Halo Laser

Why does a laser shined on brushed alumiun reflect a solid halo, when other surfaces just reflect the red dot?

The key to your answer is “brushed aluminum.”
The surfaces which reflect just a red dot are usually polished or very flat. This causes them to reflect the laser light just like a mirror, with very little scattering.
Brushed aluminum, on the other hand, has very tiny scratches on its surface. The sides of these scratches will cause the laser light to scatter in different directions, forming the “halo.”

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