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  1. The light from thermal sources such as a light bulb is unpolarized but the one from a laser is polarized. Why?

  2. Is a photon emitted in an atomic transition linearly or circularly polarized and a collection of such atoms are polarized in different directions making a beam of light unpolarized as whole?

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    $\begingroup$ I vote to close as too broad. Why ask two questions about very different subjects in the same post? Or is this homework? $\endgroup$ – Pieter Feb 17 at 9:36
  • $\begingroup$ @Pieter Not two questions. 2 is intimately related with 1. The answer of 1 may be there in the answer of 2. See the answer by user591849 $\endgroup$ – mithusengupta123 Feb 22 at 3:46
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In a typical external-cavity laser, the ends of the tube are sealed with Brewster windows, which pass linearly polarized light of one orientation much more efficiently than light of the other orientation. The light that is passed by the Brewser windows is subsequently reflected back (by the cavity mirrors) into the tube through the same window. As a result, the light that passes easily through the Brewster windows can be amplified while passing through the excited gas in the tube, so the emitted beam has the corresponding linear polarization.

If a gas laser does not have Brewster windows, but instead just has mirrors sealed directly to the ends of the tube, there is nothing to favor one polarization orientation over another, so light of all polarizations is amplified equally and the output beam is not polarized. If a circular polarizer were included inside the cavity, the laser would emit circularly polarized light.

Thermal radiation such as that from an incandescent filament is normally not polarized. Similarly, light emitted by hot gas with no fields present is not polarized. However, if a magnetic or electric field is present, the field defines a direction. The atoms or molecules in the hot gas tend to be oriented in the field direction - or sometimes perpendicular to the field direction-; and can then emit polarized light.

It's important to remember that quantum mechanics ensures that the polarization of an individual photon is usually not well determined. It has a certain probability of having this polarization or that polarization, but both probabilities are usually finite unless something forces the polarization to have a definite state. A free atom is unconstrained pretty much by definition, so emits light with no definite polarization state. On the other hand, if there is a magnetic or electric field present, emitted light is likely to be polarized. In a collection of un-oriented atoms, there will be no definite polarization in their emitted light.

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The fact is that not every lasers beam is substantially polarized.

When polarization occurs, is not intrinsically due to stimulated emission per se, but due to the specific medium and/or configuration. For instance you can pump a crystal which indeed act as a polarizer.

Further details are at https://www.rp-photonics.com/polarization_of_laser_emission.html

edit originally thought to answer to "why is laser polarized?".

I see the Q has been heavily edited. While the answer should fix Q1, for Q2 yes. Look at polarization just for a light beam anf thus as a collective property. (For an individual photon, quantum effects preclude having a well determined polarization, at least in a lasing sense). Polarization cannot emerge from an ensemble unless the latter is ordered and optically active.

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  1. The thermally excited atoms will emit photons with different phase and polarization. As a result the coherence of such sources is low to none.

The laser has a resonator two mirrors that confine the photons emitted initially as spontaneous emission. When an electron is in excited state and a photon comes the atom emits two photons with identical phases and polarization - stimulated emission. These two photons are reflected back by a mirror and become four.. Thus the beam consists of photons that are in phase mostly and with particular polarization.

  1. The polarization is related to the behavior of the field vectors in the time domain. However single photons can also have polarization. Most natural are the left and right circular polarizations $\pm \hbar$. It is also widespread that photon can have linear polarization or might be a superposition of left and right circular.
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