Question of the Week

Why would footprints in sand appear raised?

Sun, 7th Aug 2011

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Question

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?

 

Regards

 

Nikki Goodwin

Answer

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.  footprints in sandThey 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.

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Possibly an optical illusion like the hollow mask effect where the concave interior of a face mask appears to be convex ...
http://www.youtube.com/watch?v=G_Qwp2GdB1M

Your brain insists "I've seen faces/feet before and they are convex", even when what you are looking at is a concave mould of a face/foot.
RD, Tue, 10th May 2011

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.

You can notice this effect by stamping in the soft sand which is filled with water.  If you stand there you sink and can easily dig your toes it but if you stamp or jump you will jar to a halt much quicker than expected.

Lovely demonstration of custard here http://www.youtube.com/watch?v=BN2D5y-AxIY - before brainiac sold out completely and became all about bangs and idiocy.  Where is Jon Tickle now? - I know he never gave up his day job in programming, but someone should get him back on tv or radio as he was a natural educator/communicator. imatfaal, Tue, 10th May 2011

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.

That is a most extraordinary hollow face film.

Your mind has troubles determining the difference between a concave and a convex surface, and occasionally makes errors.  A photograph removes the stereo cues that might help with the assessment. 

Please post the photo if you can.
CliffordK, Tue, 10th May 2011

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, Wed, 11th May 2011

yes correct..
if i knew how to post the picture i would")
nikkig, Wed, 11th May 2011



http://www.thenakedscientists.com/forum/index.php?topic=3458.0

http://www.thenakedscientists.com/forum/index.php?topic=2893.0 RD, Thu, 12th May 2011

RD - surely those footprints are raised rather than sunken - or is the optical illusion that good. imatfaal, Thu, 12th May 2011



The illusion is very good ...

If you scroll up so only the bottom edge of the footprint photo above is seen, the half footprint looks concave,
when you scroll up further so more of the footprints are revealed your brain realises they are footprints and the convex illusion kicks in. RD, Thu, 12th May 2011



The illusion is very good ...

If you scroll up so only the bottom edge of the footprint photo above is seen, the half footprint looks concave,
when you scroll up further so more of the footprints are revealed your brain realises they are footprints and the convex illusion kicks in.


Yep - got it!  And if look between third and fourth print there is a pebble or lump - the shadow is cast to the left, ie the sun to the right and quite low in the sky; therefore the shadow on the right side of each print must be due to the fact that the shaded area is lower than the edge.  Of course that would still work if the pebble was an impression and footprint are raised, but I am in no doubt visually that the pebble is raised.  imatfaal, Fri, 13th 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


the link to view my picture
incredible right!! :) interestingly sometimes if I just look at the picture the foot prints appear 'in' the sand but if i concentrate on it they then appear 'raised':) and thanks for the info on how to load the pic!! 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....

BTW if you look at the light/shade on the log/debris nearer the water line the sun is off to the left - so shade on the left of each print must be because sunken rather than raised.  With the sun to the left if the prints were raised the shadow would be on the right hand side. imatfaal, Sat, 14th May 2011

THANKS   still I am going with the obvious stated! I am a witch 
nikkig, Sat, 14th May 2011



IMO some of the footprints on your photo appear inny (concave) some appear outy (convex):
if the footprint is deep or isnít well defined (e.g. no clear toes) then the outy illusion does not work for me.


RD, Sat, 14th May 2011



http://luci.criosweb.ro/riot/ .

RD, Sat, 14th May 2011

again thanks RD! nikkig, Sun, 15th May 2011

Odd,
That just defies all logic.

In "real life", I've never seen a raised footprint.

If I stare at the top image (tan), then flip to the bottom image (grey), then the footprints in the grey image initially appear depressed (normal).  Then they will flip to look raised.

Looking at the thumbnail, the footprints all look raised.

Part of the illusion is that the footprints are turning out to be lighter in color than the surrounding medium.

The sand is also very non-distinct, so it isn't as if you can see blades of grass and compare the footprint to the blades of grass.
CliffordK, Mon, 16th May 2011

Others have noticed this phenomenon ... http://www.flickr.com/photos/flutterbye856/114361931/

shoeprints less convincing IMO ... http://www.flickr.com/photos/eraphernalia_vintage/3416446368/in/photostream/ RD, Tue, 17th May 2011

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?
But I think you are all missing a point. If the sand is pressed in at one place, then it has to come out somewhere else, right? So at least part of the footprint has to be raised. Which is not suggesting that you are all wrong, as I suspect that a combination of different things may be involved here. rhade, Wed, 13th Jul 2011

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.

BTW: The link to the podcast is not working yet. CliffordK, Wed, 13th Jul 2011

I agree with the optical illusion but maybe its just sunken.. I'll try that on the beach..
<may you and your SPAM be lost in sands of time!> TheBeautyHouse, Thu, 14th Jul 2011

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



Would that also apply to destruction manuals provided by DIY stores in making cupboards? 

Please excuse my Shakespearean style of English at times, I did live in Warwickshire!

I was on a beech in North Wales under the influence of Magic Mushrooms, when they were legal there, even then the footprints in the sand seemed normal to me....  Aaron_Thomas, Sat, 16th Jul 2011



Yeah! Good on you. I was beginning to think the need for grammar had been abolished (by Rupert Murdoch and his pals at the News of the World, and other publications).

I wonder if he will give all of his money away to charitable organizations to compensate for the damage he has done to other people? Doesn't seem like too much to ask from someone who is obviously interested in the greater good. I suppose we'll just have wait and see.  Geezer, Sat, 16th Jul 2011



Yeah! Good on you. I was beginning to think the need for grammar had been abolished (by Rupert Murdoch and his pals at the News of the World, and other publications).

I wonder if he will give all of his money away to charitable organizations to compensate for the damage he has done to other people? Doesn't seem like too much to ask from someone who is obviously interested in the greater good. I suppose we'll just have wait and see. 


Is there a connection with raised footprints here or am I having an hallucination!    Given the average reading age of the papers Murdoch owns I suspect that his footprints are deeper then most as he is full of excrement! Aaron_Thomas, 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

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