# A variation on the Fresnel Spot experiment

• B
The black line represents the limit of the geometric shadow. In summary, Fresnel discovered that when sunlight passes through a small hole, the shadow of a small disc has a bright point in its center. This led to the experiment of looking at the shadow from the direction of the light source, which showed the edge of the disc dimly lit up due to the diffraction of light. This experiment can also be done with the shadow of the edge of a screen. The diffraction of light is a complex subject that can be studied through mathematics and experiments using microwaves. However, this is different from gravitational lensing, which was studied by Eddington in 1919 and
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.
tech99 said:
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.

tech99 said:
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?

sophiecentaur