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Chapter Twenty-Five: Light

Chapter Twenty-Five: Light. 25.1 Properties of Light 25.2 Color and Vision 25.3 Optics. 25.1 Properties of light. Light travels fast over long distances and carries energy and information.

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Chapter Twenty-Five: Light

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  1. Chapter Twenty-Five: Light • 25.1 Properties of Light • 25.2 Color and Vision • 25.3 Optics

  2. 25.1 Properties of light • Light travels fast over long distances and carries energy and information. • Light travels in straight lines, but can be bent by lenses or reflected by mirrors heat and warmth. • Light has color and can be bright or dim.

  3. The electromagnetic spectrum • Light, like sound and heat, is a form of energy. • The visible lightwe see is part of the electromagnetic spectrum. Page 604

  4. The Electromagnetic Spectrum • Visible light is a very small part of the electromagnetic spectrum.

  5. Where does sound fit into the electromagnetic spectrum? ?

  6. Page 604 Properties of light • You see book pages because light in the room reflectsfrom the page to your eyes. • Your eyes and brain use the informationcarried by the light to make a mental picture.

  7. Light is produced by atoms • Most light is produced by atoms. • When you put some energy intothe atom, it excites the atom’s electrons. • Light is produced when the electron releasesthis energy. Page 605

  8. Page 605 Incandescent light • Making light with heatis called incandescence. • Atoms in the filament convert electrical energy to heatand then to light. • Incandescent bulbs are inefficient, but their waste heat can be useful.

  9. Page 606 Fluorescent light • To make light, fluorescent bulbs use high-voltage electricityto energize atoms of gas in the bulb. • These atoms release the electrical energy directly as light (not heat), in a process called fluorescence.

  10. Page 606 Color and energy • When all the colors of the rainbow are combined, we see light withoutany color. • We call the combination of all colors white light.

  11. Page 606 Color and energy • Compare the hot, blue flame from a gas stove to the orange flame of a match. • The light from a gas flame is blue (high energy) and the light from a match is red-orange (low energy).

  12. Page 606

  13. 25.1 The speed of light • The speed at which light travels through air is about 300 millionmeters per second. • The speed of light is so important in physics that it is given its own symbol, a lower case “c”. Page 607

  14. Page 607 25.1 Speed of light • The speed at which electromagnetic waves travel through air is about 300 million meters per second. • The speed of light is so fast that when lightning strikes a few miles away, we hear the thunder afterwe see the lightning.

  15. Wavelength & Frequency of Light • Because the wavelength of light is so small, scientists measure it in nanometers. • One nanometer (nm) is one billionth of a meter (0.000000001 m). Page 608

  16. Wavelength and Frequency of Light • One THz is one trillion Hz. • 1,000,000,000,000 Hz Page 608

  17. What kind of wave is light? • A sound wave is a oscillation of air. • A water wave is an oscillation of the surfaceof water. • An oscillation of electricityor magnetismcreates electromagnetic waves. Page 609

  18. Electromagnetic waves • If you could shake the magnet up and down 450 trilliontimes per second, you would make waves of red light with a frequency of about 450 THz. Page 609

  19. Electromagnetic spectrum • The entire range of electromagnetic waves, including all possible frequencies, is called the electromagnetic spectrum. • This spectrum includes visible light and invisible waves: • radiowave • microwaves • infrared light • ultraviolet light • X-rays • gamma rays Page 610

  20. Electromagnetic spectrum Page 610

  21. End Section 25.1 Begin Section 25.2

  22. The human eye • The eye is the sensory organ used for vision. • The retina contains light-sensitive cells called photoreceptors. • Photoreceptors convert light into nerveimpulses that travel through the optic nerve to the visual cortex of the brain.

  23. Photoreceptors • The human eye has two types of photoreceptors: conesand rods. • Cones respond to color and rods respond to the intensityof light. • Rod cells “see” black, white, and shades of gray.

  24. How we see color • Our eyes work according to an additive color process • 3 photoreceptors (red, green, and blue) in the eye operate together so that we see millions of different colors.

  25. Cone Cells  • There are three types of cone cells. • One type responds best to low-energy (red) light. • Another type responds best to medium-energy (green) light. • The third type responds best to higher-energy (blue) light.

  26. Cones  • If there is a strong red signal and a weak green signal, we see orange. • If the brain gets a signal from only a green cone, we see green. • All of the light is still there. We just see orange.

  27. The Additive Primary Colors • They're called primary because any color can be made from a suitable combination of red, green, and blue.

  28. An object appears the color it reflects! White object Black object Red object Green object

  29. Reflection • An object is the color of the light reflected. • Red reflects red light.

  30. Reflection • Blue reflects blue light.

  31. Reflection • Blue light on a red object.

  32. White light • Illuminated by whitelight (a mixture of all colors) all parts of the girl's clothing show their colors because each reflectsits own part of the spectrum.

  33. Red Light • Illuminated only by red light, the girl's red top shows up red because it reflects redlight. Other clothing absorbs red light, so reflects nothing (and therefore looks black)

  34. Complementary Colors • Red + Green = Yellow • Red + Blue = Magenta • Green + blue = Cyan • (Red + Green) + Blue = White • Colors which  combine to produce white are said to be complementary.   • Therefore, yellow and blue are complementary colors. Additive Color Mixing (mixing of light)

  35. Wavelength & Frequency of Light

  36. 25.2 Making an RGB color image • A television makes different colors by lighting red, green, and blue pixels in different proportions. • Color images in TVs and computers are based on the RGB color model.

  37. RedGreen WhiteBlue • TV’s • Computer screen • Spot lights

  38. 25.2 Subtractive color process • A blue shirt looks blue because it reflectsblue light into your eyes. • Chemicals known as pigmentsin the dyes and paints absorb some colors and reflect other colors.

  39. 25.2 The CMYK color process • The subtractive color process is often called CMYK for the four pigments it uses. • CMYK stands for cyan, magenta, yellow, and black.

  40. 25.2 The CMYK color process

  41. The CMYK color process • The full color image is a combination of all four images!

  42. Mixing Colored Pigments

  43. Mixing Colored Pigments

  44. 25.2 Why plants are green • Plants absorb energy from light and convert it to chemical energy in process called photosynthesis. • Chlorophyll is the main pigment of plants absorbsred and blue light and reflects green light.

  45. 25.2 Why plants are green • Plants must reflectsome light to avoid absorbing too much energy. • A plant will dieif placed under only green light!

  46. Dustyn McKnight

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