1. Chapter 14
Properties of Light

2. Waves Wave - a moving disturbance
Moving deformation of transport medium which propagates through the medium
Example - a cork bobbing up and down ……
……...shows that a disturbance is passing rather than a flow of water (current)
Waves transport energy from one point in space to another point

4. Types of Waves
A) Mechanical waves - are those waves requiring a material medium to transport the disturbance
A1) Transverse----- medium moves at right angles to the direction of transport
A2) Longitudinal medium moves in the direction of transport of the disturbance

5. Types of Waves EM waves are transverse waves
Electromagnetic waves - are those waves requiring no material medium to transport the disturbance
EM waves are transverse waves

6. The Electromagnetic Spectrum
A range of light waves extending in wavelength from radio waves to gamma rays

8. The Electromagnetic Spectrum
Gamma Rays - most energetic
X-rays - penetrates tissue
Ultraviolet - causes sunburns
Visible Light - detected by your eyes
Infrared - “heat waves”
Microwaves - used to cook
Radio Waves - communication

10. Electromagnetic Waves
Electromagnetic Waves - energy-carrying waves emitted by vibrating electrons
Also know as…
Light
Radiation
Photons

13. Electromagnetic Wave Velocity
The speed of light is the same for all seven forms of light.
It is 300,000 kilometers per second or 186,000 miles per second.

14. Radio waves
electrons moving up and down an antenna
Visible Light
electrons changing energy states in an atom

15. The Visible Spectrum
A range of light waves extending in wavelength from about 400 to 700 namometers.

16. Let Their Be Light
Light Emission

17. light

18. Excitation and de-excitation of electrons

19. Emission Lines for the Hydrogen Atom
(Chaisson/McMillan) Astronomy

20. Excitation
When an electron is raised to a higher energy level, the atom is said to be excited.
When the electron returns to a lower energy level, energy is released in the form of photon (pellet of light).

21. Simple model of the atom
Electron absorbs some energy which causes it to “jump” to a higher energy level. This is called excitation. E2
Energy E1
Nucleus

22. Excitation E2
UV light E1
Nucleus

23. de-excitation
The electron, preferring to be in the lower energy, immediately drops back down to the lower energy level.
Energy E2 E1

24. In order to conserve energy, a photon (discrete bundle of energy) is emitted.
Photon Energy E1
Photon Energy = E2 – E1

25. .
Because of the behavior of matter and energy at the atomic scale, which is governed by Quantum Mechanics, the energy of the photon is equal to the difference in the energies of the two energy levels and is also proportional to its frequency.

26. Emission Spectrum
(Chaisson/McMillan) Astronomy

27. Emission Lines for the Hydrogen Atom
(Chaisson/McMillan) Astronomy

28. Different transitions from high levels to low levels result in different colors of light.

29. The Visible Spectrum

30. Things that create a Spectrum
Prism
Raindrops
CD’s
Diffraction Grating

31. The Kirchhoff-Bunsen Experiment
These two scientists found that burning chemicals over an open flame resulted in a spectrum with bright lines.
They found that each chemical element produced its own characteristic pattern of bright spectral lines.

32. Emission Spectra Hot gas produces a bright line emission spectrum.
Demo - hot hydrogen gas and diffraction gratings
Emission Spectrum

33. Hydrogen
Helium
Oxygen
Carbon
Every element can be “fingerprinted” by it spectra.

34. Prism and colors

35. Prism and colors

36. Why Sky is Blue……...

37. Why Sky is Blue……...Rayleigh Effect

38. Why Sky is Blue……...

39. Incandescence Hot, dense solids produce a continuous spectrum.
Demo - an incandescent light bulb and diffraction gratings
Continuous Spectrum

40. The color of light emitted by a hot object changes with its temperature.
Glowing object colors:
Reddish  coolest glowing object
Orange-ish
Yellowish
White
Bluish  hottest glowing object

41. Absorption Spectra
Cool gas in front of a continuous source of light produces an absorption line spectrum.
Fraunhofer lines in our Sun's spectrum showed that cool helium gas surrounds the Sun.
Absorption Spectrum

42. Fluorescence
Some materials that are excited by ultraviolet light can emit visible light upon de-excitation. This is fluorescence.

