[CliffordK]

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.

[RD]

Others have noticed this phenomenon ... http://www.flickr.com/photos/flutterbye856/114361931/ [try covering the toes]

shoeprints less convincing IMO ... http://www.flickr.com/photos/eraphernalia_vintage/3416446368/in/photostream/

[syhprum]

This optical effect also applies to photographs of meteor craters on other planets, they can often be difficult to interprate correctly

[rhade]

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.

[imatfaal]

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

[CliffordK]

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.

[TheBeautyHouse]

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!>

[Airthumbs]

I was going to say something about the sand possibly behaving elastically but it seems that this is actually an optical delusion.  []

[rhade]

Uh, I think an optical "delusion" is really called an hallucination.

[Airthumbs]

Uh, I think an optical "delusion" is really called an hallucination.

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....  []

[Geezer]

an hallucination

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. 

[Airthumbs]

an hallucination

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!

[rhade]

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.

[thedoc]

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.
                                        Find out more on our podcast page
[chapter podcast=3391 track=11.08.07/Naked_Scientists_Show_11.08.07_8827.mp3]  ...or Listen to the Answer[/chapter] or [download as MP3]

[Rklawton]

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