# Astronomy Picture of the Day. Why Can We See Each Other? Light emitted from other sources is reflected off of us. We don’t radiate in the visible part.

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Astronomy Picture of the Day

Why Can We See Each Other? Light emitted from other sources is reflected off of us. We don’t radiate in the visible part of the spectrum because our body temp is way too low! Also, the amount of radiation we emit is TINY compared to hotter objects like the Sun. (E α T 4 )‏ Body Temp ~ 99° F = 310 K

Why can’t we see the colors contained in white light? The colors “smear” together when they are all traveling to our eyes in the same direction - We can’t differentiate them because they reach our eyes all together all at the same time.

Kirchhoff's Laws 1. Continuous spectrum 2. Emission spectrum 3. Absorption spectrum a luminous solid or liquid, or a sufficiently dense gas, emits light of all wavelengths and produces a continuous spectrum a low-density, hot gas emits light whose spectrum consists of a series of bright emission lines that are characteristic of the composition of the gas. a cool, thin gas absorbs certain wavelengths from a continuous spectrum, leaving dark absorption lines in their place, superimposed on the continuous spectrum.

Question Which ONE of these is constant for all forms of EM radiation in a vacuum? A)amplitude B) wavelength C)frequency D)speed E)energy

Question A)It appears black, regardless of its temperature. B) It emits radiation in a continuum of wavelengths. C)Its spectrum peaks at a wavelength determined by its temperature. D)The total energy that it radiates increases rapidly with temperature. Which ONE is NOT a property of a blackbody?

Question A)It appears black, regardless of its temperature. B) It emits radiation in a continuum of wavelengths. C)Its spectrum peaks at a wavelength determined by its temperature. (Wien's Law: f peak_emission α T )‏ D)The total energy that it radiates increases rapidly with temperature. (Stefans Law: E α T 4 )‏ Which ONE is NOT a property of a blackbody? Intensity Frequency

Question You would expect the Sun's observed spectrum to be _____. A)A continuum with no lines, like the rainbow. B) A continuum with bright emission lines. C)Only absorption lines on a black background. D) Nearly a continuum with some absorption lines.

Spectrum of the Sun Complicated objects => many different elements Nearly continuous absorption spectrum What causes absorption of light at certain wavelengths?

Stellar Spectra Star Fusion generates continuous spectrum Sun's 'a tmosphere' absorbs specific wavelengths

When an atom absorbs a photon, it moves to a higher energy state briefly When it jumps back to lower energy state, it emits photon(s) in a random direction, conserving the total energy of the system. The reemission of the photon is a probabilistic process, but the “average” amount of time between absorption of the photon and reemission is ~10 -8 s.

Ionization + Hydrogen _ + _ + + Helium _ _ + + "Ion" Absorbing a high energy photon and atomic collisions can both lead to ionization. _ _

Questions How does the pitch or tone of a sound wave change when the source of the sound is moving towards or away from you? What about when you are moving towards or away from the source? Does this effect occur for all types of waves or just for sound waves?

The frequency or wavelength of a wave depends on the relative motion of the source and the observer. (Link to Demo)‏Link to Demo

Doppler Shifted Atomic Spectra Why don’t we see the color of everyday objects change as they move?

We've used spectra + doppler effect to find planets around other stars!

Star wobbling causes Doppler shift of its absorption lines. Only gives information about velocity along line of sight!

Now more than 150 extrasolar planets known. Here are the first few discovered.

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