Re: [Phys-L] Inverse Square for reflected light

[Scott Goelzer <s.goelzer@comcast.net>]

Thanks for looking at this. 
I still think the reflected ray should follow 1/r^2, but I was working on intuition. 
As you said, the reflected ray is still a ray, but perhaps a bit more dispersed. 

Background:
These are not my students, I am just running down a question from a colleague. 
So…I am not entirely familiar with the experimental setup. 
From what I could gather second hand: 
A single ray from an incandescent source collimated from slit mask. 
Detector: https://www.pasco.com/products/sensors/pasport/ps-2106 <https://www.pasco.com/products/sensors/pasport/ps-2106>
Incident on a 10cm x 10cm rear surfaced aluminum plexiglass mirror. 
Angle of incidence is at either 10,°,30°, 45° to the mirror - they did several angles . 
The detector has a collimated tube on it. 
Students are moving the detector ~0 to 50cm along the beam after the reflection. 
I think they have the beam and detector reasonably collinear. 

Since the students are getting linear results for seemingly good reasons, I feel their work needs to be addressed. 

Some dubious sites and forums that all boil down to 'if the rays leave parallel then inverse square doesn’t work'. There are more.
https://www.parabolixlight.com/debunking-the-inverse-square-law <https://www.parabolixlight.com/debunking-the-inverse-square-law>
https://www.digitalphotopro.com/technique/lighting-techniques/master-the-family-of-angles/2/ <https://www.digitalphotopro.com/technique/lighting-techniques/master-the-family-of-angles/2/>
https://www.youtube.com/watch?v=L1sV6TAhjfU <https://www.youtube.com/watch?v=L1sV6TAhjfU>

My current opinion is the the reflected ray is dispersed and they are not measuring a far enough range to see 1/r^2.

Scott



> On Dec 8, 2021, at 1:10 PM, John Denker via Phys-l <phys-l@mail.phys-l.org> wrote:
>
> On 12/8/21 9:34 AM, Scott Goelzer via Phys-l wrote:
>
> > using a Pasco light meter and a standard ray box
>
> 1) I hate to ask dumb questions, but.... 
> Have they got the geometry right?
>
> 1a) I can imagine scenarios where they move the mirror in
> such a way that the image does not "fill" the mirror; that 
> is, the edge of the mirror cuts off some of the beam.
>
> 1b) I can imagine scenarios where the sensor module blocks
> the beam on its way from the source to the mirror.
>
>
>
>
> 2) The "ray box" can be used in lots of ways.
> Much depends on what you consider "standard".
>
> It can be configured to put out a large beam that is
> more-or-less collimated:
>
> 	_______________________________
> 	_______________________________
> 	_______________________________
>
>
> Or a beam that diverges from a more-or-less point source:
>
>           /
>          /
>         /
>        /_____________
>        \
>         \
>          \
>           \
>
>
> So you can have a dependence that goes like 1/x⁰ or 1/x² or
> anything in between. In general you will get non-power-law
> behavior that only asymptotically settles down to a power law.
>
>
> > Best explanation I could give was that the mirror was not functioning
> > as a point source any longer and was producing effectively parallel
> > wave fronts.
>
> That leaves us with the obvious follow-up question: why would
> it do that?
>
> If the mirror is behaving as normal mirror, the method of
> images should apply. That is, the situation should look like
> light being emitted from the image.
>
> > some photography sites confirm the effect without explanation
>
> Reference?
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