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midterm 2

# midterm 2 - LIGHT 18:10 We emit infrared light(heat and...

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LIGHT 18:10 We emit infrared light (heat) and reflect visible light L =4piR^2sigmaT^4 Smallest unit of light called a photon – has specific wavelength and specific energy Wavelength – distance from peak to peak, called gamma Light of different colors corresponds to photons of different wavelength All light travels at the “speed of light”, regardless of its wavelength In a vacuum, that speed is 3 x 108 meters/second Use the letter “c” to represent the speed of light in equations Frequency – the number of peaks that go by a point per second f = c / λ Light actually behaves as both a particle and a wave at the same time called the wave-particle duality of light Energy of a photon E is: Energy= E =h c/λ= h f H = planck’s constant Units in joules (J) Long wavelength photons have small energies shorter wavelength photons have more energy and X-ray’s and Gamma ray’s have the most energy

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White light is really a mixture of all colors (wavelengths) of visible light Matter can: emit light -- the element of a light bulb absorb light -- a black cloth reflect scatter light – a white cloth
Planck curve describes how a body emits light according to its temperature Cold bodies emit light at infrared or radio wavelengths, warmer bodies emit more light at all wavelengths Wien’s Displacement Law –describes the wavelength of maximum emission of the Planck Curve as a function of temperature. λmax = b / T λmax is the wavelength of the peak and b = 2.9 x 10-3 meters-K Emission = σ T4 σ = 5.67 x 10-8 watts/m2/K4 Watts/m2 = Joules/sec/m2 Stephen-Boltzman Law Luminosity = surface area x σ T4 L = 4π R^2σ T4 for spherical bodies, like planets Watts = joules/sec Apparent brightness = b = L/ (4 π d2) Matter can absorb light, however no type of material uniform absorbs light of all wavelengths so the interaction typically is a combination of reflection, absorption, and transmission

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In many situations, the absorption spectrum is the complement of the reflection spectrum We see the light that is reflected so the color of the wavelengths of the reflected light is the color that we assign to the object Transmission , which is the lack of absorption, occurs when the matter does not interact with light at a particular wavelength. In general, any material is only transparent over a range of wavelengths. There are three broad types of spectra: continuous spectra Generally thermal emission from a solid body. It is characterized by emission over a wide range of wavelength without detailed structure. absorption line spectra Absorption features arise when a continuous spectrum is seen through material that absorbs light at discrete wavelengths. The simplest example is a source seen through a cloud of gas. emission line spectra Emission spectra arise when you view a gas without a continuous spectrum in the background.
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