A simple edge diffraction experiment on light is described and the observations are discussed using the wave model, particle model and a prototype unified model for light called phots.  The evidence is relevant to an investigation of whether a unified model for light might be possible.  

Introduction

Previous essays in this series challenged the assumption that wave-particle duality is just a fundamental mystery of the Universe.  The essays suggested putting old analogies aside, starting with a fresh sheet of paper and using a bottom up building process based solely on experimental evidence.  To illustrate the idea they started a prototype project called ‘phots’.

Some simple but yet fundamental experiments can be done at home.  This essay describes diffraction at an edge and diffraction through a slit.  The outcomes are examined for features not usually described or taught and then an attempt is made to describe the observations in terms of the emerging phot prototype.

Edge Diffraction – Fresnel circumstances 

When a ray of suitable light is shone on and around a sharp edge or corner of an opaque object, a set of interference fringes can be seen on a screen. Monochromatic light from a laser is a good source of the light and the edge of a knife or razor is convenient for the opaque object. 

Knife Edge Diffraction:  A narrow ray of light (ideally monochromatic) shines on and past a finely edged opaque body A and onto a screen several meters away, point m.  A pattern can be observed on the screen between P and P’.  P is in the geometric shadow.  The intensity (brightness) of the pattern is show in the magnification.  The pattern is well described by Fresnel’s equation.  It changes if the screen is positioned closer or further away.  If the source and the screen are both a long way from the Object A, the pattern is simpler and is described by the Fraunhofer equation.  There are many variations on this basic experiment. For example the object could be enveloped in a beam of light and have a large aperture in it.  

There are two ways to look at this effect.  You can look at the dark silhouette on the screen and the light surrounding it.  Or you can place your eye or camera in the shadow zone and look back at the opaque object.

Home Experiment on Edge Diffraction

Edge diffraction occurs when light from a point source passes over the sharp edge of an opaque object, such as a knife or razor.  The experiment is usually done by letting the light fall on a screen and discussing the pattern of lines and stripes that can be seen on the screen.  In this experiment the screen was replaced by a human eye looking back towards the source.  Note that the two viewpoints observe different things.  In a sense the patterns on the screen show where the photons died.  Looking back shows where they have come from. The following experiment is a looking back experiment performed in a home kitchen.

Equipment: Several kitchen knives with sharp edges, some of them curved, a knife block, some Blu-Tac and a red heat lamp.  Plus a playing card and some polaroid sunglasses.  The light source emits red light but infrared as well.  The source light is not polarised.

Method: a large knife was affixed with Blu-Tac so that it was projecting half way from the knife block.  Red light was shone on the flat side of the knife from several meters way.  Room darkened.  One eye closed and the other positioned in the shadow of the knife, about 50cm away, looking in the direction of the (obscured) lamp and focused on the edge of the knife.

Observations: 

1. A dark line could be seen to hover just outside the edge of the knife.  Then up to about ten other lines outside of that of decreasing thickness and closer spacing.  The image was probed with the playing card and it was confirmed that the fringe pattern was coming from free space outside the physical profile of the knife.  Grey fuzziness was observed above and below the fringe pattern.  The fringe pattern had some white horizontal patterning, orthogonal to the dark stripes.  The overall fringe pattern was quite bright, of uniform color and the color was the same as the lamp.  The intensity of light coming from the region of the fringe seemed comparable to the intensity of the red light arriving at that area but this was hard to determine, noting that the eye continuously adjusts to brightness and anyway there was a knife in the way.  Moving the eye from side to side and up and down had no effect.  Moving the eye closer also had no effect, until the eye could no longer focus.  Moving the eye back also had no effect, except that the knife and fringe pattern and knife block all looked smaller in the usual way.  The conclusion was that the observed pattern was coming from a region within 2 mm of the knife edge. 
2. All the above was repeated wearing the polaroid sunglasses, turned through various angles.  The image was dimmed by the lens tinting but no polarised effects could be detected.  Note that the heat lamp light source was not polarised either.
3. While experimenting with the experiment a stray flash of white light came through the gap briefly.  This had a red outer edge to it followed by some spectrum.  
4. A second knife was inserted into the same slot in the knife block, with its edge facing the first one.  Its edge was slightly curved which meant the gap between the two knives varied between zero and 3 mm.  This opposing knife edge developed a fringe pattern that was the mirror image of the first one.  This also had the same orthogonal patterning, so this was attributed to the element of the heat lamp, which was a white hot horizontal wire behind red colored safety glass. 
5. Where the gap was 3mm each knife had its own fringe.  As the gap narrowed the two fringes overlapped.  Flexing the smaller knife back and forth a bit made no discernable difference.  Where the gap was closer still the two fringes seemed to merge into one uniform fringe.  Squeezing the two knives together to try to get the two fringes to overlap, light on dark, so that the whole fringe area went dark, did not work at all.  
6. Looking to where the gap became narrower still and eventually zero an interesting effect was observed.  The light was bright except for two dark lines and no others.  Then suddenly, closer to where the knife edges touched, it all went dark.  No visible light came from the area near the V intersection area.  Furthermore the edge of this shadow area had a neat semicircular profile.
7. Later attempts to recreate this dark V effect just using two knife edges and a distant white light produced a slightly different effect.  A shadow still cloaked the intersection area but this time the shadow line was pointed.  

