A preceding essay argued that it might be possible to build a single, simple, self-consistent model for light and it would be timely to do so. As a provocation it makes a start on constructing a bottom up approach it calls ‘phots’. This essay continues the building process for the prototype phot model by doing a preliminary check of its consistency with the major types of optical experiments.
Introduction
It may be possible to develop a unified model for light by putting historical analogies aside, creating an open minded framework and letting experimental results build up the picture. This essay demonstrates this idea with a simple prototype example it calls phots. It considers a sample of the main types of optical phenomena – reflection, absorption, refraction, diffusion, Faraday and Kerr effects and various polarization effects. The following essays look at diffraction and various interference experiments.
A previous essay made a start on setting out the basic properties for phots, based on experimental evidence rather than historical opinions. The next few essays will look at some of the many experiments that have been done using interference effects, including Young’s famous double slit experiment.
The subject is much too big to do a historical review, cover all the conflicting ideas and define all the special terms in one short essay. In this essay words with special meaning will be in bold text.
The picture that starts to emerge by starting with a blank sheet and just following the evidence is rather interesting.
Reflection
Reflection in the wave model is not as straightforward as one might think.
In the wave model, light is considered to be an electromagnetic wave modelled by Maxwell’s equations. Light waves incident on a material induce small oscillations in the electron shells of atoms (or free electrons in metals), causing each particle to radiate a small secondary wave in all directions. Electric and magnetic field effects penetrate the reflecting medium and some propagate into the new medium while others form a new wave heading out again. The reflected wave heads out in a plane defined by the incident ray and the orthogonal normal vector at the point of incidence and at an angle equal to the angle of incidence. The reflected and refracted waves are each said to be carrying a fraction of the incident energy. But if this were true the reflected and refracted components would have each have less energy than the incident wave and hence would be a different (redder) colour than the incident light.
When a phot hits a surface it sets up a fluctuating electric field. This can be modelled as the combination of two sine waves of electric intensity, with a phase difference ranging between -90 degrees to +90 degrees. In the simplest case the two sine waves coincide i.e. the phase difference is zero. This defines an orientation property for the disturbance. The phot model assumes that this implies a similar property in the phot before impact. The phot can be said to be linearly polarised in that direction. If all the phots in the ray are linearly polarised in more or less the same direction the ray of phots is linearly polarised.
The disturbance has a frequency. The frequency is proportional to the energy of the phot. The constant of proportionality is Planck’s constant. (Note: In these essays there is a heretical view that it may pay to keep an open mind about whether ‘physical constants’ have the same value everywhere in the Universe, at every scale, and every epoch.)
The sine wave effects have associated magnetic effects in accordance with Maxwell’s equations. In summary there is a fluctuating magnetic intensity with direction orthogonal to the electric field and a phase difference of 90 degrees.
The disturbance affects the atoms and electrons in the top layer of the receiving substance, over a length/depth called the extinction length.
Instantaneously one of three things can occur. The phot can be reborn and sent back out and away, with an angle of reflection equal to the angle of incidence (both measured with respect to the normal). Or it can be reborn and be sent further into the new medium (called refraction – see later on). Or it can be absorbed.
The reborn phots can be thought of as child phots. They aree similar to the parent phot but not exactly. For example, a reflected phot will have an orientation that is a mirror image to its parent.
Unlike the wave model, the phot is not split into two component waves. Admittedly when light impinges on a reflective surface, especially at an angle, some of it is reflected and some refracted. In the phot model that outcome is a statistical outcome applying to a whole array of phots travelling in a narrow beam in the same direction (a ray).
Of particular interest is an angle of incidence called Brewster’s angle. At this angle the reflected ray and the refracted ray are at 90 degrees to each other, and both can be discovered to be linearly polarised. The disturbances in the receiving medium send about half of the incident phots back out as child phots. These have an orientation parallel to the surface.
If the new material is opaque and incident light consists of phots of different energy levels, then most of them will be absorbed and some particular energy levels will be reflected. This makes the substance look colored.
