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---------------------- 2723 - SPECTROSCOPY - the mystery of light?
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- We can use frequencies and wavelengths interchangeably because they are related by a constant factor, the speed of light is that constant. The product of frequency and wavelength = speed of light, f *w = c. Wavelengths per second times distance per wavelength = distance per second = a constant velocity of 186,282 miles per second in space.
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- The spectrum of visible light is the familiar rainbow of colors. It is called a continuous spectrum because it has no interruptions. Other types of spectrum are emission and absorption spectrums.
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- A cloud of gas does not produce a continuous spectrum. Rather its light is emitted only at specific wavelengths that depend on the elements in the gas and the temperature of the gas.
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- The third type of spectrum is the absorption spectrum. If a continuous spectrum of light is behind the gas, when the light travels through the gas specific wavelengths are absorbed by the elements in the gas. These show up as dark absorption lines in the continuous spectrum.
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- Each element’s atom, or molecule possesses a unique set of electrons with a unique set of energy levels. When a gas is hot, or at any temperature really, the atoms are bouncing around exchanging energy. Sometimes an electron gets bumped up to a higher energy level. When it falls back to its ground level state it emits a photon of light.
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- The energy loss from the electron must go somewhere for the Conservation of Energy. The photon always carries the exact amount of energy lost. And , since light energy is a function of frequency, it will emit a specific frequency for that exact amount of energy.
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- For example: An electron that falls from level 2 to level 1 in its orbits about the nucleus and emits an ultraviolet photon at 121.6 nanometers. An electron that falls from level 3 to level 2 in its atom emits a red-visible photon at 656.3 nanometers.
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- This same effect of the transition of energy levels is reversed when light enters a gas and the energy is absorbed to boast electrons into higher energy levels. With these emission and absorption spectrum lines each element has a unique chemical fingerprint.
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- All of these signatures have been carefully studied in laboratories. So, when astronomers see these same chemical fingerprints out in the Universe they recognize the elements that created them.
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- Temperature is simply a measure of the average kinetic energy of the atoms or molecules in an object. For large, dense objects, the photons do not easily pass through like they do with a gas. Photons get randomly absorbed or bounced around inside a dense object so many times that they eventually end up at the average kinetic energy which is the object’s temperature.
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- That is why the continuous spectrum emitting from such an object depends ONLY on its temperature. ( However, if the object is in motion the “Doppler effect” can also effect its spectrum.)
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- There are two laws governing spectrums that are affected by temperature:
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------------- (1) Each square meter of a hotter object’s surface emits more light at all wavelengths.
------------- (2) Hotter objects emit photons with higher average energy. Higher energy always means higher frequency and shorter wavelengths.
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- All bodies follow these two laws. Your body is 310 Kelvin and emits mostly infrared photons and no visible light. You do not glow in the dark, unless someone is watching you with an infrared camera.
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- A cool star is 3,000 Kelvin and emits mostly red light. Our Sun is a hot star at 5,800 Kelvin and emits a strong green light at 500 nanometers wavelength. The Sun does not look green to us because the light gets filtered by our atmosphere and our eyes see the whole visible spectrum making it appear yellow or white.
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- The hottest stars emit mostly in the ultraviolet light that we can not see and appear as blue-white to our eyes.
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- Known as the Stephen - Boltzman law: each square meter of a hotter object’s surface emits more light at all wavelengths to the forth power
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---------------- Energy emitted per square meter of surface = a constant * Kelvin^4
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---------------- Energy = 5.7*10^-8 watts / (m^2 * Kelvin^4) * (temperature)^4
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- To determine the total energy emitted for the object multiply this number by the surface area of the object.
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- Known as Wien’s law: hotter objects emit photons with higher average energy. Higher energy always means higher frequency and shorter wavelengths. The wavelength where the energy intensity is at its maximum is “inversely” related to temperature. Remember as energy increases wavelengths get shorter.
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----------------- Maximum Intensity Wavelength = the constant 2,900,000 / temperature
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----------------- Maximum Intensity Wavelength = 2,900,000 nanometers / Kelvin
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- With these two laws astronomers can measure the temperature and the size of the stars. We measure the spectrum of light from the star. The spectrum’s intensity peaks at 190 nanometers in the ultraviolet. From this wavelength at maximum we calculate the temperature to be 15,000 Kelvin. A very hot star.
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- With the temperature of 15,000 Kelvin we can calculate the energy emitted per square meter to be 2,900,000,000 watts per square meter.
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- We measure the stars total radiated power in watts and now we can calculate its total surface area in square meters and therefore its size.
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- If we find a planet its reflected light gives it its color. The absorbed light heats its surface and determines the surface temperature. If we look at the spectrum reflected off the planet Mars we see ultraviolet emission lines indicating it has a hot upper atmosphere.
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- Most of the blue light is absorbed by the surface of Mars making it appear red because the red light is mostly reflected. CO2 absorption lines indicate there is carbon dioxide in the atmosphere. An infrared peak of thermal emission indicates the surface of Mars is 225 Kelvin.
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- Neptune is colder than Mars and appears blue to our eyes. We are getting closer to being able to get a spectrum from the planets in other solar systems. Over 3,000 planets have been discovered to date using Doppler and Transit indirect methods. Soon a direct image will be made with our newer telescopes coming on line. As soon as we can get a spectrum of light we can learn a great deal more about these exoplanets.
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- April 26, 2020 843 2723
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--------------------- Sunday, April 26, 2020 -------------------------
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