Effect of Rotating Polarizing Films

[chef99]

Homework Statement

Imagine that you had two polarizing films and were holding them one on top of the other. What would the effect of rotating the two polarizing films, with respect to one another, be? Explain what would be seen, and why.[/B]

Homework Equations

n/a

The Attempt at a Solution

The first filter would polarize the light along its axis. If the second filter is aligned the same as the first, (both aligned vertically or horizontally) than the light that passed through the first filter would also pass through the second filter. This means the second filter wouldn’t affect the light emitted from the first filter. If the filters were rotated, in relation to each other, only the light parallel to the second filter would pass through. This would result in less light traveling through the second filter, making the light emitted appear dimmer. If the films were rotated perpendicular to each other, no light would be emitted through, therefore no light would be seen.Any thoughts on how this could be improved, or if it is incorrect is greatly appreciated. Thanks

 

[mfb]

Right.

You can also comment on the polarization of the light after the second filter.

 

[chef99]

Right.

You can also comment on the polarization of the light after the second filter.

So I should add:

after passing through the second filter, the polarization will be in the direction of the second filter.

Is that what you are talking about?

 

[haruspex]

If the filters were rotated, in relation to each other, only the light parallel to the second filter would pass through.

But you wrote:

The first filter would polarize the light along its axis.

If the filter axes are not parallel, and the first filter has polarized the light parallel to its own axis, then none of it is parallel to the axis of the second filter.

Any ideas on how to modify the first statement above to resolve this?

 

[chef99]

But you wrote:

If the filter axes are not parallel, and the first filter has polarized the light parallel to its own axis, then none of it is parallel to the axis of the second filter.

Any ideas on how to modify the first statement above to resolve this?

Should I say:

when the films are rotated, in relation to each other, only the component of the light that is parallel to the second filter passes through.

Does that make more sense?

 

[mfb]

That is better.

So I should add:

after passing through the second filter, the polarization will be in the direction of the second filter.

Is that what you are talking about?

That is what I meant, yes.

 

[haruspex]

when the films are rotated, in relation to each other, only the component of the light that is parallel to the second filter passes through.

That is better, but arguably still not right. That wording suggests that if the angle between the current polarisation of the light and the direction of the filter is θ then the fraction that gets through is $\cos(\theta)$. In fact, it is $\cos^2(\theta)$.
Maybe just say that the fraction that gets through is related to the angle between the two in such a way that when parallel all gets through and when at right angles... etc.

 

[chef99]

That is better, but arguably still not right. That wording suggests that if the angle between the current polarisation of the light and the direction of the filter is θ then the fraction that gets through is $\cos(\theta)$. In fact, it is $\cos^2(\theta)$.
Maybe just say that the fraction that gets through is related to the angle between the two in such a way that when parallel all gets through and when at right angles... etc.

Ok, that makes sense. So when they are rotated, the amount that gets through is related to the degree of the angle between the two films? If they are parallel then all light gets through, if they are at a right angle no light gets through, and if they are at an angle in between then the amount of light that gets through is related to the angle.

 

[haruspex]

Ok, that makes sense. So when they are rotated, the amount that gets through is related to the degree of the angle between the two films? If they are parallel then all light gets through, if they are at a right angle no light gets through, and if they are at an angle in between then the amount of light that gets through is related to the angle.

Yes.