https://www.reddit.com/r/askscience/comments/1j4bcs5/i_cant_understand_how_light_gets_polarised_and/
created by Fresh_Recognition_43 on 05/03/2025 at 19:20 UTC
225 upvotes, 10 top-level comments (showing 10)
In terms of electric and magnetic field how does a polarimeter works.
Why do optically active molecules show this rotation/how they bring about the rotation of light.
What laws it follows.
What do the half dimmed semi-circles in the polarimeter eyepiece signify ?
I can't picture light changing directions, pls explain me !!!
Comment by [deleted] at 06/03/2025 at 02:27 UTC
143 upvotes, 3 direct replies
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Comment by WhineyLobster at 06/03/2025 at 00:44 UTC*
35 upvotes, 1 direct replies
Light can have a direction the wave is traaveling and also a rotational component. If you imagine the 90 angle bewtween the electric field and magnetic field components imagine rotating that around the axis that the light is traveling on.
The rotation (its not continually rotating its just shifted by a certain amount) is usually random however there are things that create coherent light like lasers.
Even if a light wavefront is aligned it wont act as coherent unless its polarity is aligned as well. Polarity in this instance does not refer to magnetic polarity but (i believe) to polar coordinates which is how rotated something is from normal.
A polarizer filters out any light not aligned with a particular angle. Polarized glasses dim the light because it removes lots of light that is outside this particular angle. But glare is usually caused in large part by how these unaligned light rays interact with surfaces. So polarized glasses work well to reduce glare but at the expense of losing lots of brightness, but usually thats a benefit in sunglasses but can br a detriment in like a euv microchip machine for instance.
Comment by ezekielraiden at 06/03/2025 at 16:44 UTC
5 upvotes, 1 direct replies
When you make a pulse of light, that pulse contains both an electric field "bump" and a magnetic field "bump". Because of the nature of electric and magnetic fields, they are always perpendicular to each other (and to the direction the light is travelling).
Now, those "bumps" *point* in some direction. They have to, as noted, they are perpendicular to each other and to the direction the light is moving. What direction IS (say) the E-field bump pointing? That is the "polarization" of the light pulse.
So. Think of the pulse as being like a pencil, that has two arrows pointing out from the pencil, one red, one blue. For natural light, there are a zillion zillion pencils, and most of them have the red and blue arrows pointing in different directions. When you use a polarizing filter, that filter interacts with the pencils, cutting off parts of the arrows that don't "match" the way the polarizer is turned. The ones that are exactly wrong, that have no direction that way *at all*, get completely blocked. The ones that are a little bit off, just get part of their red and blue arrows shaved off, but can keep going. The ones that are *perfectly* aligned slide right on through, unchanged, like Indiana Jones passing the first Grail trial, avoiding the slicing blades.
Comment by CallMeNiel at 06/03/2025 at 04:30 UTC
11 upvotes, 0 direct replies
Moving an electric charge (up) creates a magnetic field at a right angle to the direction of the movement of the charge(left). This changing magnetic field, in turn, pushes that charge down. The now moving charge then induces another magnetic field (right). The magnetic field then causes the charge to move up again. These fluctuations in electric and magnetic fields appropriate forward at the speed of light. This is how a photon acts as a wave.
So there is a specific orientation for each photon. There's a top/bottom and a left/right with different properties. Some materials can interact with photons based on that orientation.
Comment by [deleted] at 06/03/2025 at 08:14 UTC
7 upvotes, 1 direct replies
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Comment by AberforthSpeck at 06/03/2025 at 00:29 UTC
7 upvotes, 2 direct replies
The polariser acts as a filter. It blocks most of the incorrectly aligned light, bouncing it off in random directions. Only light in the correct orientation gets through the filter. Although every filter will "leak" a bit, so you will get some randomly aligned light.
Comment by BiomeWalker at 08/03/2025 at 00:41 UTC
1 upvotes, 0 direct replies
Imagine you have a rope tied to a wall, you can shake the rope in any direction and see a wave along it's length.
Now, add a fence with vertical slats in it that the rope now passes through.
With that fence in the way, the rope can only have waves in it that move along the slats, which means vertical here.
Even if you shook the rope in a circle, the rope will still only move up and down past the fence.
Comment by ThinNeighborhood2276 at 08/03/2025 at 06:22 UTC
1 upvotes, 0 direct replies
Polarization refers to the orientation of the electric field of light waves. A polarizer filters light by allowing only waves with a specific electric field direction to pass through. Optically active molecules rotate the plane of polarization due to their chiral structure, following the laws of optical activity. The half-dimmed semi-circles in a polarimeter indicate the degree of rotation caused by the sample.
Comment by CrambleSquash at 08/03/2025 at 22:17 UTC*
1 upvotes, 0 direct replies
For optical elements, the interaction between light and matter is all about how the oscillating electric field of the light interacts with the charges (electrons) in that material.
Most materials are isotropic, i.e. more-or-less the same in every direction. But some materials are anisotropic, like certain crystals, or films of aligned carbon chains - their electronic properties are very different in different directions.
When unpolarised light, i.e. a mixture of all different orientations of these oscillating electric fields hits an anisotropic material, it's possible for it to interact differently for light of different polarisations, depending on how this light is aligned with the material's underlying structure. For example, the light that is polarised in the same direction of these carbon chains might be more strongly absorbed. Hence we can produce polarised light from unpolarised light.
The rotation of polarised light by chiral molecules is quite difficult to explain. As others have pointed out, this video explains it very well:
https://www.youtube.com/watch?v=975r9a7FMqc
In short, these molecules have some kind of twist to them, the structures are not symmetric, and in an enantiomer they're all going in the same direction. Simplifying a bit here... as the electric field of polarised light passes a single molecule, it wobbles the molecule's electrons, which will slosh along, including around the twist of the molecule. This slosh, that follows the twist - because it's oscillating charges, also radiates light, which interacts with the incident light to rotate its polarisation somewhat. This effect is really subtle, which is why it takes sooo many a long distance to rotate light a small amount.
Comment by [deleted] at 06/03/2025 at 00:42 UTC
-2 upvotes, 1 direct replies
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