VIEWS: 90 PAGES: 24 POSTED ON: 2/19/2010
Light Waves • What we call light is a small portion of the electromagnetic spectrum • All the different colors are electromagnetic waves with different wave lengths • Wave speed = frequency x wavelength • EM radiation is alternating electric and magnetic fields Electromagnetic Waves • Remember our field induction rules • Changing magnetic/electric field induces electric/magnetic field Electromagnetic Spectrum • Remember, the lower the frequency the longer the wavelength and vice versa • Wave speed of light is 300,000,000 m/s Colors • Lowest frequencies of visible light are red • Highest frequencies are violet • The order is red, orange, yellow, green, blue, violet Producing EM Waves • Need to vibrate charged particles • We shake electrons in an antenna to make radio and TV waves • We shake electrons in a magnetron to make microwaves • We shake electrons in atoms to make light (this is a bit of a simplification which we will correct later this semester Light and Materials • Electromagnetic Waves carry energy • When light strikes a material, it vibrates the lightweight electrons Light and Materials Light and Materials • If the natural vibration frequency of the electrons is close to the frequency of the incoming light wave, the electron will be set into big vibrations (resonance) • It will absorb some energy from the EM wave and will vibrate with larger amplitude • It can smack into the electrons in neighboring atoms and vibrate them too Light and Materials • This cuts down on the intensity of the light • If all the light is absorbed by shaking electrons we say the material is opaque • If the light frequency doesn’t come close to the natural frequencies of the electrons, they shake a bit, but just re- emit the energy • Materials that can’t hold the energy are Light and Materials • There is a delay in transparent materials between the time the electrons start shaking and then re-emit the energy • This means that the light travels more slowly in these materials!!! • We will use this fact later to do interesting things with lenses Light and Materials See why the light travels more slowly in a transparent material than in a vacuum? Light and Materials • Infrared EM waves shake entire atoms or molecules in materials • Microwave EM waves effect the rotation of atoms about each other + Remember our discussion of water and how it has a positive end by the hydrogen atoms and a negative end by - the oxygen Light and Materials • Things that are opaque absorb the energy carried by the light wave • This means that they heat up • Metals are opaque and shiny • The free electrons close to the surface shake and re-emit the energy back out of the material • They are reflectors!!! Light and Air • Our atmosphere is transparent to visible light, some ultraviolet light and some infrared light • The ozone layer absorbs a lot of ultraviolet light • The ozone molecule is O3 and breaks into a regular oxygen molecule O2 and an oxygen atom Colors • Colored objects are a result of selective reflection and absorption of light • Sunlight is a mixture of colors • We call this mixture white light • When white light strikes a red object, the red frequencies are reflected and the other frequencies are absorbed Sunlight Colors • White objects reflect all frequencies • Black objects absorb all frequencies • Objects can only reflect the colors of light that shine on them • If you shine blue light on a red object it will look black Colors • We can also consider shining light through semi-transparent objects • Filters used on the lights in a theater are materials that absorb some frequencies and let other frequencies pass through • Red filters transmit red light through the material while other frequencies are absorbed Mixing Colors • Our eyes and brains are very complicated color processing sensors and computers • See the discussion on page 289 in the text • Artists have learned how to mix colors to make pleasing combinations and to create a huge array of different colors • All based on what the sensors in our Visual Response Visual Response Red + Blue = Magenta Red + Green = Yellow Blue + Green = Cyan The way your computer screen makes colors. And your TV set does the same thing. Visual Response Red + Blue = Magenta Red + Green = Yellow Blue + Green = Cyan Magenta + Green = White Cyan + Red = White Yellow + Blue = White Selective Scattering • This is why the sky is blue! • The higher frequencies are scattered more by the molecules in the atmosphere • Reds and oranges just pass through, so the sky appears blue Selective Scattering Sunsets are red!
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