2 Electromagnetic waves Electromagnetic Radiation is just a name for the range of radiation….(feel free to start singing)Light is both a particle and a waveLight particles are called photons (mass-less bundles of concentrated electromagnetic energy)We only see light in the range of nmspeed of light = c = 3.0*108 m/sc = λ*f
4 Why are some things see-Through? Light is energy carried in an electromagnetic 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.
5 TransparencyIf frequency of the light ≠ natural frequency of the material electrons forced into vibration with small amplitudesThe atom holds the energy for less time less chance of collision with neighboring atoms less energy transferred as heatEnergy of the vibrating electrons reemitted as transmitted lightMaterials that transmit light are transparent.The response when light hits the material depends on the frequency of the light and the natural frequency of electrons in the material.
6 OpacityIf frequency of the light = natural frequency of the material electrons forced into vibration with large amplitudesThe atom holds the energy for more time more chance of collision with neighboring atoms more energy transferred as heatMaterials that absorb light without reemission and thus allow no light through them are opaque.
7 Glass: An example Glass is _______________ to visible light. BUT…Electrons in glass have a natural vibration frequency in the ultraviolet rangeUV 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 heatUV light does not pass through glassGlass is ____________ to UV light.
9 Glass: Example Continued 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 heatEnergy of the vibrating electrons is reemitted as transmitted light.
10 Speed of Light in Materials In a vacuum, the speed of light is a constant3 x 108 m/sAtmosphere: very close to cWater: 0.75cGlass: 0.67cDiamond: 0.40cWhen light emerges from these materials into the air, it travels at its original speed, cLight travels at different average speeds through different materials
11 Shadows 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 pathA shadow is formed where light rays cannot reachFuzzy part around the edges of the shadow happens when:Light from one source is blocked but where other light fills in orWhere light from a source is only partially blocked
12 PolarizationWhen light from a lamp or the sun shines on a polarizing filter, the light that is transmitted is polarizedLight will pass through a pair of polarizing filters when their polarization axes are aligned, but not when they are crossed at right angles.
13 3DHold an upright finger at arm’s length and see how it switches position relative to the background as you alternately close each eye.Vision in three dimensions depends on the fact that both eyes give impressions simultaneously, each eye viewing a scene from a slightly different angle.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.
14 3d Movies!Movies project the pair of views through polarization filters onto a screen.Their polarization axes are at right angles to each other, so the right eye sees only the right view and the left eye sees only the left view.Overlapping pictures look blurry to the naked eyeTo see in 3-D, the viewer wears polarizing eyeglasses with the lens axes also at right anglesEach 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
15 Light Part II: Color (and the electromagnetic spectrum)
16 And on to….Color: The Spectrum 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 rainbowspectrum = spread of colorsROYGBVwhite light = combo of all the colorsblack = absence of light
17 Why do we see different colors? The color of an opaque object is the color of the light it reflectsThe color of a transparent object is the color of the light it transmitsMost materials absorb light of some frequencies and reflect the rest.Material absorbs light of most visible frequencies & reflects red the material appears redReflects light of all the visible frequencies it will be the same color as the light that shines on itAbsorbs all the light that shines on it it reflects none and is black
18 SunlightWhite light from the sun is a composite of all the visible frequencies.The brightness of solar frequencies is unevenLowest frequencies of sunlight in the red regionNot as bright as those in the middle-range yellow and green regionYellow-green light is the brightest part of sunlight.
19 Colored lightLight of all the visible frequencies mixed together produces whiteWhite also results from the combination of only red, green, and blue lightR, G, B called additive primary colorsRed + Green light = YellowRed + Blue light = MagentaGreen + Blue light = Cyan
20 Colored pigments 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 absorbsExample:Blue paint reflects mostly blue light, but also violet and greenIt absorbs red, orange, and yellow light.Yellow paint reflects mostly yellow light, but also red, orange, and greenIt absorbs blue and violet light.When blue and yellow paints are mixed, between them they absorb all the colors except green
21 Fun things color helps us understand 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?
22 Atomic SpectraElectrons surrounding the atomic nucleus have well-defined orbitsAKA – well defined energy levels—lower energy near the atomic nucleus and higher energy farther from the nucleusAtom 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 photonWhen made to emit light, every element has its own characteristic color (the “fingerprints” of the elements)
23 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 spectrumEach line corresponds to a frequency of lightSpectral lines seen in the spectroscope are images of the slit through which the light passesMuch of the information that physicists have about atomic structure is from the study of atomic spectraThe atomic composition of common materials, the sun, and distant galaxies is revealed in the spectra of these sources.Even the element helium, the second most common element in the universe, was discovered through its “finger- print” in sunlight.