Ask @iamthe202:

You posted this image: Could you explain why it didn't occur to you to ask "How flat is my table?" or better yet "How do I check if my table is actually flat?"??

Niels Madsen Kloster
I did check it and it was flat. I also recorded a video of this experiment:
i tried to find other longer flat surfaces to do this experiment on, but couldn't find any yet, then I asked a friend to do the same on his 5 meters long table. There are the before and after pictures from both sides of the table:
You got to really go down with your camera, while the edge of the table is still not what's obstructing your view of the bottom of the object, but the vanishing point created by the table itself due to the low height of the camera.

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