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Light and Color Chapters 16, 17, 18, & 19 GPS Standards Students will analyze the properties and applications of waves. a. Explain the processes that result in the production and energy transfer of electromagnetic waves. b. Experimentally determine the behavior of waves in various media in terms of reflection, refraction, and diffraction of waves. c. Explain the relationship between the phenomena of interference and the principle of superposition. d. Demonstrate the transfer of energy through different mediums by mechanical waves. e. Determine the location and nature of images formed by the reflection or refraction of light. Journal # What is light? Write a paragraph describing your typical day without light, and what you would need to do in situations where light was necessary. Consider how the absence of light would impact your surroundings as well. Light… is the only thing we can see! has a dual personality - it can behave as both a particle and a wave. travels faster than anything else in the universe. can travel in a vacuum, unlike sound! Light and the Electromagnetic Spectrum Visible Light is an electromagnetic wave that stimulates the retina of the eye. Its wavelengths are between 400nm (violet) and 700nm (red). The Electromagnetic Spectrum is the full range of wavelengths and frequencies at which all electromagnetic radiation exists and the corresponding names that we give to certain “zones”. The Electromagnetic Spectrum Light speed – History of Discovery In a vacuum, light has a speed of 3.00 x 108m/s (symbol c) Before 17th century, people believed light traveled instantaneously or that speed of light was too fast to be measured. Roemer used the orbit of Jupiter’s moon, Io, and the orbit of Earth around the sun to first measure the speed of light in 1674. Michelson, using a specially designed experiment sending light between two mountains, more closely estimated the speed of light in 1926. His measurement is almost the exact same that we use today! Categories of Materials based on absorption or reflection of light Transparent - allow most light to pass through them. Translucent - allow some light to pass through, but some is absorbed and some is reflected. Opaque - do not allow any light to pass through, but instead absorb or reflect all light. Colors of Light White light is a combination of the spectrum of colors, each having different wavelengths. When combined, the three primary light colors of red, blue, and green will produce white light. In other combinations, they will produce other colors. Primary Additive Colors for Light Red, Green and Blue – combine to make white light Primary Pigments The three secondary colors of light are also known as the primary pigments – cyan, magenta, and yellow – and are used in pigments and dyes to produce a wide variety of colors. Why do we see colors? Pigments reflect color of light that we see. So… a shirt that has been dyed to appear red is only red because it reflects mostly red light and absorbs most of the other two primary light colors of blue and green. When mixed, the primary pigments create the secondary pigments (red, blue, and green). What’s the difference between light and pigments? Unlike light, pigments are referred to as subtractive colors. For instance, when you mix the primary pigments of yellow and cyan, the color of green is what you will see. That is because green pigment absorbs the light with colors of blue and red and reflects back only green. Thin Films Colors in soap and oil films are caused by the interference of specific wavelengths of light reflected from the front and back surfaces of the thin films. The colors actually show up because an antinode (constructive interference) is formed. Spectrum Diffraction Patterns on a CD The metallic coatings on CDs are less than 100nm thick Each coating partially reflects and partially transmits incident light. Light rays reflected from different coating boundaries interfere with each other to produce the colorful patterns Diffraction Bending of light around a barrier Diffraction pattern – a pattern of bright and dark bands produced by constructive and destructive interference; white light shows all the colors of spectrum Diffraction gratings (ex: goggles used in 1st lab) are devices made up of many single slits that bend light and form diffraction patterns See fig. 19-14 p. 528 red light versus white light Polarized light Normally, light (just like all electromagnetic radiation) vibrates in two dimensions as it travels. Polarized light consists of waves vibrating in a particular plane. Polarization is done by using VERY small filters (slits) that block vibrations from other planes. Application of Polarization – sunglasses polarized to reduce glare reflected off water or off the road Polarized Light The speed of light Speed of light in a vacuum is equal to a constant c, which equals 3.00 x 108 m/s Rearrange the speed of light formula to find wavelength of a light wave by dividing c by frequency of that light. c/ f We can also rearrange the formula to find frequency, if we know the wavelength and speed. Try p. 447, #14 and #8 p. 455 (Note that a nanometer is 10-9 m) c f Answers to practice problems P. 447 #14. 5.85 x 1014 Hz P. 455 #8. 7.43 x 1014 Hz Relative Speed of Light and the Doppler Effect with Light Doppler Effect with light determined based on relative speed Relative speed – magnitude of the difference between the velocities of the light source and the observer of the light Blue-Shifted or Red-Shifted? Blue-shifted - When light source approaches observer, there is an increase in measured frequency, so wavelength of light shortens. Called blue-shifted because the increase is towards the higher frequency (or blue) end of the color spectrum. (Ex: side of a star turning towards us as it spins) Red, green and blue channels represent the red-shifted and blue-shifted motions of the ionised material in the halo. The positions of the two galaxies C11 and C15 are marked. Red-Shifted? Red-shifted - When a light source recedes (goes away) from observer, there is a decrease in measured frequency, so wavelength lengthens. Called red-shifted because the decrease is towards the lower (or red) end of the color spectrum (Ex: the side of a star spinning away from us; distant galaxies show a red shift as they move away) How do scientists analyze light? A Spectroscope (containing a diffraction grating) is used to measure light from glowing elements and separate that light into its various frequencies. It is able to measure Doppler shift of the wavelengths of the light from elements. What does a spectroscope do? Helps chemists determine the elemental composition of heated gases or materials using the spectra they emit. Astronomers can use spectra seen to determine what elements make up the stars and galaxies and to see if they are moving towards (blueshifted) or away (red-shifted) from us. Chapter Review Questions Answer the following 19 questions: p. 438 #7 and 12 (hint: v = d/t - rearrange) P. 447 : #18 – 22 (4 questions) 13 chapter review questions on page 452-454: #25, 33, 35, 36, 37, 38, 39, 43, 48, 51, 54 (hint: remember v = d/t ), 62, and 68 (just explain in general terms, no need to solve)