Lecture 4: Light and Matter
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42 terms
Terms | Definitions |
|---|---|
 4 ways in which light can interact with matter | emission, absorption, transmission, reflection/scattering |
| emission | matter releases energy as light |
| absorption | matter takes energy from light |
| transmission | matter allows light to pass through it |
| reflection/scattering | matter repels light in another direction |
| Power | rate at which energy is used/emitted. aka luminosity. measured in Watts. |
| Watts | 1 joule per second. ex. a 100 watt light bulb radiates 100 joules of energy every second |
| Force Field | describes the strength of a force that an object experiences at any point in space |
| electric fields | surround objects with electric charges |
| Magentic Fields | surround electrical currents (i.e. moving charges) |
| an electrical field | a changing magnetic field produces |
| magnetic field | changing electrical field produces |
| electromagnetic fields | (self regenerating fields) the changing magnetic and electrical fields continue to produce each other until some object intervenes. |
| Examples of Electromagnetic Fields | Radiowaves, microwaves, visiblelight, X-rays andγ- rays |
| Light | A vibration in an electromagnetic field through which energy is transported. Can be thought of as a wave, or as a particle. Has dual nature. |
| Wavelength | distance from one peak to the next |
| Frequency | Number of peaks passing by any point each second. Units are cycles/sec or hertz |
| Amplitude | The height of each peak |
| Speed | the rate at which peaks move |
| wave | pattern revealed by interaction with particles i.e. ripple traveling on surface of water |
| fundamental equation of waves | speed=f x λ |
| speed of light | 3x10^5km/s |
| short wavelength = | high frequency (violet) |
| long wavelength = | low frequency (red) |
| photons | packets of energy. The energy carried by each photon depends on its frequency (color). E = hf = hc / λ ["h" is called Planck's Constant] h=6.626x10-34 joule x s. Bluer light carries more energy per photon |
| Left of Electromagnetic Spectrum | shortest wavelength, highest frequency, and highest energy per photon found at the end. 400 nm |
| Right of Electromagnetic Spectrum | longest wavelength, shortest frequency, and lowest energy per photon found at this end. 700nm |
| temperature, composition, velocity | By studying spectrum of an object we can learn its |
| Blackbody | ideal emitter, which absorbs all incident radiation and reradiates this energy with a characteristic spectrum. No electromagnetic radiation passes through, and none is reflected. Spectrum depends only on T, the temperature to which object is heated. |
| Glass | light goes right through, not a blackbody |
| shiny metallic surface | light gets reflected, not a blackbody |
| piece of soot | black material, light gets almost completely absorbed and material heats up, blackbody |
| continuous spectra | has energy at all wavelengths. most important example thermal (blackbody) spectrum. |
| blackbody | most astronomical objects emit spectra that is nearly? |
| Thermal Radiation | has specific spectrum shape (color) that depends on T. Has certain normalization (brightness). Isotropic. |
| Isotropic | same emission in all directions |
| 7000k (thermal spectra) | all colors appear brighter, but blue is brightest, objects look blue |
| 6000k (thermal spectra) | all objects appear the same brightness, objects appear white |
| 5000k (thermal spectra) | all colors fainter, but red is brightest, objects appear red |
| Rules for Emission for Opaque Objects | Hotter objects emit bluer photons (with a higher average energy). -Wiens Law Hotter objects emit more total radiation per unit surface area (each square meter of hotter object's surface emits more light at all wavelengths) - Stefan Boltzman Law |
| Wiens Law | λmax =2.9x10^6 /T(K) nm |
| Stefan Boltzman Law | F = σT^4 [J/s/m^2] |
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