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Footsteps in the sand

Updated: Jun 27, 2020

Yesterday, I went to the beach with my son, and he kept running around & splashing water at us. The weather was perfect so I started taking some pictures. I noticed he had made a trail of footsteps, which made me think of taking pictures of all our footsteps together. Yes, I am very cheesy that way! My husband used to refuse to join into these before, but now he agrees. I am yet to find the reason for the change, but moving on…

Me and my son usually look at the pictures afterwards & talk about it. I noticed that the footsteps looked raised at some angles and sunken at some angles! If you scroll the image you will see that effect.

Footsteps appear raised in the sand

Their raised appearance is an illusion caused by the pattern of shadows. These shadows are ambiguous. They could result either from bumps lit from the top of the picture plane, or from indentations lit from the bottom of the picture plane. In the face of this ambiguity, the brain makes its best guess as to which is more likely and that is what we see. With this particular image, our brains make the wrong call. Why? Because they have a built-in bias to assume that light comes from above.

So the problem is that our brains have a bias toward top down illumination which means that the brain tends to assume light is coming from above. This bias is so strong that it often competes and overcomes the clues our vision is giving us about relative depths. So when light comes from a slightly different angle, in the case of the footprints in low sun, our brain tries to tell us they’re convex instead of concave.

Now I know what to tell him if he asks that question! Probably not now, but in a few years.

I got this information from

#sand #illusion #physics #footsteps #beach

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