Scattering
If a ray of phots in air (say) encounters (for example) a cloud of smoke particles or a fog of water droplets they get scattered in all directions. The wave model treats this as a process of absorption and re-emission. In 1923, using X-rays and clouds of free electrons, Arthur Compton showed that the photon model, plus a bit of quantum dynamics and special relativity, gave a better explanation than the classical approach. More generally, this is true for a range of effects involving phots at various energy levels and interactions at the atomic level, nuclear level or just free electrons.
For scattering effects the phot model follows the photon model, but without imagining the phots are like little balls.
Note: Photons are not little billiard balls and classical Newtonian mechanics does not apply exactly as taught in high school. Relativistic effects apply. For example mass can become energy and vice versa. Nevertheless classical laws of conservation of energy, linear momentum and angular momentum remain as good general guides.
Absorption
Low energy radio waves/photons can go through a wall that stops visible light. This is evident every time you use a cellphone indoors. But why is it so? Naively you might expect higher energy light to have greater penetration ability.
Both the wave model and the photon model handle this easily. Absorption occurs when electron in atoms are excited to a higher state of energy. Radio waves are too weak to excite such electrons (except in very light elements). Visible light however has the right amount of energy to interact with most types of everyday matter.
Glass is transparent because its does not present energy bands that can be excited by the energy levels in white light. But ultra violet light with high energy levels can interact with most types of glass and hence gets stopped by it.
Metals present a loose sea of electrons that are easily moved by all types of light and hence (apart from very thin foils) are opaque across the whole spectrum.
The phot model pretty much follows the photon story, with the exception that a phot is conceived as having a two dimensional lateral presence that decreases with radius. So it can be thought of as having a flat wave-front.
Refractive Index
Light travels at its full maximum speed in a vacuum. Radio waves, microwaves, blue light, red light, infra-red light or gamma rays all travel close to 300 million metres/sec in a vacuum. Other environments are not so simple. For a start they may not be transparent. Generally speaking, low energy light can penetrate quite a lot of (non-metallic) substances but high energy light cannot. Visible light travels easily through air, water, some other liquids, ice, glass, some plastics and some crystals. Transparency is related to how readily the light imparts its energy.
In an optical medium other than a vacuum light travels at a slower rate. The slowdown is expressed as a dimensionless number n called the refractive index (or refraction index). n = c/v where c is the speed of light in a vacuum and v is the speed of light in the optical medium.
However, the higher the energy of the light the slower it travels. So the quoted refractive index is given assuming the light is red. The refractive index of air, ice, water, glass and diamonds respectively is 1.0003, 1.309, 1.333, 1-5-1.6 and 2.4.
If you ask textbooks or the internet – why does light travel more slowly in glass, you can get a variety of conflicting replies.
Note that sound travels faster in water than air, but light travels faster in air than water. So any appeal to the mass density of the new medium is a non starter.
In one type of explanation the wave excites a large number of electrons into becoming little antennae passing on wavelets with a slight phase delay and the superposition of these wavelets creates an overall wave of the same frequency but lower wavelength. A variation on this ‘catch and release’ theme is that the waves or photons are continuously absorbed and re-emitted but with a small time delay on each occasion.
Another type of explanation suggests that photons are scattered slightly over and over again, like a pinball or someone pushing through a crowd, and this lengthens the overall path they take and hence slows them down.
None of these explanations seems ideal. The wavelet model seems to forget that most electrons in non-metallic materials exist in quantised orbitals. Not like the free electrons in antennae. They tend to absorb and emit at very specific frequencies. And probably in random directions. All the competing and contradicting explanations seem to predict a degree of scattering. This would imply that coherent laser light would turn into a cone as it travels, which isn’t observed, or that the spatial and/or temporal coherency would break up rapidly, which isn’t observed either. And none provide an intuitive answer for why the refractive index varies smoothly with frequency.
In situations like this the bottom up approach for building the phot model for light recommends collecting additional evidence before prejudging the outcome.
