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I was having fun with my laser. What causes this reflection?

Edit: I had an idea and tilted the surface of this plaque by 90°. I now get a straight line, such as the reflection from the plaque below:

enter image description here

Yes, they're different plaques but THE SAME MATERIAL. The second picture is just a 90 degree rotation of the first! It's as if I turned the first plaque sideways.

Why does the material of the plaque cause the reflection to project a semicircle when tilted? And why is the reflection a line and not a single point, such as when reflected off a mirror?

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    $\begingroup$ For comparison, can you show what the reflection looks like on the other surfaces, and on a hand-held mirror? This is almost certainly caused by the curvature of the plaque, but with an uncontrolled experiment like this it can be impossible to tell for sure. $\endgroup$
    – Emilio Pisanty
    Commented Oct 8, 2019 at 15:18
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    $\begingroup$ @EmilioPisanty that's a good point. I'll fix it later tonight. For some reason, physics.SE is smarter and more constructive than all other SEs... $\endgroup$
    – Jossie Calderon
    Commented Oct 8, 2019 at 17:46
  • $\begingroup$ @EmilioPisanty see edit $\endgroup$
    – Jossie Calderon
    Commented Oct 9, 2019 at 2:21
  • $\begingroup$ I get circles like that when I shine a laser on a needle. Does your plaque have a texture (like vertical needles laid side-by-side)? Do you get the circles if you rotate the plaque sideways (or shine the laser in the horizontal plane)? $\endgroup$
    – Keith McClary
    Commented Oct 9, 2019 at 15:26
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    $\begingroup$ It's quite hard to understand the geometry of your projections from words. Could you add a sketch, showing position of the laser, the beam, the reflecting surface and the surface you're projecting the reflection onto? And it'd also be good if you showed on the sketch what you mean by "tilting by 90 degrees". $\endgroup$
    – Ruslan
    Commented Oct 11, 2019 at 15:58

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Your description of rotating by 90 degrees doesn't make any sense. You show an image from one plaque "5th place" then talk about rotating 90 degrees and show an image from another plaque "4th place". It's not clear if this is meant to show the "90 degrees rotated" behavior or just that you see similar behavior for different plaques.

In any case what you are seeing is an example of specular reflection. The surface off which the light reflects is not perfectly flat so you don't get a direct reflection of the beam you send in, rather you get a distorted version. It is like a funhouse mirror.

Funhouse mirror

Notice how the curvature of the surface "stretches" the reflection in various directions depending on the direction of the curvature.

It is possible that the metal plaque off which the light is reflecting was polished (before engraving/labeling) using a circular polisher tool. This polishing might leave a very minimal circular pattern in the surface topography of the metal surface. When the laser beam hits this circular pattern it becomes distorted/stretched in the directions along the curvature of this pattern.

It is a bit difficult to say what exactly is going on without more information.

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    $\begingroup$ @JossieCalderon per the hypothesis in my answer I would need information about the granular structure of the plaques at the location where the laser beam is hitting the particular plaque. That is, what matters is the granular structure where the beam hits the plaque. What pattern do you see if you shine the light on a particular location of a particular plaque at many different angles? What if you shine it on many different locations at different angles? What if you repeat the same set of experiments on many other plaques? $\endgroup$
    – Jagerber48
    Commented Oct 11, 2019 at 23:25
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    $\begingroup$ Also the 4th place plaque looks like it is vertical to me. I am basing this on the fact that I assume the plaque is mounted on a wall and then surface above the plaque is the ceiling. Then, given how the text is oriented, it looks to me like that plaque is mounted vertically. I don't know why you're saying it's horizontal. $\endgroup$
    – Jagerber48
    Commented Oct 11, 2019 at 23:26
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    $\begingroup$ @JossieCalderon A close up image of the plaque material would help make it more clear what you are talking about. In any case, what you say lends more evidence to my idea that small scale granular structure (possibly from the polishing process for the metal) creates a sort of fun house effect on the light which stretches it depending on the granular orientation of the roughness. Is there anything you do not understand about my answer? $\endgroup$
    – Jagerber48
    Commented Oct 12, 2019 at 17:21
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    $\begingroup$ @JossieCalderon The reason the pattern changes when you rotate the plaque 90 degrees is because whatever the small structure surface pattern is on the plaque is not isotropic so when light hits at different angles it makes different patterns. This hypothesis is correct. Maybe you can propose your own hypotheses as you perform more tests. $\endgroup$
    – Jagerber48
    Commented Oct 13, 2019 at 2:01
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    $\begingroup$ @JossieCalderon Please include diagrams of what you are describing if you would like any more detailed responses than you've received so far. You are not clearly communicating your setup (and I'm not the first person to tell you this in this question). The diagrams should indicate the orientation of the plaque the direction from which the laser beam approaches and leaves the plaque as well as the declination angle between the laser beam and surface normal and how the beam looks as a function of these things. All of those details are relevant to understand the phenomena you are observing. $\endgroup$
    – Jagerber48
    Commented Oct 13, 2019 at 19:04
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The laser projects onto the plaque which causes a reflection onto the roof.

On the vertical plane, the laser projects directly into a slit which causes diffraction of the light. This is why the second picture looks like a "stick" of light.

But on the horizontal plane, think backwards. The projection onto the roof comes from the laser hitting it at an angle. For example, think of the point furthest away from the middle. If the laser were to directly shine onto the plaque to reflect on this point ("A"), you'd have to hit the plaque at an angle.

But that's exactly what the diffraction of the laser through the slit does on the horizontal plane.

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  • $\begingroup$ Thank you! This is exactly what I was looking for! $\endgroup$
    – Jossie Calderon
    Commented Oct 19, 2019 at 15:19

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