Hewitt/Suchocki/Hewitt Conceptual Physical Science Fourth Edition

1. Hewitt/Suchocki/HewittConceptual Physical Science Fourth Edition Chapter 11: LIGHT

2. This lecture will help you understand: • Electromagnetic Spectrum • Transparent and Opaque Materials • Reflection • Refraction • Color • Dispersion • Polarization

3. Electromagnetic Nature of Light Light: • electromagnetic waves created by vibrating electric charges having frequencies that fall within the range of sight • frequency of vibrating electrons equals the frequency of the light • travels nearly a million times faster than sound in air • light and all electromagnetic waves are transverse waves

4. Electromagnetic Spectrum Electromagnetic wave • made up of vibrating electric and magnetic fields that regenerate each other by electromagnetic induction

5. Electromagnetic Spectrum The Electromagnetic Spectrum

6. Electromagnetic Spectrum CHECK YOUR NEIGHBOR The electromagnetic spectrum spans waves ranging from lowest to highest frequencies. The smallest portion of the electromagnetic spectrum is that of A. radio waves. • microwaves. • visible light. • gamma rays.

7. Electromagnetic Spectrum CHECK YOUR ANSWER The electromagnetic spectrum spans waves ranging from lowest to highest frequencies. The smallest portion of the electromagnetic spectrum is that of A. radio waves. • microwaves. • visible light. • gamma rays. Explanation: This can be inferred by a careful study of the spectrum and its regions in Figure 11.3.

8. Electromagnetic Nature of Light Order of increasing frequency of visible light: • red • violet—nearly twice the frequency of red • ultraviolet—cause of sunburns • X-rays • gamma rays

9. Electromagnetic Nature of Light A situation to ponder… A photographer wishes to photograph a lightning bolt by setting a camera so that it is triggered by the sound of thunder.

10. A situation to ponder… CHECK YOUR NEIGHBOR Is this a good idea or a poor idea? A. Good idea for nearby lightning strikes. • Good idea for all strikes. • Poor idea for nearby lightning strikes. • Poor idea for all strikes.

11. A situation to ponder… CHECK YOUR ANSWER Is this a good idea or a poor idea? A. Good idea for nearby lightning strikes. • Good idea for all strikes. • Poor idea for nearby lightning strikes. • Poor idea for all strikes. Explanation: Light travels about a million times faster than sound. By the time the sound of thunder arrives, the lightning bolt is long gone.

12. Transparent and Opaque Materials Opaque materials: colored glass is opaque to much of incident white light

13. Transparent and Opaque Materials Light incident on: • dry surfaces bounces directly to your eye • wet surfaces bounces inside the transparent wet region, absorbing energy with each bounce, and reaches your eye darker than from a dry surface

14. Transparent and Opaque Materials Light is transmitted similar to sound • light incident on matter forces some electrons in matter to vibrate

15. Transparent and Opaque Materials How light penetrates a pane of glass • electrons in atoms of glass are forced into vibration • energy is momentarily absorbed and vibrates electrons in the glass • a vibrating electron either emits a photon or transfers the energy as heat • light slows due to time delay between absorption and reemission of photons

16. Transparent and Opaque Materials Average speed of light through different materials • vacuum—c (300,000,000 m/s) • atmosphere—slightly less than c (but rounded off to c) • water—0.75 c • glass—0.67 c, depending on material • diamond—0.41 c

17. Transparent and Opaque Materials CHECK YOUR NEIGHBOR Strictly speaking, the photons of light incident on glass are A. also the ones that travel through and exit the other side. • not the ones that travel through and exit the other side. • absorbed and transformed to thermal energy. • diffracted.

18. Transparent and Opaque Materials CHECK YOUR ANSWER Strictly speaking, the photons of light incident on glass are A. also the ones that travel through and exit the other side. • not the ones that travel through and exit the other side. • absorbed and transformed to thermal energy. • diffracted. Explanation: Figure 11.6 illustrates this nicely. A photon that exits the glass is not the same photon that began the process of absorption and re-emission.

19. Reflection Reflection: the returning of a wave to the medium through which it came when encountering a reflective surface Law of reflection: angle of incidence = angle of reflection

20. Reflection Virtual image • same size as object, formed behind a mirror, and located at the position where the extended reflected rays converge • as far behind the mirror as the object is in front of the mirror

21. Reflection Plane mirror • note, the only axis reversed in an image is the front-back axis

22. Reflection Curved mirrors form a different virtual image • convex mirror (that curves outward): virtual image is smaller and closer to the mirror than the object • concave mirror (that curves inward): virtual image is larger and farther away than the object

23. Reflection Diffuse reflection • light striking a rough or irregular surface reflects in many directions • an undesirable circumstance is the ghost image that occurs on a non-cable TV set when TV signals bounce off buildings and other obstructions

24. The open-mesh parabolic dish is a diffuse reflector for short-wavelength light. It is also a polished reflector for long-wavelength radio waves. Diffuse Reflection Application

25. Diffuse Reflection Different road surfaces determine amount of diffuse reflection • Rough road surface—diffuse reflection of illumination from your car headlights lets you see road ahead at night • Wet road surface is smooth—more plane mirror than diffuse, so seeing road ahead is more difficult Now you know!

