Properties of Visible Light


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Properties of Visible Light

  1. 1. Properties of Visible Light
  2. 2. Light waves • All waves in the electromagnetic spectrum are light waves • Visible light – part of the spectrum that humans CAN see • Human eyes are equipped with special cone-shaped cells that act as receivers to the wavelengths of visible light. • Human eyes detect the brightness, or intensity, of visible light, which is determined by the amplitude of a light wave • The range of wavelengths of visible light is from about 380 nanometers to about 740 nanometers; human eyes do not detect wavelengths outside of this range • The visible light range further divides into smaller regions depending on wavelength; humans recognize these smaller regions as colors.
  3. 3. “Color” Discovery Education Streaming Movie (Look at the video at the right) 1. What happens to visible light as it passes through a prism that produce colored light? • Visible light refracts, or bends., as it enters and leaves a prism. The different colors refract by lightly different amounts, so they separate. 2. What is the order of the colors of refracted light? • Red, orange, yellow, green, blue, indigo, and violet. 3. How do rainbows form? • Rainbows form when sunlight passes through raindrops, which act as prisms, that separate the light into colors 4. Why does a yellow flower appear yellow? • A yellow flower appears yellow because it absorbs all the colors of visible light, except yellow, which it reflects 5. Why does a black notebook appear black? • A black notebook appears black because it absorbs ALL the colors of visible light and does NOT reflect any color. Watch the video ^^
  4. 4. Transparent, Translucent, and Opaque • Light waves travel in strait lines from their source. • When a light wave strikes an object, the light will do three things: transmit (go through an object), reflect (to bounce off an object), and/or absorb (the energy of the light wave transforms into heat). • Certain descriptions of matter are based on the way light interacts with it. Transparent objects: • Lets nearly all the light that strikes them to pass through. • Example: a piece of glass • Are see-through Translucent objects: • Scatters the light that passes through them • Example: a frosted window • Are partially see-through • You can usually see something behind a translucent object, but not the details Opaque objects: • Either reflect or absorb all the light that strikes them • Example: brick wall • Are NOT see-through because light cannot pass through them • The color of an opaque object comes from the color it reflects (look back at slide 4)
  5. 5. Translucent panes of glass Opaque object (brick wall) Transparent object (see- through glass)
  6. 6. Watch the video ^^ and answer the questions below. 1. Match the type of material with its proper description: Wax paper a. transparent Aluminum foil b. translucent Clear plastic c. opaque 2. How did persistence of vision allow you to see the image reflected from the dowel rod? ***Answers are on next slide. 
  7. 7. Answer to number 1: - Wax paper – translucent - Aluminum foil – opaque - Clear plastic – transparent Answer to number 2 - Persistence of vision is a phenomenon whereby your eyes retain an image for a fraction of a second. This enabled you to see a whole image even though the reflected light came to you in pieces on the dowel rod. Answers to slide 7
  8. 8. Albedo • Different materials reflect different amounts of light from their surfaces. Albedo is the percentage of light reflected off a surface. The chart to the right shows the albedo, or reflectivity, of various surfaces. Albedo of Various Surfaces Fresh snow 80-95% Thick clouds 60-90% Thin clouds 30-50% Sand 15-45% Ice 30-40% Earth and atmosphere 30% Grass 10-30% Dry soil 5-20% Wet soil 10% Water 10% (daily average)
  9. 9. Look at the video at the right. What is the albedo of moon dust? - The albedo of moon dust is between 7- 10%. Look at the chart on the previous slide. Which substances have about the same reflectivity as moon dust? - Water, wet soil, and grass have about the same reflectivity as moon dust. How did simulating the albedo of the moon’s surface help the scientists in the movie? - The scientists were able to prove that a photograph of the lunar landing was not a fake because the albedo of the moon’s surface provided sufficient lighting from reflected sunlight to make the photograph possible. “Replicating the Moon’s Reflective Surface Discovery Education Video
  10. 10. Other Properties of Light • Remember, when light waves encounter a medium (or object) they can be transmitted, reflected, or absorbed. • When light rays are transmitted, they will refract in the new medium. (To make it easier, when light waves go through an object, the light wave changes speed and/or direction). **Make sure you remember the difference between wave refraction and reflection. Also remember what a medium is. Reflection happens when a wave bounces off a medium without entering it. Refraction occurs when a wave changes speed and/or direction upon entering a new medium . • When light waves encounter a medium, they can reflect from the medium in two ways. Look below: Regular reflection  Occurs when parallel rays of light bounce off a smooth medium or surface at the same angle, producing an image on the surface of the medium  Is also called specular reflection Diffuse reflection  Occurs when parallel rays of light bounce off of a bumpy or uneven medium or surface at different angles, producing either an unclear image or no image at all.
  11. 11. Lets take a closer look at reflection of light waves True or False: The reason you are able to see anything is because light reflects off objects. • True What happens to light waves when they reflect off a rough surface? • The light scatters in different directions. What happens to light waves when they reflect off a smooth surface? • The light bounces off in one direction. What results when light waves bounce off a smooth surface in one direction? • An image of the object reflects from the surface. What makes something appear dull instead of shiny? • If the surface of an object is not smooth, light reflecting from the surface will scatter in different directions, and the object will appear dull instead of shiny. Watch the “Light Reflection: Dependent on Surface” Video
  12. 12. Now take a closer look at refraction of light waves Why does the pencil in the video appear to bend in water? • Light waves bouncing off the lower end of the pencil bend as they travel from the water to the air. This makes the pencil appear bent. Why does the seashell in the video appear to be in a location in which it really was not? • Light waves bouncing off the seashell bend as they travel from the water to the air. This makes the seashell appear to be in a different location. Look at the “Refraction: The Bending of Light Waves” video
  13. 13. Index of refraction, angle of incidence, and angle of refraction index of refraction of a medium:  Measures how many times SLOWER a light wave will travel in a medium versus in a vacuum.  Example: the index of refraction of water is 1.33 at 20 degrees Celsius. This means that a light wave will travel 1.33 times slower in water than in a vacuum  The larger the index of refraction, the slower the wave travels in a medium Angle of incidence  The angle at which a light waves enters a medium Angle of refraction  The angle at which a light wave refracts, or bends, as it travels through the new medium
  14. 14. Both the angle of incidence and the angle of refraction are measured between the light ray and the dotted line called the normal.