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a physics puzzle
So last night after Thanksgiving dinner, we went to see "Bolt". Which was a lot of fun, and heart-warming, and all, and gorgeous computer graphics and all, but that's for another post.
It's also in 3-D, which is usually accomplished by showing slightly different pictures to the left and right eyes to create parallax. Now, a few decades ago, this was done with one picture in red and the other in green; everybody wore glasses that were red on one side and green on the other (which puts severe limits on the use of color in the movie!). Obviously, all right lenses in the theater must behave the same, and all left lenses must behave the same, in order for different customers to see roughly the same effect. And, insofar as possible, light that passes through left lenses must not pass through right lenses, and vice versa, in order to achieve cleanly separate pictures.
We brought home two pairs of glasses. I had heard that it was being done now with polarization these days, to allow a full range of colors. I assumed that right lenses would be vertically polarized and left lenses horizontally (or vice versa). This would produce the following effects, if you positioned two lenses in the same line of sight between you and a light source:
R to R, right side up or one upside-down -> fairly clear
L to L, right side up or one upside-down -> fairly clear
R to L, right side up or one upside-down -> dark
R to R, one rotated 90 degrees -> dark
L to L, one rotated 90 degrees -> dark
R to L, one rotated 90 degrees -> fairly clear
So just to confirm that this was what they were doing, I checked. Here's what I actually observed:
R to R, right side up or one upside-down -> fairly clear
L to L, right side up or one upside-down -> fairly clear
R to L, right side up or one upside-down -> somewhat dark, purple
R to R, one rotated 90 degrees -> fairly clear but bluer
L to L, one rotated 90 degrees -> fairly clear but bluer
R to L, one rotated 90 degrees -> dark
WTF?
Two questions occur to me. First, how did they achieve this combination of effects? (My last physics class was in 1981....) Second, why? I would think that to maximize the 3-D effect, you would want any light that got through one lens to be maximally stopped by the other, i.e. "dark", not "somewhat dark". If we assume that they did do that, then one of the lenses must be not simply passing light through but modifying it.
In which case the lenses might behave differently front-to-back than back-to-front. All the above observations were made face-to-face, i.e. with the "outside" of one lens facing the other, so light was going in the back of one lens, out the front, in the front of the other and out the back. So let's try
back-to-back, so the light is going in the front of one lens, out the back, in the back of the other and out the front.
R to R, right side up or one upside-down -> fairly clear
L to L, right side up or one upside-down -> fairly clear
R to L, right side up or one upside-down -> fairly clear
R to R, one rotated 90 degrees -> dark
L to L, one rotated 90 degrees -> dark
R to L, one rotated 90 degrees -> dark
Now let's try both facing the same direction.
R to R, right side up or one upside-down -> fairly clear but yellower
L to L, right side up or one upside-down -> fairly clear but yellower
R to L, right side up or one upside-down -> fairly clear but yellower
R to R, one rotated 90 degrees -> fairly clear but bluer
L to L, one rotated 90 degrees -> fairly clear but bluer
R to L, one rotated 90 degrees -> fairly clear but bluer
(BTW, none of these depend on whether the lenses are both facing the light, or both facing me.)
Let me point out the most surprising observations again:
R to L right-side up, back to back -> fairly clear
R to L right-side up, front to front -> somewhat dark, purple
That is, the light is going through the exact same two lenses, in the same direction, but it makes a difference which one it goes through first.
R to R right-side up, back to back or front to front -> fairly clear
R to R right-side up, front to back -> fairly clear but yellower
This time, the light is going through the exact same two lenses, in the same order, but it makes a difference whether it goes through them in the same direction or opposite directions.
R to R, one rotated 90 degrees, back to back -> dark
R to R, one rotated 90 degrees, front to front -> fairly clear but bluer
R to R, one rotated 90 degrees, front to back -> fairly clear but bluer
The light is going through the exact same two lenses, but if the front of either or both is facing the other, you get a different effect from if the backs of both are facing one another. It's an OR gate.
Then think about the color shifts. I could imagine a certain combination of polarized lenses or diffraction gratings or something selectively stopping low-frequency light while allowing through high-frequency light whose wavelength is smaller than the grating. But what would selectively stop high-frequency light? Or is it a matter not of "stopping" but of destructive interference at particular frequencies and constructive interference at others?
OK, so what can I conclude?