General Discussion

Textbooks invariably use the Huygens principle of imagining that light consists of wave fronts and that these can be modeled as a superposition of innumerable little wavelets and that differences in arrival time at a screen allow for constructive and destructive interference effects and so create the dark and light fringe pattern observed on the screen.  The principle was probably inspired by water waves.

Discussions of knife edge diffraction usually concentrate on the interference pattern observable on a screen. The mathematical formulae based on Huygens’ ideas and developed by Fresnel, Fraunhofer, Cornu etc. explain the intensity distribution very well right across the pattern and at any distance from the screen.  This includes a tiny bit of light arriving on the screen within the geometric shadow of the knife.  

We now know that light is discrete in nature.  Just looking at the splash of its destruction on the screen wastes a key piece of information – where did it come from?  Our eyes are very good at detecting this so it is interesting to replace the screen with a camera, or set of eyes (suitable protected if using laser light or sunlight).

Simply looking back along the rays coming from diffraction over a knife edge reveals that a dark and light pattern can be seen in close proximity to the knife edge itself.  The light emerging from this pattern then progresses to the screen, where it is absorbed and reflected in various ways to produce the intensity patterns normally talked about and modeled by Fresnel etc.  Looking back towards the knife and the source gives a fresh perspective to this type of experiment.

Another way to explore the actual path of the light between edge and screen is to use a smoke machine to scatter some of the light towards an observer. 

It seems slightly odd that no polarisation effects were observed. The classic wave explanation for how a linear polariser works is that some orientations are allowed through while others are blocked.  Here we have two highly conductive knife edges on either side of the light ray and yet no apparent polarisation.  

Discussion in terms of Phots

The first band between the knife edge and the first dark line is quite bright.  The phot model suggest that this is simply a case of phots ‘tripping’ over the edge of the knife and changing direction as a result.  Phots have a lateral effective width.  They slow down according to refractive index. However, here the knife tip is too thin for these phots to be captured and absorbed.

Some phots are deflected downwards when grazing across the top of a knife edge.  Low energy pots will be deflected more than lower energy phots, simply because they are “wider”. The diagram shows the lower part of each phot bending as if it were refracting, but this is artistic license because the metal knife has an enormous refractive index as far as visible light is concerned.  There are a lot of free electrons in metal so the bottom part of the phots might flow over and around the tip rather than through it.  

This effect is reminiscent of Arago’s spot.  It might account for the phots that end up on the screen within the geometric shadow of the knife.  

The brief glimpse of color dispersion that was noticed when white light passed through the slits is consistent with the above narrative.  Note that the dispersion in “upside down” or “negative” compared to diffraction through a prism in that red light is bent more than the more energetic blue light. 

 The main feature of interest is the fringe pattern that could be seen directly above the knife edge. 

Simple Knife Edge diffraction experiment using a back looking perspective.   A fringe pattern can be seen in the vicinity of the knife edge.  Photons reach the eye from the gaps between the dark areas, but not from the dark areas themselves.

