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Light

Chapter 16. Light. Light. The Ray Model of Light was introduced as a way to study how light interacts with matter Ray= a straight line that represents the linear path of a narrow bean of light Rays can change direction if reflected or refracted. Light Sources.

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Light

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  1. Chapter 16 Light

  2. Light • The Ray Model of Light was introduced as a way to study how light interacts with matter • Ray= a straight line that represents the linear path of a narrow bean of light • Rays can change direction if reflected or refracted

  3. Light Sources • There are MANY different sources of light but there are only two TYPES of sources • 1. Luminous Source = an object that emits light (such as the sun or a candle) • 2. Illuminated Source = object that becomes visible as a result of the light reflecting off it (such as the Moon)

  4. Properties of Light • The illuminance produced by a point source is proportional to 1/r2 (the inverse square law)

  5. More “Stuff” • Speed of Light (c) = 3.00 x 108 m/s • Diffraction = the bending of light around a barrier

  6. Electromagnetic Spectrum • As the wavelength of visible light decreases, the color changes from red to violet • As wavelength decreases, the frequency increases, and the energy of the wave increases

  7. Electromagnetic Spectrum

  8. Primary Colors of Light • Primary colors of light = red, green, and blue • Secondary colors=yellow, cyan, and magenta

  9. Color • Complementary colors = 2 colors of light that can be combined to make white light • Objects appear a certain color because they reflect that color light and absorb all the others

  10. Polarization • Polarization is the production of light in a single plane of oscillation

  11. Doppler Shift • Doppler Shift= the difference between the observed wavelength of light and the actual wavelength • A positive change in λ = red shift • The relative velocity of the source is away from the observer • A negative change in λ = blue shift • The relative velocity of the source is towards the observer

  12. Doppler Shift • Stellar motion • take the spectrum of a star • compare observed wavelengths of absorption lines to lab values (H, Fe, Na, etc.) • calculate star’s radial motion (need distance and tangential angular motion to get space motion) • NO, you won’t have to calculate this!!!

  13. Reflection • Reflection is the change in direction of a wave at an interface between two different media so that the wave returns into the medium from which it originated. • Law of reflection: the angle of reflection=the angle of incidence • θr=θi

  14. Reflection • Specular reflection = when light hits a smooth surface the rays are reflected in parallel • Diffuse reflection = when light hits a surface that is rough (on the level of the wavelength of light) the light scatters

  15. Reflection • Reflected rays of light that enter the eye appear to originate at a point behind the mirror • Virtual image= a type of image formed by diverging light rays • Always on the opposite side of the mirror from the object

  16. Refraction • Refraction= the bending of light as it passes into a new medium • Index of Refraction= the ratio of the speed of light in a vacuum to the speed of light in that medium • The index of refraction determines how much the light bends/refracts

  17. Total Internal Reflection • Phenomenon that occurs when light traveling from a region of a higher index of refraction to a region of lower index of refraction strikes the boundary at an angle greater than the critical angle such that all light reflects back into the region of higher index • Critical angle = the angle of incidence above which total internal reflection occurs • This is how fiber optic cables work

  18. Total Internal Reflection

  19. A. Images • Light reflecting from an object to your eye • Real image • When light rays converge to form an image • Virtual image • An image your brain perceives though no light passes through it

  20. C. Plane Mirrors • Flat, smooth, reflecting surface • Upright image • Image is same distance as you are from mirror • Image is virtual

  21. D. Concave Mirrors • Surface of mirror is curved inward • Forms real and virtual images • Distance of object from mirror determines size and type of image formed • Object is closer than focal length upright virtual image • Object is further than focal length  upside down real image

  22. E. Convex Mirrors • Curves outward like the back of spoon • Forms a virtual image • Image is upright • Image is smaller than actual object

  23. F. Lenses • Transparent material with at least one curved surface that refracts light rays

  24. G. Concave Lenses • Thinner in middle and thicker at edges • Rays diverge • Image is virtual, upright and smaller • Used in some eyeglasses and telescopes

  25. H. Convex Lenses • Thicker in middle and thinner at edges • Refracts rays toward center of lens • Rays converge • Image depends on location of object

  26. Wave-Particle Duality of Light

  27. Atoms and light-Bohr Model • Electrons only orbit in certain shells. • Electrons jump from shell to shell when the atom absorbs and gives up energy. • The ground state is the state the electron is in when it has the smallest allowable amount of energy • The excited state is any energy level about the ground state, where the electron has more energy • When the electron transfers from a higher energy level to a lower energy level energy is given off in the form of light • This is how we get the emission spectra

  28. Energy a la Einstein • Mass can be converted into energy with a yield governed by the Einstein relationship: E=mc2 • E=energy (Joules) • M=mass (kg) • c=speed of light

  29. Particle Model of Waves • The idea of light being simply a wave caused problems for physicists because the wave nature of light could not explain several important phenomenon • The absorption and emission of electromagnetic radiation could not be explain using this “wave nature” • These phenomena would eventually be explained by the “particle nature” of light

  30. Photoelectric Effect • Heinrich Hertz first observed this photoelectric effect in 1887. • Hertz had observed that, under the right conditions, when light is shined on a metal, electrons are released. • Light falling on a metal can cause electrons to be ejected from the metal. This is known as the photoelectric effect.

  31. Photoelectric Effect • Einstein proposed that the energy in the light was not spread uniformly throughout the beam of light. Rather, the energy of the light is contained in "packets" or quanta • He said that each quanta has a specific amount of energy found by E = h f • h is Planck's constant 6.62606957(29)×10−34 • f is the frequency of the light. • From the conservation of energy, we would expect the electron to leave with kinetic energy KE given by KE = h f – W

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