A variation on the Fresnel Spot experiment

[sophiecentaur]

the light hitting the disc causes movement of the electrons, or the flow of currents, on the surface of the disc.

I don't think the details of the mechanism relevant (first order). A conducting or non conducting obstacle will both produce the effect. The limits of a simple definite integral will do for predicting the spot size.

Edit: I'm not confident that what I just said is correct, so please ignore me. I'm leaving this post here for continuity of the thread.

I think you are right to strike it through. Just think how, when you look at an object in a lens, you see the object somewhere in the centre and not 'from any particular part of the lens'. What you see is a result of diffraction (yes - diffraction from all over the lens is a 'thing'). It even works with a stereo pair of speakers; the image is perceived as in the middle - where there's nothing.

With an obscuring a beam with a disc, the resulting spot is very faint and a normal camera sensor will not have the contrast capabilities to handle the range but multiple exposures can be stacked to produce a highly enhanced contrast ratio (which can be gamma'd to display the whole range). It will have been done many times.

 

[Bernadette]

Thanks Sophie

In the usual experience of the shadow of a disc, we look at this shadow (and its central point...). The question was: what do we see when looking towards the source of light hidden by the disc? In diffraction, light is considered to be a wave. This makes it possible to "understand", to calculate the intensity of light at each point of the shadow. In other experiments on light, we look in the direction of the source of light, and we look where it comes from; and possibly whether it has been diverted. And the light is in these experiences, considered as a particle. The question was therefore to design an experience where we can both look at where the light comes from and where it arrives. That of the Fresnel spot seems appropriate to me.

Bernadette

 

[tech99]

Remember, however, that in order to resolve an image of the disc, the detector cannot be a point, but must have finite dimensions. The detector itself is then subject to interference effects, so the disc must be large enough to allow an effective directional detector to be employed.

 

[Drakkith]

The detector itself is then subject to interference effects,

I'm not sure I'm following you. What interference effects is a camera sensor subject to?

 

[tech99]

I'm not sure I'm following you. What interference effects is a camera sensor subject to?

I do not know what the consequence is for this experiment, but if the camera aperture is made very small, so it is small compared to the spot, then the beamwidth of the camera will be increased, lowering the resolution. We have interference between rays from the centre and edges of the aperture, and these should cancel at the edges of the beam. The effect is described by Rayleigh.

 

[sophiecentaur]

And the light is in these experiences, considered as a particle.

I can't imagine what you mean by this. Every measurement or 'experience' of light relies on quanta of energy interacting with 'matter'. Diffraction calculations will give you the answer to the question of the energy distribution over any sensor.

Remember, however, that in order to resolve an image of the disc, the detector cannot be a point, but must have finite dimensions. The detector itself is then subject to interference effects, so the disc must be large enough to allow an effective directional detector to be employed.

I'm assuming this 'detector' is the eye or a camera so it would have sufficient resolution to identify the Fresnel Spot but it wouldn't necessarily be capable of measuring the actual angle subtended by the spot.
I'd actually question the phrase "image of the disc". I would say that what you are seeing, when looking towards the light source, is actually a modified Image of the light source. This would be just the same as if you were looking through a lens or just through a hole. The 'position' of the spot, along the prime axis, would not be where the disc is - any more than any image is in the plane of a lens or even in the plane of your shaving mirror.

Imo, the nearest thing to this Fresnel Spot thing is what you see through a zone plate. In that case, the plate is much like a lens.

 

[Drakkith]

I do not know what the consequence is for this experiment, but if the camera aperture is made very small, so it is small compared to the spot, then the beamwidth of the camera will be increased, lowering the resolution. We have interference between rays from the centre and edges of the aperture, and these should cancel at the edges of the beam. The effect is described by Rayleigh.

Sure I guess, but what does this have to do with the rest of the thread? Were your previous posts just saying that the disk (or sphere) needs to be large enough to be resolvable by the camera in more than a handful of pixels?