Nikki Goodwin asked:
Dear Naked Scientist..
I recently took this photograph of my foot prints in the sand and it was only after I noted that the "footprints" are not IN the sand they appear to be "raised" on the sand? How could this be?
We put this to Dr Rob Jenkins, a Cognitive Psychologist at the University of Glasgow...
Rob - The questioner has sent in a wonderful photograph of footprints on a sandy beach. What's striking about the photograph is that the footprints seems to rise up from the surface of the sand rather than sinking in. In fact, the footprints are normal footprints. They're sunk in as you would expect. 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. This is a sensible rule of thumb because sunlight generally does come from above, but not in this photograph. Here, the sun is setting behind the photographer, below the bottom of the picture plane. This is evident from clumps of sand in the foreground that act as mini sundials. Under these lighting conditions, only indented footprints could create the pattern of shadows we see. So the footprints must be indentations after all.
Diana - 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.
Possibly an optical illusion like the hollow mask effect where the concave interior of a face mask appears to be convex ...
Sand is compacted under foot. Subsequently erosion by water or wind removes the surrounding looser sand. MikeS, Tue, 10th May 2011
If it is very wet sand - then it might be acting as a non-newtonian fluid (like cornstarch rich custard). Non-newtonian fluids under sudden force tend to behave more like a solid than a liquid - perhaps if you are close to the water line the rest of the sand has settled back down as the water has receded with the wave, but the solidified/compacted surface under your foot changed enough to resist that settling back. I would have thought that three or four foot prints back (that have have more than wave) will have disappeared entirely.
I'm going with RD as it being an optical illusion as the footprints I've seen in the sand always stay lower than the surrounding sand, although they may fill with water.
Hello.. all new to me.. haven't a clue how to post the photo. Did you not see it then? Its so freaky. I would agree that its an optical illusion but knowing what I know and all who know me - perhaps its just because I am a witch. For sometimes when I look at the photo the foot prints are IN the sand.. but mostly the look like they are raised :) nikkig, Wed, 11th May 2011
This sort of thing ? ...
RD - surely those footprints are raised rather than sunken - or is the optical illusion that good. imatfaal, Thu, 12th May 2011
Thank you for that - it appears the photo I took is too large to attach now I dont know how to reduce the file size nikkig, Sat, 14th May 2011
thanks RD !! nikkig, Sat, 14th May 2011
Great photo - thanks for posting Nikki. I see what you mean - if it hadn't been for RD's example I would have sworn....
THANKS still I am going with the obvious stated! I am a witch
again thanks RD! nikkig, Sun, 15th May 2011
Others have noticed this phenomenon ... http://www.flickr.com/photos/flutterbye856/114361931/
This optical effect also applies to photographs of meteor craters on other planets, they can often be difficult to interprate correctly syhprum, Mon, 11th Jul 2011
Wow! Who would have thought that such an apparently simple thing could raise so many complex aspects?
rhade - not necessarily; just as an example you could just compact the sand which you stood on, ie you increase the density by removing the air holes and the only thing to come out is air imatfaal, Wed, 13th Jul 2011
You may see a little ridging outside of the footprint from the pressure, not in the footprint itself. However, this is almost purely an optical illusion.
I agree with the optical illusion but maybe its just sunken.. I'll try that on the beach..
I was going to say something about the sand possibly behaving elastically but it seems that this is actually an optical delusion. Aaron_Thomas, Thu, 14th Jul 2011
Uh, I think an optical "delusion" is really called an hallucination. rhade, Sat, 16th Jul 2011
Personally, I blame poor education for falling english standards. I don't know how much we can blame Murdoch for the editorial policy of his papers, also let us not forget he has been propping up the Times at a loss. Not to suggest that he is some kind of philanthropist. I'm sure he's just a businessman, and I won't mourn the loss of one of our seedier tabloids. Sorry if this thread is getting off topic, but I guess it just proves we all care about the world beyond science. I suspect some moderator would jump in if he felt we were breaking some rule. rhade, Sat, 16th Jul 2011
I'm the photographer who took the picture featured in Rob's reply. The prints are concave. Perhaps it's because I took the picture, but I've never seen them as anything but concave. ~Rklawton Rklawton, Mon, 11th Aug 2014