2013 Yesterday, we did a lab activity. Today's objective: Review the concepts of light, color, and reflection/refraction. Many of you need to catch up on some assignments, so you will be given some class time to complete these.

Review: Reflection vs. Refraction You should be able to describe which direction the light rays are traveling, points of reflection, points of refraction, and the path that the light ultimately takes as it travels through a medium such as a prism.

You must be able to identify and label the components of the ray diagram: the normal, incident ray, reflected ray, angle of incidence, angle of reflection, direction the light energy is traveling, etc.

Drawing ray diagrams

Refraction Light slows down when it enters a denser medium (due to "delays" with absorption and re-emission of the light energy). Because it changes speed, it will also change its direction or path. Depending on the frequencies of the light and the type of medium it enters, the ray will bend a certain angle.

Ch. 27 - Light - Review Questions #7-23, 32 assigned on 3-25-13, 5 points possible 7. Is the color spectrum simply a small segment of the electromagnetic spectrum? Defend your answer? - Yes; the spectrum is also made up of all the other EM waves: radio, IR, UV, X, gamma 8. How do the frequencies of infrared, visible, and ultraviolet light compare? - Frequencies increase from IR to visible to UV.

9. How does the role of inertia relate to the rate at which electric charges can be forced into vibration? - Low inertia permits rapid vibration. 10. Different bells and tuning forks have their own natural vibrations and frequencies, and emit their own unique tones when struck. How is this analogous to atoms, molecules, and light? - Vibrating electrons in atoms and molecules emit their own unique colors of light.

11. Light incident upon a pane of glass slows down in passing through the glass. Does it emerge at a slower speed or at its initial speed? Explain. - It returns to its initial speed because there are no longer any absorption/re-emission delays to slow down the light energy outside of the glass. 12. Will glass be transparent to frequencies of light that match its own natural frequencies? - No, energy is absorbed and then converted to heat.

13. Does the time delay between the absorption and re-emission of light affect the average speed of light in a material? Explain. - Yes; the small delays from absorption and re- emission result in a lower average speed of the light. 14. Why would you expect the speed of light to be slightly less in the atmosphere than in a vacuum? - There are interaction delays due to particles in the atmosphere that slow it down.

15. When light encounters a material, it can build up vibrations in the electrons of certain atoms that may be intense enough to last over a long period of time. Will the energy of these vibrations tend to be absorbed and turned into heat, or absorbed and reemitted as light? - They are absorbed and turned into heat. 16. What determines whether or not a material is transparent or opaque? - Whether light energy is re-emitted or absorbed.

17. Why are metals shiny in appearance? - Metals have "free electrons" that absorb, vibrate, stop, and re-emit light in the form of a reflection. 18. Distinguish between an umbra and a penumbra. - Umbra is a total shadow, penumbra is a partial shadow.

19. a. Distinguish between a solar eclipse and a lunar eclipse. - Solar is moon's shadow on Earth. - Lunar is Earth's shadow on the moon. b. Which type of eclipse is dangerous to your eyes if viewed directly? - Solar

20. What is the difference between light that is polarized and light that is not? - Polarized: vibrations occur in one plane; nonpolarized, the vibrations occur in all planes 21. Why is light from a common lamp or from a candle flame nonpolarized? - Electrons vibrate randomly, so light alignment is random. 22. In what direction is the polarization of the glare that reflects from a horizontal surface? - Horizontal

23. How do polarizing filters allow each eye to see seperate images in the projection of three- dimensional slides or movies? - The Polaroid pairs in the projector and the glasses are at right angles. 32. Suppose that sunlight is incident upon both a pair of reading glasses and a pair of sunglasses. Which pair would you expect to be warmer, and why? - Sunglasses, because they absorb more sunlight energy and hence warm up more.

The Sun The Sun is our ultimate source of energy and is a great example that sums up many aspects of physics: gravity, light, other types of energy such as nuclear, etc. We are getting closer to discussing more and more Modern Physics (Einstein, relativity, nuclear concepts such as fission and fusion, etc.), so the Sun will make more sense to us in the coming weeks.

Where does the sun get its energy?

For the rest of the class period: Be sure you've submitted the assignments from this week, listed below... Monday - Ch. 28 Color questions (worksheet) Tuesday - Lab: making color with light Wednesday - Review Questions Ch. 29 Thursday - Lab: Reflection vs. Refraction