[2810713]

Why is the plane of PPL rotated?  What are the factors that lead to rotation in the planes of electric and magnetic field in the light??? is it electron density
that causes this.Can this be related to quantum chemistry??? the plane of a photon!!!??? sounds weired , as i do imagine a photon as a spherical massless enegry packages.

help required.

hrushikesh

[Chrataxe]

Err..I don't really understand the question, but I think the answer has to do with stereo chemistry and racemic mixtures and what not else.

[2810713]

Sorry, i think i didn't explain properly.
It is to do with stereochem. The plane of poarized rotated by only  the moleculs having chiral centre always by a specefic angle. I'm asking why does that angle gets changed that too only by the molecules having chiral centre and why is it independent of angle of incidense of light or any such external factor i.e. depends only upon the molecular structure.
PLease help.

hrushikesh

[Chrataxe]

When looking at stereochemistry, when light hits the molecule, it will be rotated.  If the molcules are enatiomers, when the light is rotated by one isomer, the other will rotate it back...and so on and so forth.  Now, if they are all the same stereo-isomer, they will continue to rotate the light and instead of bending it back to its original.

[2810713]

Yes but why is the angle of rotation independent of arrangement of molecules, no.of molecules, angle of incidence of light. How to imagine this process correctly...


hrushikesh

[Chrataxe]

Ok...let's use a telescope as an example.  Reflector telescopes have 2 mirrors...the primary and the secondary.  Ususally, the primary is a parabolic mirror.  So, when the light comes in, it hits the parabolic mirror and all light is focused and reflected back to the secondary mirror.  So, it doesn't matter the angle at which the light comes in at, it will all be reflected back to the same point b/c the primary mirror is a parabola.  

That is essentially what is going on.  The angle of rotation is independent of everything except the molecule b/c the molecule will rotate the light at a certain angle.  It is independent of the arrangement of the molecules b/c its not the arrangement the bends the light...its the stereochemical molecule itself...thus the number of molecules and angle of incidence doesn't matter or anything else doesn't matter.

[2810713]

Means if we consider a sigle molecule it can rotate the plane of PPL. But  see if two molecules are kept besides each other in such a way that the net rotation of plane of PPL is zero like two lenses [one converging and another diverging ]. This means that arrangement of molecs may also matter here[ if this is a perfect analogy] Same way i am still not getting
why the angle of incidense doesn't matter.

Please help,this is my genuine difficulty and i'm really very curious about the ans...

hrushikesh

[Yggdrasil]

Here's my attempt at an answer.  Note that I'm just a biochemist and a physicist would probably be able to answer the question more completely:

Plane polarized light can be considered to be the superposition of two counter-rotating electrical field oscilations.  Molecules which have symmetry elements which prevent them from being chiral will interact equally with each electrical field component.  However, chiral molecules will interact more strongly with one electrical field component.  Therefore, the two components travel at different velocities through an optically-active (chiral) environment.  Since one component arrives at the observer later than the other, the plane of the light appears rotated.

Now, it is probably true that different molecules in different orientations will interract with  PPL differently.  However, in a liquid, the molecules will be fairly randomly oriented, so the rotation of the PPL light will essentially be averaged out.  Just as one would expect a coin to land on heads 50% of the time if flipped a million times, one would expect the same rotation of PPL when passed through a million molecules  in random orientations.

[2810713]

Thanks you people are really trying to help me, thanks again.

That average-out concept really helped me and relaxed me to...
But then the one thing comes out- the rotation by single molec. may or may not be the av. rotation. And when we take diff. conc. solutions of these we still get same av. rotation i.e. no. of molecules/unit volume doesn't matter here.

Let's we think something more
Also what u said about diff velocities of components...
i'm trying to imagine it....[ actually that dual nature of light isn't imaginable for me yet]
 but why the diff velocities make it appear rotated...

See if the new plane makes acute anangle of r with the original plane, while still the resultant of field has the same magnitude [R] as before. We can say that-
 R² = [Rsinr]² + [Rcosr]² + R[2.sinr.cosr]

 R² = [Rsinr]² + [Rcosr]² + R[sin(2r)]

          R²  = R² + Rsin2r

thus Rsin2r=0 and R not = 0 thus 2r=0  r=0 !!!
Thus actually the plane cannpt be rotated still keeping the resultant at same value the same or can be rotated by an angle r=0!!!
Thus either the magnitude chnges or as u say actually it just appears to be rotated...

I think we should keep thinking and when we get some clue we should communicate.

Shrei

[Winga]

Plane polarized light can be considered to be the superposition of two counter-rotating electrical field oscilations.

What is the meaning of "the superposition of two counter-rotating electrical field"?

You mean an electric field and a magnetic field are perpendicular to each other?