# Photon absorbtion and conservation

• B
Gold Member
If a (polarized) photon is absorbed by a polarization filter, does its energy go into the filter?

I am wondering if that is the case to obey conservation laws.

And if it passes, is its original polarisation direction somehow conserved?

## Answers and Replies

jbriggs444
Homework Helper
If a (polarized) photon is absorbed by a polarization filter, does its energy go into the filter?

I am wondering if that is the case to obey conservation laws.

And if it passes, is its original polarisation direction somehow conserved?
One can take a polarized light beam, pass it through a polarized filter at a 45 degree angle and then through a second filter at a further 45 degree angle. The beam emerging from the second filter is now polarized 90 degrees different from the original (with reduced intensity by a factor of 2 if I recall correctly). So no, I would say that polarization is not conserved.

Gold Member
One can take a polarized light beam, pass it through a polarized filter at a 45 degree angle and then through a second filter at a further 45 degree angle. The beam emerging from the second filter is now polarized 90 degrees different from the original (with reduced intensity by a factor of 2 if I recall correctly). So no, I would say that polarization is not conserved.
It occurred to me that, in quantum mechanics, a photon passing a polarisation filter can be viewed as in superposition of being passed and blocked. The polarisation-components of the passed part and the blocked part add up to the original polarisation (vector). So if we view the photon as being in superposition, polarisation would perhaps be conserved?

DrClaude
Mentor
It occurred to me that, in quantum mechanics, a photon passing a polarisation filter can be viewed as in superposition of being passed and blocked.
That's not correct. A polarising filter is macroscopic, so it acts like a measurement device. If the photon goes through, then it is polarised in the direction set by the polariser.

Nugatory
Mentor
If a (polarized) photon is absorbed by a polarization filter, does its energy go into the filter?
Yes. That's generally true when light is absorbed.

How we learn about light is through its interaction with matter. In fact, as Bohren said, we wouldn't really have noticed polarisation were it not that the different polarisation states interact with matter differently. Absorption refers to a change from its original state (light wave) to, for example heat (an increase in the kinetic energy of molecules). Energy is conserved but has many different forms.
In regard to a light wave retaining its polarisation state after passing through a polarisation filter we must first be on the same page with just what polarisation actually is. So, i want you to imagine holding a thin rope letting it dangle. I place a large sheet of paper underneath and i ink the end of the rope closest to the floor. I then waggle the rope so the end moves back and forth and i allow it to draw a pattern on the paper. This pattern is called the vibration ellipse and if at any given moment the pattern is the same, the light (rope) is polarised. So, if i'm super careful i could draw a line with my rope waggle - this is plane polarised. If the planes are different at any given moment, even though they remain in a line the light is unpolarised. By the way, lines and circles are special cases of ellipses. By waggling the rope in different ways i can get the typical ellipse or again if i'm very careful a circle. Polarisation refers to this pattern in time and requires that it is the same at any given moment. Strictly speaking nothing is completely polarised it is always partially polarised. Ok, a polarising filter works by having a grid (series of parallel lines adjacent to each other) made of metal. When the rope (light wave) waggles in the same plane as the grid lines the metal absorbs the light. If the rope waggle is at right angles the light can pass as it is not absorbed. This seems as if i have it muddled - this is genuinely how they work. How light interacts with matter to produce different polarisation states requires some subtle thinking, suffice to say that filters affect amplitudes of waves whereby other materials which are termed retarders affect phases. These latter materials are responsible for changing the shape of the vibration ellipse.

Gold Member
To generate entangled electrons, I believe that one can use radioactive elements that decay into other particles, in case of which if there result two electrons with spin, the spin must be conserved because the decaying particle was spin-neutral, so that the electrons have necessarily opposite spins. But if spin is conserved, why then not generally polarisation of photons?

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DrClaude
Mentor
To generate entangled electrons, I believe that one can use radioactive elements that decay into other particles, in case of which if there result two electrons with spin, the spin must be conserved because the decaying particle was spin-neutral, so that the electrons have necessarily opposite spins. But if spin is conserved, why then not generally polarisation of photons?
Because there is no law that says that polarization should be conserved.

At best you have conservation of angular momentum. The spin angular momentum of the photon must be conserved, so absorption of a photon changes the angular momentum of, say, the atom that absorbs it. Circular polarization also as angular momentum associated with it.

entropy1
Gold Member
Because there is no law that says that polarization should be conserved.
The probability amplitudes of the photon's polarization direction in the filter's basis add up to the original polarization direction of the photon. If we take an ensemble of identically liniarly polarized photons, on average the polarization direction is conserved, right? (if we take blocked photons' polarization direction as perpendicular to passing ones)

DrClaude
Mentor
If we take an ensemble of identically liniarly polarized photons, on average the polarization direction is conserved, right? (if we take blocked photons' polarization direction as perpendicular to passing ones)
I don't understand. If you have bunch of photons with the same linear polarization and a linear polarizer at an angle with respect to the photon polarization, then you get as output photons that have a different linear polarization. As the blocked photons no longer exist, I don't see how you can say that polarization is conserved.

It is also possible to change the polarization angle without losing photons:
https://en.wikipedia.org/wiki/Optical_rotation
https://en.wikipedia.org/wiki/Polarization_rotator

entropy1
sophiecentaur