Polarizer question. Help needed please.

[sacapuntas]

When you polarize a laser beam, then polarize it again with a second polarizer - perpendicular to the first - no light comes through, right? But when you put a third polarizer in the lineup and adjust the angle, the beam reappears. Why is this? What makes it come back?

 

[deam_87]

where did you see this? was it by experimentation?..... and what do you mean by "adjusting the angle"??

 

[sacapuntas]

Yes, I did it in a lab. Sorry, I didn't explain that very well did I? By adjusting the angle, I meant rotating the third polarizer to some angle other than that of the first and second. For example, the first would be adjusted to "0 degrees", the second would be perpendicular to that, "90 degrees", and the third would be anywhere in between those two. It was the weirdest thing. The first two completely blocked out the beam, which makes perfect sense, because putting two polarizers together, one perpendicular to the other, blocks all incoming light. But I can't figure out how adding a third polarizer makes the beam reappear (intensity of the beam is significantly less). Does that help?

 

[deam_87]

.... no haha, ive never heard of that before, but ima research it...


Edit...
Seems pretty darn interesting, but there is quantum physics involved in the explanation ... here is a link that describes it at "our level" hahaha

http://alienryderflex.com/polarizer/

 

[rathat]

Because it doesn't just block light not polarized to the filter, but actually changes the angle of some of it, in a way. Its weird. Light closest to 0 degrees will go through and light at a further angle going through will come out with a lower magnitude. from 0 to 90 none will go through but from 0 to 45 lets say the light looses 50% of the magnitude and 45 more to 90 will loose another 50% now that is not 100% the light at the end will have 25% of the original magnitude.

 

[HIMNL9]

Are you sure that the middle plate is still a polarizer ?

After all LCD works in this way (the liquid layer rotate 90 degrees the polarization of the passing light), so maybe also other substances can do this ..... just a thought

 

[Benm]

It does work like this.

What a polarizer does, is take whatever fraction of the total amplitude of the incoming light is in its pass-orientation. This fraction of the light is then emitted from the other end - polarized in the pass direction.

If you have 2 polarizers at 90 degree angle to eachother, they will block all light.

If you add a third at 45 degree angle between the two, that will take half of the light of the first polarizer, and rotate it to 45 degrees. The final polarizer will be able to pass half of that, and emit it at a total rotation of 90 degrees.

Notice that while the beam does 'pass', it will have lost 75% intensity in the process.

 

[Deleted member 8382]

ahhh, the third one is put between the other 2! I thought you meant you added this last one at the end and that the light was just "appearing" lol

 

[Benm]

That would have been very odd indeed.

A nice thing is that other materials affect optical rotation too. If you set up 2 polarizers in a row such that they block all light, you can see stress lines in pieces of (plexi)glass you put in between, appearing as rather pretty coloured shapes.

 

[Helmholtz Watson]

ahhh, the third one is put between the other 2! I thought you meant you added this last one at the end and that the light was just "appearing" lol

This is what I thought too!

Everyone should have done this experiment in school: Optical Rotatory Dispersion: Polarized Light and Sucrose

Shows how polarization works including demonstration of rotation of poplar plane.

 

[LordKaramazov]

i wasnt understanding it at first too...just like the light hopped from one place to another... and i was starting to think like the zeeman effect and the fact that u cant get one electron to have just one component (xy or z) of spin, was something similiar to this... but now that i know the 3rd polarizer goes in the middle it does make perfect sense. im just goin to repeat what someone said, just because i dont have anything better to do

we can look at this like vectors. if u have a given vector on a xy plane, and if u can project this vector on the xx axis, ull have a new vector with direction being the same as the xx axis. this was the 1st polarizer. now if u apply the second (90º) polarizer, ull have to transform this last vector into its projection on the yy axis, which is, of course, 0.
But now lets assume u havent applied the 2nd polarizer yet, and get a 3rd (45º) polarizer first. one can in fact project a vector that only has a xx component on vector that bisects the angle between both axis. This projection gives you a vector that has equal ammounts on both directions. if u now apply the yy polarizer, ull get only the yy of this last vector to pass.

This process really does cut the light energy, but it makes it look like for some people that the light is rotating xD