The phot model doesn’t have an explanation yet. However, it is inclining to a view like this: An incoming phot interacts with the new medium in its surface layer. If it is not reflected or absorbed it progresses into the medium. However it is now in a new environment. There are clouds of electrons around each atom. There are ionic or covalent bonds between the molecules. We know the speed of light in a vacuum bears a simple relationship to the permittivity and permeability of free space (c = the inverse of the product of permittivity and permeability) so why should this not also be true in the different medium?)
Refraction
Refraction is the bending of the path of a ray of light after it encounters a new medium.
Refraction was part of the debate between leading scientists in the 17th century. They struggled with it because the answer depends on the speed of light in optically dense media. And because the speed of light is so enormous it was not possible to measure it in detail until the mid 19th century.
A higher refractive index corresponds to a lower speed of light. Furthermore, the speed of light is no longer independent of energy levels. In an optically dense medium, lower energy light (lower frequency, redder light) travels faster than higher energy light. Furthermore the path of less energetic light bends less. This is the origin of rainbows and Newton’s spectrum
The basic equation describing the path taken by refracted light is called Snell’s Law, after the Dutchman that came up with it. It is simply that sinø1/sinø2. = v1/v2 = n2/n1 where the angles are the offsets from the normal before and after the refraction, v1 and v2 are respective speeds of light in the two mediums and n1 and n2 are the respective refractive indices.
The phot model provides an intuitively simple explanation for why the path of phots becomes steeper when they encounter an optically denser medium and shallower again when they exit. This is because a phot has width. So suppose a phot arrives at denser medium and impacts it at an angle. One side of the phot encounters the new medium sooner, slewing the whole phot around. It is as simple as that. See diagram below
Phot of width W has angle of incidence ø1. Time taken to be fully absorbed is d1/v1 = d2/v2 where v2 is the velocity of light in medium 2. But d1 = H sinø1 and d2 = H sinø2. So d1/d2 = sinø1/sinø2 = v1/v2 = n2/n1 (The last part comes from the definition of what is meant by a refractive index.) Note that W drops out of the relationship, having served its explanatory purpose.
Total Internal ‘Reflection’
Total internal reflection occurs when light travelling inside a dense medium approaches an interface to a less dense medium at a high angle of incidence. For example, if you shine an underwater torch directly upwards, the light will travel into the air above, but if you shine it obliquely it will bounce off the interface and be reflected downwards again. The effect is used to create optical fibres. The smallest angle of incidence that gives total internal reflection is called the critical angle.
The effect works both ways. Hence a fish can see objects above the surface of the water if the objects are somewhere inside a cone shaped zone expanding upwards from the eyes of the fish. Outside that cone, what the fish sees is a reflected panorama of the seafloor.
The refractive index n1 is higher than n2 . Light reaching the interface is refracted into a path closer to the surface. At the critical angle θc the refracted light is parallel to the surface. At larger angles (measured from the normal) all the light is ‘reflected’ back into medium 1.
Applying Snell’s law it is easy to see that the critical angle θc = Arcsin (n2 / n1), which is equivalent to saying that the sine of the critical angle is the ratio of the less dense refractive index to the denser one.
Wave model: Explanations of total internal reflection and associated phenomena have been put forward by great thinkers as far back as the 14th century, including Kepler, Descartes, Hooke, Newton, Huygens, Wollaston, Laplace, Malus and Fresnel.
When a wave hits the interface its electric field vector has a particular orientation. This can be thought of as the sum of a vector in the plane of incidence (called the p component) and a vector square to the plane of incidence (called the s component). The p component is in the same plane as your screen or page, and the s component is square to this, i.e. towards and away from you. Now trace the electric vector components as time unfolds and use that to guide the wave model explanation of what is happening.
The electro-magnetic field effects at the interface are somewhat complicated. Note that there is no phase shift in the reflection when going from an optically dense medium to one with a lower refractive index. If the fish can see a seashell image up at the interface between the water and air then it will be without reversal of the spiral in the shell.
An interesting aspect is that a temporary companion wave is created in the less dense medium as part of the process that sends the totally internally reflected wave on its way. This so-called ‘evanescent wave’ loses its presence within a few wavelengths of the interface. It is possible to rob some of the energy in this wave with a suitable absorber placed just outside the interface, with the result that the totally reflected wave becomes weaker.