43. Fluorescence
Fluorescence occurs when UV light is absorbed by the electrons of an atom and visible light is emitted upon de-excitation.

44. Fluorescent Lamps
Primary excitation - electron collisions with low pressure mercury vapor, and ultraviolet light is given off
Secondary excitation - ultraviolet light is absorbed by phosphors and these emit visible light

45. Fluorescent Lamps e- Fluorescence High-speed electron Hg atom
UV Photon
Phosphor
Coating
Fluorescence

46. Phosphorescence
Phosphorescence - a type of light emission that is the same as fluorescence except for a delay between excitation and de-excitation.
Electrons get "stuck" in an excited state and de-excite gradually.
Demos - glow-in-the-dark objects

47. Phosphorescence
Atoms or molecules are excited by incident visible light and remain in an excited state for a long time before they de-excite.
This allows phosphorescent materials to glow in the dark.

48. Phosphorescent Dove

49. Glow in the Dark Dove

50. Incoherent and Coherent light
White light from an incandescent light bulb is called incoherent light because the light is composed of different colors and the waves are out of phase with each other.

51. If we pass the white light through a filter, we could get only one color (monochromatic) light which would still be out of phase. (incoherent red light)

52. Monochromatic, Coherent light
By getting all the red light in phase, we would have monochromatic, coherent light.
This is the kind of light emitted by a LASER.

53. Monochromatic, Coherent Red light

54. Lasers
Light
Amplification
Stimulated by
Emission
Radiation of

55. Lasers Laser Light amplification by stimulated emission of radiation
Lasers produce coherent light.
Coherent light means that all the light waves have the frequency, phase and direction.
Demo - Laser and chalk dust
Demo - Laser and prism

56. Transparent Materials
Transparent - the term applied to materials through which light can pass in straight lines

57. Opaque Materials
Opaque - the term applied to materials that absorb light

58. Example Questions Are clouds transparent or opaque to visible light?
Answer: opaque
Are clouds transparent or opaque to ultraviolet light?
Answer: transparent

59. Shadows
Umbra - the darker part of a shadow where all the light is blocked
Penumbra - a partial shadow
These terms also apply to Solar Eclipses and Lunar Eclipses.

60. Solar Eclipse
Umbra
Sun
Full Shadow
Earth
Moon
Partial Shadow
Penumbra
A solar eclipse occurs when the Moon passes in front of the Sun.

61. Lunar Eclipse
Sun
Earth
Moon
A lunar eclipse occurs when the Moon passes into the Earth's shadow.

62. Questions
Which type of eclipse is dangerous to view with the unprotected eye?
Why are lunar eclipses more commonly seen than solar eclipses? *

63. SEEING LIGHT - THE EYE Cornea - does most of the focusing Iris -
has the eye color and controls light intensity
Pupil -
the hole in the eye
Lens -
does remainder of focusing
Retina -
location of light sensors, has rods and cones
Fovea -
center of vision, predominantly cones
Blind spot -
optic nerve exit, no light sensors

64. Color Deficiency

65. Myopia (Near-Sightedness)
People with near-sightedness cannot see clearly at distance.

66. Hyperopia (Farsightedness)
People with far-sightedness cannot see clearly up close.

67. The speed of light... (a) has never been measured
(b) is about the same as that of sound
(c) is infinitely fast
(d) is very fast, but not infinite
(d) is very fast, but not infinite

68. In the dark at late evening, no color is seen because of lack of stimulation of
a. rods.
b. cones.
c. cornea.
d. crystalline lens.
b. cones.