Properties of Light

1. Properties of Light

2. Electromagnetic Radiation Light Photons

3. These three are the same… • Light • pure energy • Electromagnetic Waves • energy-carrying waves emitted by vibrating electrons • Photons • particles of light

6. Electromagnetic Waves • Demo • Tesla Coil, Radio, Fluorescent light

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

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

9. EM Waves Sources THE ELECTROMAGNETIC SPECTRUM Radio Waves Vibrating charges Molecular vibrations Microwaves Infrared Molecular vibrations Visible Atomic vibrations Ultraviolet Atomic vibrations Atomic vibrations X-rays Gamma rays Nuclear vibrations

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

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

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

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

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

16. 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.

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

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

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

20. 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 (red eye demo) • Lens - does remainder of focusing • Retina - location of light sensors, has rods and cones center of vision, predominantly cones • Fovea - • Blind spot - optic nerve exit, no light sensors

24. Color Deficiency

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

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

27. You will observe a total eclipse of the sun when... (a) you stand in the penumbra of the moon’s shadow (b) you stand in the umbra of the moon’s shadow (c) sunlight diffracts around the moon (d) sunlight reflects from the moon to the earth

28. 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

29. 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.

30. Light Emission • The following slides will illustrate facts about Light and electrons as they relate to atomic structure and release of energy

31. Structure of the Atom • Proton • Neutron • Electron • Energy Levels

32. Excitation • When an electron is raised to a higher energy level, the atom is said to be excited.

33. Emission • When the electron returns to a lower energy level, energy is released in the form of light.

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

35. Gustav Kirchhoff Robert Bunsen “The Taste Test” versus “The Flame Test”

36. 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.

37. Emission Spectra of Hydrogen Discrete Emission Spectrum Slit Film Low Density Glowing Hydrogen Gas Prism Photographic Film

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

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

40. Emission Spectra Continuous Emission Spectrum Slit White Light Source Prism Photographic Film

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

42. The brightness and 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

43. Absorption Spectraof Hydrogen Discrete Emission Spectrum Discrete Absorption Spectrum Slit Hydrogen Gas Film White Light Source Prism Photographic Film

44. 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

45. Matching Questions Type of Spectrum Appearance 1. Emission Spectra a. All Colors 2. Continuous Spectra b. Dark Lines 3. Absorption Spectra c. Bright Lines

47. Matching Questions 1. Emission Spectra a. Hot Solids 2. Continuous Spectra b. Hot Stars 3. Absorption Spectra c. Hot Gases

48. Fluorescence • Some materials that are excited by ultraviolet light can emit visible light upon de-excitation. This is fluorescence. • Demo - Black light and chalk • See Figures 30.10 & 30.11

49. 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

50. 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