Another interesting discovery, first identified by Augustin-Jean Fresnel between 1817 and 1823, is that total internal reflection advances the phase of one of the sub-component waves with respect to the other. This changes the polarization characteristics of the internally reflected wave. Fresnel used the effect to explain the observation that a rhomboid prism made out of a particular crystal can change linearly polarized light to circularly polarized light and vice versa – (see Fresnel Rhomb).
Particle Model: Centuries earlier Isaac Newton favoured a corpuscular model for light and hypothesised that when a corpuscle (particle) tried to move into the less dense medium it encountered a force that pushed it back toward the denser medium. At the critical angle, the force (possibly an attraction by the denser medium) was just sufficient to prevent the corpuscle escaping. Newton noted that total internal reflection could be frustrated by placing a suitable third medium just outside of the interaction zone (e.g. in the above diagram put the third medium where the word ‘reflection’ is printed). This is the region where there is a mysterious ‘evanescent wave’ in the wave model explanation. Newton thought of light as being comprised of discrete particles, but allowed them to create wave-light effects in the surrounding ‘aether’. Hence he may have been open to the idea that the corpuscle creates waves which reflect at the dense/less dense interface and contribute to the subsequent behaviour of the particles.
The phot model: The phot model is trying to gather evidence before reaching conclusions. The wave model is particularly good at dealing with all sorts of polarisation phenomena, as well as all sorts of interference phenomena. It is not so good at explaining how these phenomena arise when the incident light intensity is so low that the experiment can be thought of as occurring one photon at a time.
In the case of total internal reflection, the phot model suggests that when the incident phots meet the interface there is a drawn out interaction. One edge of the phot encounters the lower impedance of the outside environment and accelerates, producing a severe slewing effect. The phot executes a high speed turn and heads back into the denser medium. It is not actually a reflection at all, which explains why there is no phase reversal. It is more like a severe refraction. The path of the phot is just bent. The evanescent wave is simply bits of electric and magnetic field potential that stick out of dense medium while the child phot is executing its turn. That is why it is possible to frustrate the internal ‘reflection’ by placing a third substance just above the interface.
Twisting Phots
If a beam of plane polarized phots traverses through a transparent crystal and a strong magnetic field is applied in the direction of travel, the plane of orientation is turned through a small angle that is directly proportional to the path length, the magnetic field intensity and something called the Verdet constant (which is related to the dielectric properties of the material).
The experiment was first described by Michael Faraday in 1845 and helped firm up the idea that light has a close association with electro-magnetism. It is called the Faraday effect. If you imagine the light is moving in the same direction as the magnetic field line caused by a surrounding solenoid, the turning effect is in the same direction as the current (using the convention that a current is a flow of positive charges). The effect does not work in free space and therefore has something to do with the dielectric medium. It does however work in strong magnetic fields and ionised gases and so it plays a useful role in optical astronomy studying the neighbourhood of stars.
Wave Model explanation: A plane polarized wave is imagined to comprise two counter balancing circularly polarised waves. One of the circularly polarized sub-components moves electrons one way, creating a magnetic field that adds to the external magnetic field, and the other sub-component wave does the opposite. This causes one sub-component to slow more than the other. The sub-components recombine on leaving the medium, recreating plane polarized light with an orientation different to that of the incident wave.
Particle Model explanation: The photon model uses a quantum electrodynamics mathematical description. A plain English description of what is envisaged to be happening is hard to find. Remember how circular polarisation can be modelled by using two sub-component wave-like disturbances that add together with 90 degree phase differences. Well, quantum electrodynamic models do it the other way around. They imagine that the basic sub-components are left and right circularly polarized disturbances and that plane polarisation is a balanced superposition of the two states. They then proceed as per the wave model.
Both the wave and particle model explanations involve breaking light into components with opposite handedness, which then interact in opposite ways with the external magnetic field. While the explanations work, are they convincing?
If light is a discrete package, would not the above explanations eventually tear the package apart as one component lags further and further behind the other in very long apparatus configurations?
