Kap Test 5, Polarized light (Warning, spoiler)

[Sammy1024]

I thought that since the light is going straight through, and there's no angle given for the polarizer, that 100% of the light would pass through?

---

Members don't see this ad.

 

[type12]

View attachment 184686

View attachment 184687

I thought that since the light is going straight through, and there's no angle given for the polarizer, that 100% of the light would pass through?

In short, the MCAT wants you to know that the act of polarizing light reduces the intensity (aka the number of photos that can pass through) by half. The reason for this requires a 3D image, which I cannot do. So:

http://www.physics.utoronto.ca/~phy225h/experiments/polarization-of-light/polar.pdf

On the second page, you can see why this is the case.

The MCAT, though it does hide simple concepts in seemingly complex ways, it does tend to be clearer than this question. The fact that it says P1 and P2 are polarizers was supposed to be the hint.

Hope that helps!

 

[justadream]

Just wondering:

You wouldn't be able to determine the intensity at point B without knowing the orientation of the polarization filter P2 relative to P1, correct?

 

[Sammy1024]

I understand that a polarizer decreases the amount of intensity of light, but if i'm not given any angles, how do I know that it is reduced by 1/2 or 1/4?

 

[justadream]

@Sammy1024

Well the incident light here is unpolarized so you don't need to worry about angles.

 

[type12]

Just wondering:

You wouldn't be able to determine the intensity at point B without knowing the orientation of the polarization filter P2 relative to P1, correct?

100% correct! I think you got this concept downpat. If this was written by AAMC, then they would provide that information (or have an option where it says it cannot be determined).

 

[justadream]

@type12

lol I owe it to TBR.

Let's assume AAMC provides the scenario in which they do provide the angle. I imagine they would also provide some type of formula? Or, is there some intuitive way to figure it out?

 

[type12]

@type12

lol I owe it to TBR.

Let's assume AAMC provides the scenario in which they do provide the angle. I imagine they would also provide some type of formula? Or, is there some intuitive way to figure it out?

Frankly, they probably won't have two polarizers. If they did, they would PROBABLY describe the relationship in such a way that there was proportional "trick," worded like, "Only 50% of the light polarized will be transmitted through the second polarizer. How much intensity of the original, unpolarized light will be at point B?" Which, then it's not 50%, but 50% * 50%. You get the idea.

Equations for light, especially as a wave phenomenon are largely probabilistic, so the MCAT tends to avoid them where possible.

 

[Sammy1024]

So in this question this unpolarized light is going through a polarizer and then we're supposed to determine the intensity. How can you determine it to be 1/2? I looked at the linked page but it didn't really say.

 

[type12]

So in this question this unpolarized light is going through a polarizer and then we're supposed to determine the intensity. How can you determine it to be 1/2? I looked at the linked page but it didn't really say.

Errr... This is so hard to show not in person... This link might help?
http://www.physicsclassroom.com/class/light/Lesson-1/Polarization

Basically, if you think of all 360 degrees of light waves, then only those with a y (and only their y-components) can get through. Or, x, or some angle thereof.

 

[Sammy1024]

Sorry that i'm probably making this more complicated than it has to be. Okay, looking at the explanation again, a polarized light as electric fields that are perpendicular to each other. So when the light goes through, only 1/2 of the light is able to go through because all the electric fields perpendicular can't get through?

 

[type12]

Sorry that i'm probably making this more complicated than it has to be. Okay, looking at the explanation again, a polarized light as electric fields that are perpendicular to each other. So when the light goes through, only 1/2 of the light is able to go through because all the electric fields perpendicular can't get through?

Good, just one fix: polarized light just means the electric fields are only along a single axis (which results in perpendicular magnetic fields). But yes, correct, only half of unpolarized light can make it through.

I wish I could come up with an analog. Anyone think of one?

 

[Sammy1024]

So if the light was originally polarized, then they would give angles and yadiyada?

 

[justadream]

Sorry that i'm probably making this more complicated than it has to be. Okay, looking at the explanation again, a polarized light as electric fields that are perpendicular to each other. So when the light goes through, only 1/2 of the light is able to go through because all the electric fields perpendicular can't get through?

I believe the way TBR explained it is to think about light in terms of its components.

The original light is unpolarized. You can represent it in terms of its components (e.g., component of light parallel to the filter and the component perpendicular to the filter). Those two components are equal and make up the total light intensity (with each component making up half the intensity).

So if you remove one of the components, you are only left with half the original intensity.

 

[type12]

So if the light was originally polarized, then they would give angles and yadiyada?

For the MCAT, it's EXTREMELY unlikely you will ever polarize already-polarized light. In the EXTREMELY unlikely case this happens, you would need the polarizers angle with respect to the light's polarized axis.

