# Coming to you in PowerPoint format, by request… Try to leave a light on When I’m gone Even in the daylight Shine on And when it’s late at night You can.

## Presentation on theme: "Coming to you in PowerPoint format, by request… Try to leave a light on When I’m gone Even in the daylight Shine on And when it’s late at night You can."— Presentation transcript:

Coming to you in PowerPoint format, by request… Try to leave a light on When I’m gone Even in the daylight Shine on And when it’s late at night You can look inside You won’t feel So alone

 Electromagnetic Radiation is just a name for the range of radiation….(feel free to start singing)  Light is both a particle and a wave  We only see light in the range of 400-700 nm speed of light = c = 3.0*10 8 m/s c = λ*f

 Light is energy carried in an EM wave that is generated by vibrating electric charges.  Visualize the electrons in an atom as connected by imaginary springs.  When light hits the electrons, they vibrate.  The natural vibration frequencies of an electron depend on how strongly it is attached to a nearby nucleus.

 If frequency of the light ≠ natural frequency of the material  electrons forced into vibration with small amplitudes  The atom holds the energy for less time  less chance of collision with neighboring atoms  less energy transferred as heat  Energy of the vibrating electrons reemitted as transmitted light  Materials that transmit light are transparent.

 If frequency of the light = natural frequency of the material  electrons forced into vibration with large amplitudes  The atom holds the energy for more time  more chance of collision with neighboring atoms  more energy transferred as heat  Materials that absorb light without reemission and thus allow no light through them are opaque.

 Glass is transparent to visible light  BUT…Electrons in glass have a natural vibration frequency in the ultraviolet range  UV light shines on glass  resonance occurs  e - forced into vibration with higher amplitudes  atoms hold energy for more time  collide with neighboring atoms  energy lost to heat  UV light does not pass through glass

 BUT…when the EM wave has a lower frequency than UV, as visible light does, e- are forced into vibration with smaller amplitudes  atom holds the energy for less time  less chance of collision with neighboring atoms  less energy is transferred as heat  Energy of the vibrating electrons is reemitted as transmitted light  Time delay results in a lower average speed of light through a transparent material

 In a vacuum, the speed of light is a constant 3 x 10 8 m/s  Atmosphere: very close to c  Water: 0.75c  Glass: 0.67c  Diamond: 0.40c  When light emerges from these materials into the air, it travels at its original speed, c

 Lab time!!!  Shadows and Polarization (a few things to know before you start):  A thin beam of light is called a ray  Shadows form where light rays cannot reach  EM waves are partly magnetic and partly electric

Coming to you in PowerPoint format (again), by request… A picture’s worth a thousand words But you can’t see what those shades of gray keep covered You should have seen it in color

 Electromagnetic Radiation is just a name for the range of radiation….(feel free to start singing)  Light is both a particle and a wave  We only see light in the range of 400-700 nm speed of light = c = 3.0*10 8 m/s c = λ*f

 Materials that transmit light are transparent.  Materials that absorb light without reemission and thus allow no light through them are opaque.  Why can you not get a tan through a glass window?

 A thin beam of light is called a ray  When light shines on an object, some of the rays may be stopped while others pass on in a straight line path  A shadow is formed where light rays cannot reach  Fuzzy part around the edges of the shadow happens when:  Light from one source is blocked but where other light fills in or  Where light from a source is only partially blocked

 When light from a lamp or the sun shines on a polarizing filter, the light that is transmitted is polarized  Light will pass through a pair of polarizing filters when their polarization axes are aligned, but not when they are crossed at right angles.

 Vision in three dimensions depends on the fact that both eyes give impressions simultaneously, each eye viewing a scene from a slightly different angle.  Hold an upright finger at arm’s length and see how it switches position relative to the background as you alternately close each eye.  The view seen by each eye is different.  The combination of views in the eye-brain system gives depth.  A pair of photographs or movie frames, taken a short distance apart (about average eye spacing), can be seen in 3-D when the left eye sees only the left view and the right eye sees only the right view.

 Movies project the pair of views through polarization filters onto a screen  Their polarization axes are at right angles to each other, and the overlapping pictures look blurry to the naked eye  To see in 3-D, the viewer wears polarizing eyeglasses with the lens axes also at right angles  Each eye sees a separate picture, just as in real life. The brain interprets the two pictures as a single picture with a feeling of depth

 By passing a narrow beam of sunlight through a triangular-shaped glass prism, Newton showed that sunlight is composed of a mixture of all the colors of the rainbow  spectrum = spread of colors  ROYGBV  white light = combo of all the colors  black = absence of light

 The color of an opaque object is the color of the light it reflects  The color of a transparent object is the color of the light it transmits  Most materials absorb light of some frequencies and reflect the rest.  Material absorbs light of most visible frequencies & reflects red  the material appears red  Reflects light of all the visible frequencies  it will be the same color as the light that shines on it  Absorbs all the light that shines on it  it reflects none and is black

 White light from the sun is a composite of all the visible frequencies.  The brightness of solar frequencies is uneven  Lowest frequencies of sunlight in the red region  Not as bright as those in the middle-range yellow and green region  Yellow-green light is the brightest part of sunlight.

 Light of all the visible frequencies mixed together produces white  White also results from the combination of only red, green, and blue light  R, G, B called additive primary colors  Red + Green light = Yellow  Red + Blue light = Magenta  Green + Blue light = Cyan

 Red + green + blue paint  muddy dark brown  Not white!  The mixing of paints and dyes is an entirely different process from the mixing of colored light.  When paints or dyes are mixed, the mixture absorbs all the frequencies each paint or dye in it absorbs  Example:  Blue paint reflects mostly blue light, but also violet and green  It absorbs red, orange, and yellow light.  Yellow paint reflects mostly yellow light, but also red, orange, and green  It absorbs blue and violet light.  When blue and yellow paints are mixed, between them they absorb all the colors except green

 How do color television sets work?  How is color printing done?  Why is the sky blue?  Why are sunsets red?  Why is water greenish blue?

 Electrons surrounding the atomic nucleus have well- defined orbits  AKA – well defined energy levels—lower energy near the atomic nucleus and higher energy farther from the nucleus  Atom absorbs external energy  one or more of its electrons is boosted to a higher energy level  ‘excited state’  Electron quickly drawn back to its original or a lower level  atom emits a pulse of light called a photon  When made to emit light, every element has its own characteristic color (the “fingerprints” of the elements)

 The light is analyzed using a spectroscope  The spectrum of an element appears not as a continuous band of color but as a series of lines called a line spectrum  Each line corresponds to a frequency of light  Spectral lines seen in the spectroscope are images of the slit through which the light passes  Much of the information that physicists have about atomic structure is from the study of atomic spectra

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