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Light

Light. Introduction. Light. What is LIGHT? Where does it come from? How is it different than other waves?. What is Light?. Light is a wave, or rather acts like a wave. How do we know? Reflection Refraction Dispersion Diffraction Interference Polarization. What is Light?.

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Light

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  1. Light Introduction

  2. Light • What is LIGHT? • Where does it come from? • How is it different than other waves?

  3. What is Light? • Light is a wave, or rather acts like a wave. • How do we know? • Reflection • Refraction • Dispersion • Diffraction • Interference • Polarization

  4. What is Light? • Light is a special type of wave • What we know as light or VISIBLE LIGHT is actually a type of something called ELECTROMAGNETIC RADIATION. • ELECTROMAGNETIC WAVES are made up of a propagating electric and magnetic field. • Because the two fields are orientated perpendicular to each other EM waves are TRANSVERSE WAVES.

  5. Electromagnetic Waves

  6. Electromagnetic Waves • James Clerk Maxwell first proposed the idea of an EM wave. • The Maxwell Equations mathematically show that a changing ELECTRIC FIELD will create a MAGNETIC FIELD and vice versa. (Induction). • Thus changes induced by the E-Field will cause changes in the M-Field, and changes in M-field will cause changes in E-Field, and The EM waves propagates out forever.

  7. Electromagnetic Spectrum • What we perceive as “light” is actually called visible light and is a very small portion of the ELECTROMAGNETIC SPECTRUM.

  8. Electromagnetic Waves • Until the late 19th Century light was thought to require a medium like all waves. • Scientist proposed that the universe was filled with elementary particles called ETHER that provided light waves with a medium through which to propagate. • Eventually the idea of a “luminiferous ether” was disproven and quantum physics picked up where classical physics left off.

  9. Photon • Unlike mechanical waves, EM waves do not require a medium. In fact, they have their own. PHOTONS! • Photons are mass-less, charge-less elementary particles, that make up light and all electromagnetic radiation. • Photons are emitted and absorbed by electrons. • Where and how exactly they are created is still being debated by scientists

  10. Speed of Light • We know from last chapter that v = f x λ. • We also learned that the speed of the wave is constant in the same medium. • Because all electromagnetic radiation is made up of the same thing (photons) thespeed for all EM waves is constant. • The speed of light in a vacuum has been experimentally proven to be 2.9792458 x 108 m/s, but for us; c = 3.00 x 108 m/s

  11. Electromagnetic Spectrum • The speed of light will always be constant in a vacuum. • While that speed can change depending on the medium it can never be greater than c. • Theory of General Relativity • And since c = f x λ; all EM radiation will have a corresponding frequency and wavelength.

  12. Electromagnetic Spectrum

  13. Electromagnetic Spectrum • While radio waves are harmless to humans, EM radiation with a higher frequency such as gamma rays are tremendously dangerous. • Even X-rays can be fatal after long-term exposure. • Photons, as far as we know, are pure energy and our bodies are not built to absorb that kind of energy. Think fatal sunburn!

  14. Colors • “White light” or visible light is the small portion of the EM spectrum that we can perceive with our eyes. • White light can be separated into 7 colors (ROY G BIV). • When we see an object with a certain color we are not seeing an inherent property of that object but rather the reflected wavelengths that are not absorbed.

  15. Colors • The mnemonic device ROY G BIV was created by Sir Isaac Newton. • His “color wheel” originally had only five colors but he later added orange and indigo so that there were the same amount of colors as notes in a musical major scale; seven.

  16. Color • Newton performed experiments using narrow beams of sunlight passed through a glass prism. • Newton called the ordered arrangement of colors a spectrum. • Newton eventually concluded that white light was made of all colors of the spectrum.

  17. Color • What are the Primary Colors? • In fact, the primary colors according to science are different from those in the world of art. • The primary colors are actually; Red, Blue, and Green. • The secondary colors are; Yellow, Cyan, and Magenta. • From the three primary colors we can make any color in the visible spectrum.

  18. Color Color Addition Color Subtraction We see colors as a result of color subtraction. A pigment is a material that determines the reflected and absorbed light of an object. A primary pigment absorbs one primary color and reflects the other two A secondary pigment absorbs two primary colors and reflects only one. • We can form white light from colored light in a variety of ways. • Complimentary Colors are two colors of light that combine to form white light. • Red + Cyan • Green + Magenta • Blue + Yellow

  19. Color • Televisions and computer screens consist of dots/pixels that are either red, green, or blue. • The human eyes has cone cells/color receptors that are sensitive to red, green, and blue. • All the colors in our world are made up of combinations of these three colors.

  20. Color • Illuminated sources are visible because light is reflected off or transmitted through the object to you. • Opaque media only reflects and does not transmit any light (brick and wood). • Transparent media allow light to pass through easily (air and glass). • Translucent media transmit light but do not allow objects to be seen clearly through them (lampshades and curtains).

  21. Luminous Flux • The rate at which light energy is emitted from a source is called the luminous flux, (P). • The SI unit for luminous flux is the lumen (lm). • You can think of flux as the rate at which light waves come out of a source. • Flux is not dependent on distance from the source. • A typical 100-W lightbulb emits 1750 lm.

  22. Luminous Flux

  23. Illuminance • The intensity (Illuminance) of light is equal to the luminous flux of the light source incident on a surface. • Unlike flux, illuminanceis greatly dependent on the distance from a source. • Since light propagate out in a 3-D sphere the equation for illuminance is; • Illuminance is in lux (lx) which is equal to lumens/m2.

  24. Example Problem • What is the illuminance of your desktop if it is lighted by a 1750-lm lamp 2.50 m above your desk? • If public schools require an illuminance of 160 lx at the surface of each students desk and the school’s architect placed the lights 2.0 m above the desks. What is minimum luminous flux the lights must produce?

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