In short, don't worry about polarizing polarized light; just be aware you lose half the intensity (aka photons) when you polarize unpolarized light.

 

[Sammy1024]

Okay, so lose half of the light and then if it were to ask about P2, that would depend on the angles right?

 

[type12]

Okay, so lose half of the light and then if it were to ask about P2, that would depend on the angles right?

Yep!

 

[Sammy1024]

This makes sooooooo much more sense now! Thank you everyone for helping through this! :]

 

[Czarcasm]

I believe the way TBR explained it is to think about light in terms of its components.

The original light is unpolarized. You can represent it in terms of its components (e.g., component of light parallel to the filter and the component perpendicular to the filter). Those two components are equal and make up the total light intensity (with each component making up half the intensity).

So if you remove one of the components, you are only left with half the original intensity.

This is a great explanation, but what I have trouble understanding is this. I thought unpolarized light has electric fields radiating from every direction (not strictly perpendicular to each other, ie. strictly X vs. Y axis, as in their example). My interpretation of unpolarized light was that electric fields radiated at all points of a 360 degree circle (perpendicular to the direction of propogation). Here, you guys are saying that to find the intensity of polarized right - we kinda simplify it down to just two electric field components perpendicular to each other - and with this logic, it makes sense the intensity decreases by half, but how does this explain what's really happening (multiple electric fields radiating outwards). I guess what's confusing me is, if only a small proportion of these electric fields are able to pass through the filter, it would seem more intuitive if the intensity would decrease more than 50%. Taking this a step further, if we have polarized light (e-fields point up and down y axis) pass through a second filter oriented between X and Y axis - would this allow some of the light to pass through or block it entirely? I figured it would block it entirely and prevent any light from going through, but you guys seem to be saying a certain percentage of light would pass depending on the angle? I'm really confused, lol. I hope this makes sense to you guys, and hopefully someone knows how to make sense of it.

 

[Czarcasm]

@type12 Maybe you can provide some insight too hopefully

 

[type12]

This is a great explanation, but what I have trouble understanding is this. I thought unpolarized light has electric fields radiating from every direction (not strictly perpendicular to each other, ie. strictly X vs. Y axis, as in their example). My interpretation of unpolarized light was that electric fields radiated at all points of a 360 degree circle (perpendicular to the direction of propogation). Here, you guys are saying that to find the intensity of polarized right - we kinda simplify it down to just two electric field components perpendicular to each other - and with this logic, it makes sense the intensity decreases by half, but how does this explain what's really happening (multiple electric fields radiating outwards). I guess what's confusing me is, if only a small proportion of these electric fields are able to pass through the filter, it would seem more intuitive if the intensity would decrease more than 50%. I hope this makes sense to you guys, and hopefully someone knows how to make sense of it.

It's like, think of a force at an angle: you are sliding a box, along the ground, but your force is down and to the right. The right portion "gets through," while the other portion does not.

This is the wave phenomenon of light, so you can't think of it as discrete photons, but more like something that can be broken into components (which, yes, a photon "can't" be broken down into components, but a wave can). So yeah, any wave with a - say - y-component will have that y-component left after polarization, but not its x-component.

Does that make sense?

 

[justadream]

@Czarcasm

lol I was thinking the exact same thing while I was reading this in TBR.

I think if you have a polarizing filter and you have incident light that NOT perpendicular (by a slight angle), then some percentage of the light would go through.

As for intuitively why, type12 provides some explanation but yeah I agree, it's just not something that is very intuitive. I just accept it as it is lol.

 

[Czarcasm]

It's like, think of a force at an angle: you are sliding a box, along the ground, but your force is down and to the right. The right portion "gets through," while the other portion does not.

This is the wave phenomenon of light, so you can't think of it as discrete photons, but more like something that can be broken into components (which, yes, a photon "can't" be broken down into components, but a wave can). So yeah, any wave with a - say - y-component will have that y-component left after polarization, but not its x-component.

I see what you're saying, but... take a look at this picture:

If only the vertical light is able to get through, and all the other light radiating outwards is not - why then do we strictly focus on only two perpendicular electric fields when considering intensity? Is this just a principle we have to know? I kinda figured it was because that the light, even though it's radiating in every direction, it's also spinning - so at some point, some half of this light is passing -X and -Y (rather than static, always radiating in the same direction). That's the only way I can really make sense of the explanation. Just wasn't sure if that was right.

 

[Czarcasm]

@Czarcasm

lol I was thinking the exact same thing while I was reading this in TBR.

I think if you have a polarizing filter and you have incident light that NOT perpendicular (by a slight angle), then some percentage of the light would go through.

As for intuitively why, type12 provides some explanation but yeah I agree, it's just not something that is very intuitive. I just accept it as it is lol.