26. Reflection CHECK YOUR NEIGHBOR When you stand 2 meters in front of a plane mirror, your image is A. 2 meters in back of the mirror. • 4 meters from you. • Both A and B. • None of the above.

27. Reflection CHECK YOUR ANSWER When you stand 2 meters in front of a plane mirror, your image is A. 2 meters in back of the mirror. • 4 meters from you. • Both A and B. • None of the above.

28. Refraction Refraction • bending of light when it passes from one medium to another • caused by change in speed of light

29. Refraction Here we see light rays pass from air into water and from water into air • pathways are reversible for both reflection and refraction

30. Refraction CHECK YOUR NEIGHBOR Refracted light that bends toward the normal is light that has A. slowed down. • sped up. • nearly been absorbed. • diffracted.

31. Refraction CHECK YOUR ANSWER Refracted light that bends toward the normal is light that has A. slowed down. • sped up. • nearly been absorbed. • diffracted.

32. Refraction CHECK YOUR NEIGHBOR Refracted light that bends away from the normal is light that has A. slowed down. • sped up. • nearly been absorbed. • diffracted.

33. Refraction CHECK YOUR ANSWER Refracted light that bends away from the normal is light that has A. slowed down. • sped up. • nearly been absorbed. • diffracted. Explanation: This question is a consistency check with the question that asks about light bending toward the normal when slowing.

34. Refraction Illusions caused by refraction • Objects submerged in water appear closer to the surface.

35. Refraction Illusions caused by refraction (continued) • Objects such as the Sun seen through air are displaced because of atmospheric refraction.

36. Refraction Illusions caused by refraction (continued) • Atmospheric refraction is the cause of mirages.

37. Refraction CHECK YOUR NEIGHBOR Which of these would not exist if light didn’t slow in transparent materials? A. Rainbows. • Mirages. • Magnifying glasses. • All wouldn’t be.

38. Refraction CHECK YOUR ANSWER Which of these would not exist if light didn’t slow in transparent materials? A. Rainbows. • Mirages. • Magnifying glasses. • All wouldn’t be.

39. Color Color we see depends on frequency of light ranging from lowest (red) to highest (violet). In between are colors of the rainbow. Hues in seven colors: red, orange, yellow, green, blue, indigo, and violet. Grouped together, they add to appear white.

40. Color Selective Reflection Most objects don’t emit light, but reflect light. A material may absorb some of the light and reflect the rest. Selective Transmission The color of a transparent object depends on the color of the light it transmits.

41. Color Mixed Color Lights Three types of cone receptors in our eyes perceive color—each stimulated by only certain frequencies of light: • lower-frequency light stimulate cones sensitive to low frequencies (red) • Middle-frequency light stimulate cones sensitive to mid-frequencies (green) • High-frequency light stimulate cones sensitive to high frequencies (blue) • Stimulation of all three cones equally, we see white light

42. Color Additive primary colors (red, blue, green): red + blue = magenta red + green = yellow blue + green = cyan

43. Color Opposites of primary colors: opposite of green is magenta opposite of red is cyan opposite of blue is yellow The addition of any color to its opposite color results in white.

44. Dispersion Dispersion • process of separation of light into colors arranged by frequency • Components of white light are dispersed in a prism (and also in a diffraction grating)

45. Dispersion CHECK YOUR NEIGHBOR When white light passes through a prism, green light is bent more than A. blue light. • violet light. • red light. • None of the above.

46. Dispersion CHECK YOUR ANSWER When white light passes through a prism, green light is bent more than A. blue light. • violet light. • red light. • None of the above

47. Dispersion Rainbows • a colorful example of dispersion

48. Dispersion Rainbow detail

49. Dispersion Rainbow facts • an observer is in a position to see only a single color from any one droplet of water • your rainbow is slightly different from the rainbow seen by others • your rainbow moves with you • disk within the bow is brighter because of overlapping of multiple refractions (which don’t occur outside the disk)

50. Dispersion Rainbow facts (continued) • secondary rainbow is fainter (due to two internal reflections and refracted light loss). • secondary bow is reversed in color (due to the extra internal reflection)