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Without light, we can’t see things
Without light, we can’t see things. (Turn off lights; then, turn on necklace or Xmas lights) Did you know that white light is a mixture of all the colors of the rainbow? White light is hitting this basketball, but all we see is orange & white light is hitting this, etc. White light is hitting my lab coat, but all we see is white…wait a minute, why do we not see a color? The light that we receive from the Sun is called visible light, or white light. But it actually is composed of many different colors. However because they are all traveling at the same velocity, we see them as a combined color, white sunlight. But light gets slowed down when it travels through a medium.
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Terms You Need to Understand
Absorption Reflection Dispersion Diffraction Scattering Refraction Interference
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If you pass white light through a prism, it separates into its component colors.
Spectrum This is what happens when you shine white light through a glass prism. The light gets slowed down, but not uniformly at all wavelengths. Isaac Newton was the first person to demonstrate this. Newton passed sunlight through a glass prism to separate the colors into a rainbow spectrum. Different wavelengths of light get slowed down by different amounts. Blue light gets slowed down more than red light. This slowing down happens because the light gets bent in the prism. Blue light gets bent more so it travels slower than red light. This bending of light is called diffraction of light. This diffraction causes us to see a rainbow of colors ranging from red to violet. That is what happens when you see a rainbow in the sky after it rains. There sunlight which is called visible light or white light gets broken down into its component colors when it hits the water molecules in the sky. Because the light is hitting something, different wavelengths get slowed down by different amounts and the different colors that make up white light gets dispersed by different amounts. Astronomers have developed a mnemonic to help you remember the colors that make up visible light. Starting from longer wavelength to shorter wavelength: It is Roy G. Biv. That is Red, Orange, Yellow, Green, Blue, Indigo, and Violet. These are all the colors of the rainbow. Of course in reality, each individual color is more finely divided and they show up as different tints of the color. spectra- when light gets dispersed into component colors or wavelengths. R.O.Y. G. B.I.V !! long wavelengths short wavelengths
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Dispersion The separation of light into colors arranged according to their frequency, by interaction with a prism or diffraction grating. Pass out diffraction gratings and ask them to list colors they see. Bring spotlights to demonstrate that colors of rainbow make up white light. I’m handing out what are called diffraction gratings. This is just some material with fine grooves on it so that when light hits it, it bends in different amounts. Just hold it over your head and then you should be able to see a rainbow of colors. This is what happens when white light gets diffracted. Talk about Rainbows: Rainbow demo? Show demo that if you bring different colors together, they combine to form white light. It is not a coincidence, but evolutionary that our eyes are tuned to the light that our Sun mainly emits which is visible light. It emits at other wavelengths too, but very little. So if our eyes were not tuned to visible light, we could barely see anything. Perhaps we can be like some animals which can detect infrared light. This is the light that occurs due to heat. Because we all have a temperature, our bodies are radiating in the infrared. After play, ask “Why is my apple red?” Why is the banana yellow?
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Apple absorbs all colors but reflects red
Banana reflects yellow Due to chemicals in skin of fruits and veggies. Chemicals change with age. Gummi: Red shines thru red; green thru green What happens to red on green & visa versa? Colors have been absorbed. Finger-show green doesn’t pass thru, red does, white shows red only Why is my lab coat white? Why does your table appear black?
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Double Rainbow Raindrops reflect the sun’s light noticeably inward from the rainbow arc, and correspondingly out of the secondary bow, so that the dark band is seen between the bows. This effect, called Alexander’s band, was first described by the Greek philosopher Alexander of Aphrodisias in the 3rd century. The sky below the primary (lower) rainbow, and above the secondary (higher) bow, is brighter as a result. In a double rainbow, raindrops reflect the sun’s light noticeably inward from the rainbow arc, and correspondingly out of the secondary bow, so that the dark band is seen between the bows. This effect, called Alexander’s band, was first described by the Greek philosopher Alexander of Aphrodisias in the 3rd century. The sky below the primary (lower) rainbow, and above the secondary (higher) bow, is brighter as a result.
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This photograph illustrates the actual appearance of a double rainbow, with the bright area below the primary bow and the dark Alexander band between the bows. The colors of the bright primary rainbow (lower) run from violet on the inside to red on the outside. In the secondary (higher) rainbow the color sequence is reversed, with red on the inside and violet on the outside.
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Light Absorption When photons of light hit atoms and cause them to increase their vibrations (causing them to heat up.) A fabric that we see as black has absorbed all colors of the spectrum and is hotter than a white fabric that reflects all colors. REMEMBER THIS AS YOU GET DRESSED THIS SUMMER!
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Light Reflection When an incoming light wave bounces off of an object.
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What is light scattering?
In the lab… Notes: In the laboratory we can control the conditions to retrieve detailed information about the light scattering. We can choose the wavelength (l), polarization, and intensity (Ii) of the incident light. The size of the laser beam and the field of view of the detector define a scattering volume. We can detect the scattered light (Is) from this volume as a function of angle (q) and polarization. With such exquisite control of the experimental parameters, we can use light scattering to retrieve fundamental physical properties of the scattering medium. When light bounces off of an object in a variety of directions. © Wyatt Technology Corporation All Rights Reserved
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Light Scattering In nature… red sunsets blue sky and clouds
Notes: Simple questions about our everyday world, such as “Why is the sky blue?”, “How can we see clouds?”, or “Why are sunsets red?” have interesting answers that depend upon light scattering. In fact, it was the question of the blue sky and the polarization of skylight that lead Lord Rayleigh to develop a theoretical description of light scattering in 1871. When light passes through matter, most of the light continues in its original direction. However, some of the light is scattered into new directions. A careful analysis of the scattered light can yield detailed information about the scattering system. blue sky and clouds © Wyatt Technology Corporation All Rights Reserved
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Why is the sky blue? What Makes a Red Sunset?
Sunlight reaches Earth's atmosphere and is scattered in all directions by all the gases and particles in the air. Blue light is scattered in all directions by the tiny molecules of air in Earth's atmosphere. Blue is scattered more than other colors because it travels as shorter, smaller waves. This is why we see a blue sky most of the time. As the Sun gets lower in the sky, its light is passing through more of the atmosphere to reach you. Even more of the blue light is scattered, allowing the reds and yellows to pass straight through to your eyes.
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Today’s Assignment: Conceptual Physics Book
Read the sections and complete the end of chapter Check Concepts questions for the following sections: 28.1 through Q#2, 6,& 8 28.8 through Q#16-20
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