Phot model: The phot model (at this stage of its development) suggests the following tentative explanation. Note that the effect requires using a crystal as the medium and it does not work in free space. This suggests that the medium itself drives the outcomes. The external magnetic fields will have some effect on the electron shells in the crystal. This will combine with the strong ionic bonds in the crystal and ‘warp’ the electro-magnetic environment through which the phot is travelling. The final child-phot emerging from the situation will have its plane of polarization rotated.
As a side comment – since the magnets twist the phots a little bit, the phots must twist the magnets and anything they are attached to a little bit the other way.
Waveplates
Waveplates are transparent crystals with structures that affect circularly polarized light according the direction of rotation. A quarter-wave plate can be used to turn linearly polarized light into plane polarized light and vice versa. A half-wave plate can change the plane of polarisation of linearly polarised light.
Quote from Wikipedia: “A waveplate works by shifting the phase between two perpendicular polarization components of the light wave. A typical waveplate is simply a bi-refringent crystal with a carefully chosen orientation and thickness. The crystal is cut into a plate, with the orientation of the cut chosen so that the optic axis of the crystal is parallel to the surfaces of the plate. This results in two axes in the plane of the cut: the ordinary axis, with index of refraction no, and the extraordinary axis, with index of refraction ne. The ordinary axis is perpendicular to the optic axis. The extraordinary axis is parallel to the optic axis.
For a light wave normally incident upon the plate, the polarization component along the ordinary axis travels through the crystal with a speed vo = c/no, while the polarization component along the extraordinary axis travels with a speed ve = c/ne. This leads to a phase difference between the two components as they exit the crystal. When ne < no, as in calcite, the extraordinary axis is called the fast axis and the ordinary axis is called the slow axis. For ne > no the situation is reversed. Depending on the thickness of the crystal, light with polarization components along both axes will emerge in a different polarization state.”
Particle Model explanation: This author is not clear how the particle model, or its successor the photon model, can explain this optical experiment’s results.
Phot Model explanation: The phot model explanation is similar to that of the wave model. Start with circularly polarized light passing through a quarter waveplate as in the explanation given above. At first contact the phot turns into a child phot and the crystal’s peculiar properties affect the two sub-components differently, slowing one more than the other and thus bringing their phases closer to the other. Same explanation for how circular polarization can become linear polarization.
Spin and angular momentum
As quantum mechanics was being developed physicists realised that electrons had to have a property they called spin, always the same in magnitude. Protons and neutrons turned out to have this property also. Wolfgang Pauli developed the Pauli exclusion principle that says electrons can share the same energy level orbits around a nucleus only if they have opposite spin.
Spin is a mysterious property. It works well in the mathematics of quantum mechanics applied to atomic and subatomic physics. But is has proved difficult to describe in plain English. It isn’t thought to be the case that electrons, protons and neutrons are actually spinning like little balls. They would have to do so at impossible speeds to produce the right amount of angular momentum effects observed.
Einstein and others came to realise that photons delivered twice as much spin as this whenever they interacted. Exactly h/2π where h is Planck’s constant equal to 6.63×10-34 Joule.sec. This amount does not vary with the energy level of the photon. Which is remarkable because gamma rays have energies more than a million billion times stronger than low frequency radio waves.
Note that Planck’s constant is also the ratio between any photon’s energy and the frequency of its electromagnetic disturbances upon absorption.
Spin is even more mysterious for photons than it is for electrons because photons have no mass. Some texts try to link spin to circular polarisation, but the phot model does not. Mainly because a linearly polarised phot also has spin of plus or minus 1.
We know from experiments that a beam of light can impart angular momentum to small absorbing particles i.e. it subjects them to a degree of optical torque. Does this come purely from the net spin of the phots in the beam, or can a beam of light have angular momentum other than the net aggregate of the spins of its constituent photons?
It seems that the answer is yes. In a well collimated beam the ‘spin angular momentum’ is identified with its integral polarization and anything left over is assigned to ‘intrinsic orbital angular momentum’. The wave model constructs helical patterns in an overall wave front and use that to explain some of the observed effects.
