# Light

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## Light

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##### Presentation Transcript

1. Light
2. Outline Wave characteristics of light The electromagnetic spectrum Atmospheric absorption Doppler shift Light scattering: Why is the sky blue? Spectroscopy
3. Light as a Wave Light (orelectromagnetic radiation), can be thought of as either a particle or a wave. As a wave, light has a wavelength,  (distance between waves) a frequency, (number of waves passing you each second) a speed, c =  (this is always the same: 300,000 km/s) an energy, E = h  (where h is just a constant) Note that because the speed of light is a constant, , , and E are linked: if you know one, you know the other two.
4. The Electromagnetic Spectrum
5. Atmospheric Windows Not all light from space makes it through the earth’s atmosphere. In fact, only visible light, radio waves, and some infrared light makes it to the ground. The rest of the electromagnetic spectrum can only be observed from space.
6. The Doppler Shift The wavelength emitted by an object is not always the wavelength you observe. If you are moving towards an object, you will see more waves per second (i.e., a higher frequency, like swimming upstream). The light will appear bluer and be blueshifted. Conversely, if you are moving away from an object, its light will be redshifted. v  c The faster the relative motion, the larger the red or blue shift.
7. The Doppler Shift The wavelength emitted by an object is not always the wavelength you observe. If you are moving towards an object, you will see more waves per second (i.e., a higher frequency, like swimming upstream). The light will appear bluer and be blueshifted. Conversely, if you are moving away from an object, its light will be redshifted. v  c The faster the relative motion, the larger the red or blue shift.
8. Inverse Square Law of Light Photons emitted from a source will spread out in all directions at the speed of light. Since the amount of area surrounding a source increases as the distance squared, the density of photons will decrease as 1 / d2. This is theinverse square law of light.
9. Inverse Square Law of Light
10. Scattering of Light Dust in the Earth’s atmosphere (or in space) can scatter light. In general, short wavelength (blue) light gets scattered more than red light. That’s why the sky is blue.
11. Scattering of Light Dust in the Earth’s atmosphere (or in space) can scatter light. In general, short wavelength (blue) light gets scattered more than red light. That’s why the Sun is red at sunset. The long path through the atmosphere means all the blue photons are scattered away.
12. 1st Type of Radiation: Blackbody (or Thermal) Anything that is hot (i.e., above absolute zero) produces light at all wavelengths – acontinuous spectrum. But the amount of light given off at each wavelength is very sensitive to the object’s temperature. For hotter objects: Peak intensity shifts to shorter (bluer) wavelengths peak1/T more light is created Total energyT4 http://astro.unl.edu/naap/blackbody/animations/blackbody.html
13. 1st Type of Radiation: Blackbody (or Thermal)
14. Blackbody Spectra
15. Electrons live in orbitals that have specific energies. E4 E3  E2 E1 E1  E2 E3 E4
16. The orbital with the lowest energy is called the ground state. - E4 E3  E2 E1 - E1  E2 E3 E4
17. A collision with another atom can cause the electron to jump to a higher level, called an excited state. - - E4 E3  - E2 E1 E1  E2 E3 E4
18. An electron in an excited state can spontaneously drop to one of the lower levels, releasing a photon. - - E4 E3  E2 - E1 E1  E2 E3 E4
19. The photon’s energy equals the difference between the energies of the old and new levels. Ephoton = E3-E2 - - E4 E3  E2 - E1 E1  Ephoton = E3-E2 E2 E3 E4
20. Transitions between different pairs of levels produce photons at different energies (or wavelengths). 4-1 3-1 2-1  E1 E2 E3 This is an emission line spectrum E4 3-1 4-1 2-1
21. Different elements and molecules have different energy levels, so they produce emission lines at different wavelengths.    E1 E1 E2 E2 E3 E4 E3 E4
22. Emission Line Spectra Since every element has a different set of atomic orbital energies, the emission line spectrum of every element is different. They are as unique as fingerprints!
23. 2nd Type of Radiation: Emission Line
24. 2nd Type of Radiation: Emission Line
25. 2nd Type of Radiation: Emission Line
26. To produce emission lines, a gas needs to have electrons that are excited to the higher levels. One way to excite electrons is heating the gas since this increases collisions between atoms.
27. There’s a second way of exciting an electron to a higher level. Begin with a source of light, a blackbody producing a continuous spectrum in this example.
28. Shine the light on a cloud of gas (or a single atom). -  E1 E2 E3
29. If one of the approaching photons has the exact energy needed to excite the electron to a higher level (E2-E1, E3-E1, etc.), then the electron can absorb that photon and jump to a higher level. -  E1 E2 E3
30. Most of the light passes through the gas, but the photons that were absorbed by electrons are now missing. These missing photons in the spectrum are called absorption lines. - E2-E1  E1 E2 E3
31. Absorption Line Spectra
32. Absorption Line Spectra An absorption line spectrum is produced when light passes through a cloud of gas.
33. Absorption Line Spectra from the Sun
34. Continuous, Emission, and Absorption Spectra