The image seen by the observer shows a fringe pattern of dark lines just outside the edges of the knife or knives.  A bit like a fence made out of a lot of dark wires.  The dark lines seem to be above the knife edge but might be slightly closer to the observer. 

Simple Knife Edge diffraction experiment:  This is the view as seen by an observer looking back towards the source.  The picture stays the same when looked at from different angles and distances.  The diagram is about 50% scale.

A fringe pattern can be seen in the vicinity of the knife edges.  Phots reach the eye from the bright bands between the dark lines but not from the dark lines areas themselves. There are separate fringes where the gap is about 3mm.  In the 2mm zone the fringes cross each other without any noticeable diminution of the light intensity compared to the 3mm zone.  In the 1mm zone the fringe pattern is uncluttered and symmetrical. 

Near the junction of the two knives there is a dark zone.  It is illuminated from behind but no phots reach the observer.  The edge of the dark zone is curved, a bit like a concave meniscus. The evidence suggests that phots incident on this zone are absorbed by the knives.  Another possibility is that instead of being refracted from side to side they suddenly begin to be refracted upwards. 

Where the gap is wider there is just one fringe pattern, symmetric between the knives and consisting of quite strong lines.  This suggests that the knives are equal partners in creating a single effect.  Above that the two sets of lines overlap. Above there is a fringe pattern on each side a gap between them. The two patterns look like mirror images.

Three possibilities suggest themselves.  In principle, each dark line could be caused by:

1. something  absorbing the phots,
2. something bending the path of the phots so that the dark lines are a multiple mirage of the knife edge, or something like that,
3. something is causing phots whose path goes through the dark areas to be scattered, 
4. all the incident light is transmitted via the bright areas and not from the dark areas.

Option 1 is not favored because the overall intensity seems strong and because moving the patterns across each other did not cause darkening.

Option 2 seems better.  But no hypothesis could be thought of to produce such an effect.  Note that the image did not change when the observing eye was moved closer or further away.  Option 3 is not favored because the picture was sharply defined.  Option 4 seems the most likely.

Speculative hypothesis: Any hypothesis about what is being observed needs to take into account other evidence about phots.  The following suggestion may seem a little bit strange, but perhaps not.  It is easiest to describe for the ~1mm gap zone where just four or five strong dark lines were observed in the gap.  On either side are the two knife edges.  The knives themselves are being bombarded with phots, many of which are absorbed. This is imparting electro magnetic energy to the knives. There are free electrons in the metal.  The knife edges are like antennae.  It is possible that they set up resonant field across the air gap.  A standing wave.  About five wavelengths long (i.e. infrared/microwave energy levels).  Stretching right across the gap and along it in each direction.  Like a multi-wire fence, but invisible.

(This may seem a bit strange but strange things happen in the nanometer zone around solid surfaces, especially metal ones.  See for instance the Casimir effect.)  

In the bright bands, phots are passing through the fence with ease.  In the dark line, phots are being disrupted.  They interact with the standing wave.  The result that their paths are displaced to one side or the other. They join the phots in the bright bands.  This is what makes the dark lines dark.

But that is not all that is happening.  Something happens to their phase as well.  All along the fence phots are having their phase altered slightly with the effect that they leave the fence with same phase as each other.  

The overall effect is to create something that strongly resembles Huygens’ wavelets idea.  Lots of phots with the same energy and phase going forward in a synchronised manner.

The rest of this story follows the standard Fresnel/Fraunhofer explanations and mathematics.

There are a few nice things about this hypothesis.  If true it means that it is possible to ‘see’ a microwave standing wave using nothing more than a small bright source of monochromatic light and two sharp metal edges, e.g. two razors held in a clamp with a sub-millimeter gap between them.  The other nice thing is that it suggest some experiments to check if it possible to ‘organise’ a ray of monochromatic light with a suitable transverse field of some sort.

This is an initial hypothesis.  It remains to be seen if it has any explanatory usefulness against a wide range of other interference phenomena.  If the idea seems promising the interaction between sharp edges and nearby beams of phots would bear a lot more investigation. 

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