There is third type of angular momentum arising if the photon or phot has its momentum vector off-centre in whatever system of coordinates is in use. Think of a skater gliding past a pole in a straight line. If the skater reaches out and grabs the pole, then the angular momentum is readily apparent. It is called extrinsic angular momentum.
Spin and orbital angular momentum effects are an ongoing area of research in optics at a nanometer scale (see nano-otics, photoics and plasmonics). Look for spin-Hall effects in which the spin creates its own electro-magnetic interaction with the wider world. These are said to occur in any optical interface and produce tiny effects such as deflecting reflected and refracted beams slightly away from the plane of incidence.
Particle Model Description: The quantized particle model of light uses probabilities for the two spins states of particular photons. Some texts say that a photon is linearly polarized (plane polarized) when it can be described as a superposition of equal amounts of the left-handed and right-handed spin states. Circular polarization occurs when just one of the spin states is present. But how does this account for the experimental fact that circularly polarised light can be converted into linearly polarised light?
Recall that polarisation effects are only manifest when photons are destroyed. However people cannot help but try to imagine what is going on the photon before it is destroyed.
Phot Model Description: The phot model is basically an attempt to come up with an intuitive model that fits all the evidence. In the phot model, phots do not do anything when they travel except go very fast, changing their phase as they travel. The phot model downplays what phots look like when they travel because this is fundamentally unobservable. All we can observe are bits and pieces of evidence arising from the destruction of a great number of phots.
Nevertheless it is tempting to speculate that a phot is a package of energy travelling very fast and that it is a carrying spin in the form of a slow rotation of the whole package. Then maybe negative spin is a slow rotation in the other direction.
Or maybe spin is not a property of the phot at all. Maybe phots travel in a vacuum energy density field or something like that and put a ‘twist’ in that field and the twist travels with them. When the phot interacts with something and delivers its package of energy and associated effects, the twist delivers the spin to the something. An analogy might be a clock spring. Spin can be incorporated into a wound up clock spring and be released latter on.
Spin is finally delivered into a receiver, usually an electron shell. The electron may have spin = -½ until the phot interacts with it. It ends up with the extra energy from the phot and spin = +½.
If we are talking about a beam of phots, then group effects come into play. Macro patterns. For example, suppose a beam takes the form of a thick tube with an impact pattern in the shape of a ring. It is conceivable that phots arriving at the 12 o’clock point have a phase slightly ahead of those arriving the 1 o’clock point and so on around the dial. The overall effect would be a pattern that goes round and round the edge of the clock face. You could even imagine the tube of phots to be helical. But none of the phots in it would be travelling a helical path.
In summary, spin is a bit paradoxical in the world of quantum physics because an intuitive plain English description of it remains elusive. If a naive phot model can do better than this then it would be a feather in its cap.
Three Polarisers Experiment
The three polarisers experiment is very simple and yet very interesting. All you need is three polarising filters. These are often sold as camera filters. Polarized sunglasses will do at a pinch.
Line up a polariser and look through it at a lamp. The filter is only letting about half of the light through. It lets through any light whose polarisation lines up with it own angle of polarisation, and less and less light as the incoming light has a polarisation angle different to its own.
Now look through the second polariser at the light coming through the first. Rotate it around to see what happens. At one angle it makes no difference. But at an angle at 90 degrees to that it cuts out any light getting through at all. The filter looks dark. If the filters have an indication on them as to their polarisation orientation you will see the blackout happens when the two filters have their lines of orientation at right angles to each other.
Now comes the most interesting bit. Insert the third filter between the other two. You would think that adding yet another filter can only stop even more light getting through. Rotate the middle filter to check this. You will find that at an angle midway between the other too, the dark third filter suddenly brightens. About a quarter of the incident is making its way to your eye.
How can adding a third filter make a positive difference? It is just a simple linear filter like the others. (The filters contain long lines of molecules that act like metals, all aligned with each other. Light waves give up their energy to these molecules if their electric vector vibrates parallel to the lines of conductive molecules. Cross two filters and nothing should get through.)