I really think it's because, even though light is radiating in every direction (perpendicular to propogation), the light radiating outwards is also rotating - such that at some point, half the light points in the X direction, half in the Y direction. That's just me trying to reason it out - never read that anywhere lol. Ah well, somethings are probably left unquestioned lol. Thanks guys.

 

[justadream]

@Czarcasm

Found this on Yahoo answers (yes, I know, not super trustworthy) but the guy seems to know what he's talking about.

The materials in a polarizing filter are special in that their electrons are all oscillating on a single plane. When a light wave which is oriented along that same plane strikes an electron, all of its energy is absorbed by the electron. It makes the electron vibrate faster. But when a light wave which is perpendicular to that plane strikes an electron, its energy is transmitted to the neighboring electrons, and passed on from electron to electron, until it is finally re-emtted out the other side of the filter. Depending upon how each light wave entering the filter is oriented, relative to the plane that the materials electrons are oscillating on, a certain percentage of their energy will be absorbed, and a correspnding percentage will be transmitted. So a wave oriented at a 45 degree angle to the electrons, will have half of its energy absorbed, and half of it transmitted. A wave oriented parallel to the electrons will have all of its energy absorbed, and a wave oriented perpendicular to the electrons will have all of its energy transmitted. All of the transmitted energy ends up getting re-emitted out the other side of the filter. If the light entering the filter is non-polarized, then half of its light will be absorbed, and half will pass through the filter, and be re-emitted on the other side.

https://answers.yahoo.com/question/index?qid=20090116053436AA8P1Ct

 

[type12]

I see what you're saying, but... take a look at this picture:

If only the vertical light is able to get through, and all the other light radiating outwards is not - why then do we strictly focus on only two perpendicular electric fields when considering intensity? Is this just a principle we have to know? I kinda figured it was because that the light, even though it's radiating in every direction, it's also spinning - so at some point, some half of this light is passing -X and -Y (rather than static, always radiating in the same direction). That's the only way I can really make sense of the explanation. Just wasn't sure if that was right.

Yeah, so you see the waves that are diagonal that don't seem to make it? Well, they do, at least a portion of it does.

They have a y and x component: the x component won't make it through, but the y will. The y-components from all angles add together and make it through, and all the x-components from all angles are removed. I think @justadream 's latest post explains this further, though I'm not sure if I agree with the explanation.

 

[Czarcasm]

That explanation seems really theoretical and who knows, he's probably right. Sounds like he knows what he's talking about. I think this is just one of those instances where over-thinking it does more harm than good. Gonna just try to keep it simple and hope this helps. Thanks again guys.

 

[Czarcasm]

@Czarcasm

Found this on Yahoo answers (yes, I know, not super trustworthy) but the guy seems to know what he's talking about.

The materials in a polarizing filter are special in that their electrons are all oscillating on a single plane. When a light wave which is oriented along that same plane strikes an electron, all of its energy is absorbed by the electron. It makes the electron vibrate faster. But when a light wave which is perpendicular to that plane strikes an electron, its energy is transmitted to the neighboring electrons, and passed on from electron to electron, until it is finally re-emtted out the other side of the filter. Depending upon how each light wave entering the filter is oriented, relative to the plane that the materials electrons are oscillating on, a certain percentage of their energy will be absorbed, and a correspnding percentage will be transmitted. So a wave oriented at a 45 degree angle to the electrons, will have half of its energy absorbed, and half of it transmitted. A wave oriented parallel to the electrons will have all of its energy absorbed, and a wave oriented perpendicular to the electrons will have all of its energy transmitted. All of the transmitted energy ends up getting re-emitted out the other side of the filter. If the light entering the filter is non-polarized, then half of its light will be absorbed, and half will pass through the filter, and be re-emitted on the other side.

https://answers.yahoo.com/question/index?qid=20090116053436AA8P1Ct

Check out this:

"Linear polarization is merely a special case of circularly polarized light. Circularly polarized light = a condition caused by two waves whose electric field components are 90 degrees out of phase, causing an effective rotation of the electric field about an axis in direction of propagation." Source: http://plc.cwru.edu/tutorial/enhanced/files/lc/light/light.htm

So I guess there was some truth to my theory, lol.

 

[justadream]

Check out this:

"Linear polarization is merely a special case of circularly polarized light. Circularly polarized light = a condition caused by two waves whose electric field components are 90 degrees out of phase, causing an effective rotation of the electric field about an axis in direction of propagation." Source: http://plc.cwru.edu/tutorial/enhanced/files/lc/light/light.htm

So I guess there was some truth to my theory, lol.

Might as well rename it the "Czarcasm theory of light polarization".

 

[Czarcasm]

Might as well rename it the "Czarcasm theory of light polarization".

Lmao, wooo hoo!