It's also in 3-D, which is usually accomplished by showing slightly different pictures to the left and right eyes to create parallax. Now, a few decades ago, this was done with one picture in red and the other in green; everybody wore glasses that were red on one side and green on the other (which puts severe limits on the use of color in the movie!). Obviously, all right lenses in the theater must behave the same, and all left lenses must behave the same, in order for different customers to see roughly the same effect. And, insofar as possible, light that passes through left lenses must not pass through right lenses, and vice versa, in order to achieve cleanly separate pictures.
We brought home two pairs of glasses. I had heard that it was being done now with polarization these days, to allow a full range of colors. I assumed that right lenses would be vertically polarized and left lenses horizontally (or vice versa). This would produce the following effects, if you positioned two lenses in the same line of sight between you and a light source:
R to R, right side up or one upside-down -> fairly clear
L to L, right side up or one upside-down -> fairly clear
R to L, right side up or one upside-down -> dark
R to R, one rotated 90 degrees -> dark
L to L, one rotated 90 degrees -> dark
R to L, one rotated 90 degrees -> fairly clear
So just to confirm that this was what they were doing, I checked. Here's what I actually observed:
R to R, right side up or one upside-down -> fairly clear
L to L, right side up or one upside-down -> fairly clear
R to L, right side up or one upside-down -> somewhat dark, purple
R to R, one rotated 90 degrees -> fairly clear but bluer
L to L, one rotated 90 degrees -> fairly clear but bluer
R to L, one rotated 90 degrees -> dark
WTF?
Two questions occur to me. First, how did they achieve this combination of effects? (My last physics class was in 1981....) Second, why? I would think that to maximize the 3-D effect, you would want any light that got through one lens to be maximally stopped by the other, i.e. "dark", not "somewhat dark". If we assume that they did do that, then one of the lenses must be not simply passing light through but modifying it.
In which case the lenses might behave differently front-to-back than back-to-front. All the above observations were made face-to-face, i.e. with the "outside" of one lens facing the other, so light was going in the back of one lens, out the front, in the front of the other and out the back. So let's try
back-to-back, so the light is going in the front of one lens, out the back, in the back of the other and out the front.
R to R, right side up or one upside-down -> fairly clear
L to L, right side up or one upside-down -> fairly clear
R to L, right side up or one upside-down -> fairly clear
R to R, one rotated 90 degrees -> dark
L to L, one rotated 90 degrees -> dark
R to L, one rotated 90 degrees -> dark
Now let's try both facing the same direction.
R to R, right side up or one upside-down -> fairly clear but yellower
L to L, right side up or one upside-down -> fairly clear but yellower
R to L, right side up or one upside-down -> fairly clear but yellower
R to R, one rotated 90 degrees -> fairly clear but bluer
L to L, one rotated 90 degrees -> fairly clear but bluer
R to L, one rotated 90 degrees -> fairly clear but bluer
(BTW, none of these depend on whether the lenses are both facing the light, or both facing me.)
Let me point out the most surprising observations again:
R to L right-side up, back to back -> fairly clear
R to L right-side up, front to front -> somewhat dark, purple
That is, the light is going through the exact same two lenses, in the same direction, but it makes a difference which one it goes through first.
R to R right-side up, back to back or front to front -> fairly clear
R to R right-side up, front to back -> fairly clear but yellower
This time, the light is going through the exact same two lenses, in the same order, but it makes a difference whether it goes through them in the same direction or opposite directions.
R to R, one rotated 90 degrees, back to back -> dark
R to R, one rotated 90 degrees, front to front -> fairly clear but bluer
R to R, one rotated 90 degrees, front to back -> fairly clear but bluer
The light is going through the exact same two lenses, but if the front of either or both is facing the other, you get a different effect from if the backs of both are facing one another. It's an OR gate.
Then think about the color shifts. I could imagine a certain combination of polarized lenses or diffraction gratings or something selectively stopping low-frequency light while allowing through high-frequency light whose wavelength is smaller than the grating. But what would selectively stop high-frequency light? Or is it a matter not of "stopping" but of destructive interference at particular frequencies and constructive interference at others?
OK, so what can I conclude?
- the lenses do not simply "pass or not" a given photon; they change it, so the lenses aren't idempotent (i.e. passing through several of the same lens in a row isn't the same as passing through one).
- the change passing through a lens from front to back is considerably different from the change passing through from back to front of the same lens.
- 90 degree rotation does make a difference (unlike with the red/green lenses), but...
- the change is not equivalent to a 90 degree rotation, and
- one lens is not equivalent to a 90 degree rotation of the other.
- it's too late at night to figure this out.
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