Evidence against Einstein? Einstein was inclined to believe that photons and sub-atomic particles acquire definite properties when they are created and that these inherent properties pre-determine what happens when those particles interact with subsequent detection equipment. If not precisely, then at least in a probabilistic way.
The Copenhagen interpretation of quantum mechanics is that particles have a variety of potential properties all superimposed together and when an observation is made one of the states for one of the properties triumphs and all the other possibilities for that particular disappear.
Some physicists/authors inclined to the Copenhagen Interpretation (CI) say that the three polarisers experiment is evidence against the Einstein viewpoint. They say that the experiment consists of photons passing through or not passing through a sequence of filters. Each photon is either absorbed in a filter or it passes through that filter and on to the next one.
If Einstein’s Hidden Variables (HV) view is correct, then the likelihood that a particular photon will pass through first filter is already pre-coded in that photon. It will also have pre-coded instructions for what do if it meets the middle filter and pre-coded instructions for what do if it meets the last filter.
The instructions for whether to pass through the last filter or not is not affected by the first two filters because the photon passed straight through those two filters. So they say that the HV view cannot explain why the presence or otherwise of the middle filter is making a difference to the outcomes at the third filter.
Conventional explanations: For clarity, here is a repeat of the facts. If you pass a normal beam of light through a reasonably good polarising filter about half of it gets through. If you then try to pass the emerging beam through another extra polariser turned through 90 degrees then no light gets through. But if you insert a third polariser between the other two, aligned at 45 degrees to both of them, then about 25% of the incident light comes out of the third and final polariser.
The essential point is that adding an extra filter results in more light coming at the end, not less.
Try reading multiple textbooks or online lectures in an effort to find a simple explanation of this simple experiment. You will discover a mixture of different explanations that contradict each other and create complicated confusion.
Suggested Explanation using Phots: In the phot model an incident phot is absorbed into the polariser and is either absorbed or it gives rise to a new and very similar ‘child phot’ travelling on the other side of the polariser in the same direction as the incident photon. The process is effectively instantaneous.
It is now quite easy to describe the three polarisers experiment. Let us start with light that is initially unpolarised. In other words it consists of phots that are polarised in random orientations. The first polariser absorbs all the phots and recreates about half as many child phots. The phots that arrive attuned to the first polariser’s plane of polarisation are the ones that tend to give rise to child phots with a similar persuasion. The others phots are the ones that are absorbed.
The middle polariser absorbs all the phots it receives and emits many (about half for 45 degree relative orientation) ‘grandchild’ phots. The grandchild phots have an average orientation more aligned to the plane of polarisation of the middle polariser. This gives the effect of twisting the beam of phots.
Likewise at the third polariser. The ‘grandchild’ phots emitted by the middle polariser are oriented in a way that leads to about half as many ‘great grandchild’ phots being created and emitted by the final polariser.
The hidden variables idea is not being contradicted at all. Simply for the reason than none of the initial phots reach the second polariser let alone the third. It is their ‘offspring’ that reach the next polariser, not them. And like all offspring (!) the new generation of phots has its own ideas about what to do next.
Summary
A preceding essay argued that it might be possible to develop a better understanding of light and an improvement over the internally inconsistent wave-particle duality approach. It suggested abandoning misleading analogies, starting with a fresh sheet of paper and building up a picture based purely on the evidence of experiment. To avoid mental baggage from the past it says that light is made up of ‘phots’. Then, as an example, it started to start build a framework in which a model for phots might emerge.
In this essay the most common types of optical effects are examined and used to construct the basic framework. The essay looked at reflection, absorption, scattering, various polarization effects and the three polarisers experiment.
The idea that a single model might be able to explain all of these phenomena seemed to fare reasonably well. The explanation of spin is still work in progress.
The next essay takes a look at the plethora of optical experiments that use interference effects in some way, including the one which is particularly problematic for the particle model – Thomas Young’s double slit experiment.
Reference
Van de Vusse, Sjoerd B.A., 2024, Some ideas and experiments for issues affecting modern physics, https://hereticalphysics.com.au
Author contact: SBAvan@utas.edu.au
Author’s location: Hobart, Australia
