runny4
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http://fizx.wdfiles.com/local--files...2009%20QP1.pdf
For question 3bii in the paper above , in the mark scheme below it says (absorption line) atoms take in radiation and re-radiate in all directions. Why would you say re-radiate in all directions?

http://www.theredhillacademy.org.uk/...ark_Scheme.pdf
-Page 29 of mark scheme
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phyzard
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(Original post by runny4)
http://fizx.wdfiles.com/local--files...2009%20QP1.pdf
For question 3bii in the paper above , in the mark scheme below it says (absorption line) atoms take in radiation and re-radiate in all directions. Why would you say re-radiate in all directions?

http://www.theredhillacademy.org.uk/...ark_Scheme.pdf
-Page 29 of mark scheme

When an atom absorbs radiation you can think of it as an electron absorbing the photon and jumping to a higher energy state, but it must then find a way of restoring thermal equilibrium with its surroundings. So for this to happen the excited electron spontaneously re emits a photon and transitions back down to the lower energy state. This photon can be emitted into any direction and so for a black body which is emitting many of these photons, the light goes in all directions. I guess the markers want you to show understanding that the light emitted from a black body source is emitted in all directions, rather than a highly directional light source such as a laser.Hope this helps
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runny4
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(Original post by phyzard)
When an atom absorbs radiation you can think of it as an electron absorbing the photon and jumping to a higher energy state, but it must then find a way of restoring thermal equilibrium with its surroundings. So for this to happen the excited electron spontaneously re emits a photon and transitions back down to the lower energy state. This photon can be emitted into any direction and so for a black body which is emitting many of these photons, the light goes in all directions. I guess the markers want you to show understanding that the light emitted from a black body source is emitted in all directions, rather than a highly directional light source such as a laser.Hope this helps
But isn't light emitted from a black body source to do with a line spectrum and not an absorption spectrum
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phyzard
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(Original post by runny4)
But isn't light emitted from a black body source to do with a line spectrum and not an absorption spectrum
They are really not that different, in an absorption spectrum you are seeing what wavelengths of an object is absorbing, and in an emission (or line) spectrum you see what the black body is emitting. The point is that if you radiate a black body with light, it absorbs at certain wavelengths, giving you the lines you see in absorption spectrum. That light is then re radiated in all directions, and if someone else happened to measure that emission spectrum of the same object they would see the corresponding line.

To answer the actual question which is how is the absorption spectrum formed, I am guessing that they want you to be clear that we see a line because the object absorbed that bit of the light and it was re radiated in a different direction so that it doesn't ultimately end up in our spectrum again.

Its a hand-wavey explanation i know, hope it helps a little
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Joinedup
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(Original post by runny4)
http://fizx.wdfiles.com/local--files...2009%20QP1.pdf
For question 3bii in the paper above , in the mark scheme below it says (absorption line) atoms take in radiation and re-radiate in all directions. Why would you say re-radiate in all directions?

http://www.theredhillacademy.org.uk/...ark_Scheme.pdf
-Page 29 of mark scheme
I think this page explains it well http://www.atnf.csiro.au/outreach/ed...tro_types.html

for absorption see the diagram at the top - the spectrum we receive from the star is made of the photons that are traveling into our telescope and not at other directions - so if there is a lot of gas between us an the star, photons of certain frequencies which would otherwise be heading into our telescope are absorbed and re emitted in any direction, which reduces the number of photons of those frequencies going from the star into our detector.
this includes gas that is quite near to the star - the outer stellar atmosphere.

emission spectra are useful when you've got a gas in a nebula being ionised by something, the photons are emitted when the electrons re combine with a nucleus and then step down to lower energy levels. this will tell you the composition of the gas in the nebula and also that something is shining ionising radiation into it.
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runny4
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(Original post by Joinedup)
I think this page explains it well http://www.atnf.csiro.au/outreach/ed...tro_types.html

for absorption see the diagram at the top - the spectrum we receive from the star is made of the photons that are traveling into our telescope and not at other directions - so if there is a lot of gas between us an the star, photons of certain frequencies which would otherwise be heading into our telescope are absorbed and re emitted in any direction, which reduces the number of photons of those frequencies going from the star into our detector.
this includes gas that is quite near to the star - the outer stellar atmosphere.

emission spectra are useful when you've got a gas in a nebula being ionised by something, the photons are emitted when the electrons re combine with a nucleus and then step down to lower energy levels. this will tell you the composition of the gas in the nebula and also that something is shining ionising radiation into it.
thanks
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