Author Topic: QotW - 11.08.07 - Why would footprints in sand appear raised? (Read 12469 times)

thedoc · **on:** 09/08/2011 13:56:26

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
Asked by Nikki Goodwin

Find out more on our podcast page

RD · **Reply #1 on:** 10/05/2011 00:25:59

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.

MikeS · **Reply #2 on:** 10/05/2011 07:52:46

Sand is compacted under foot. Subsequently erosion by water or wind removes the surrounding looser sand.

imatfaal · **Reply #3 on:** 10/05/2011 10:31:34

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.

CliffordK · **Reply #4 on:** 10/05/2011 15:33:19

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.

nikkig · **Reply #5 on:** 11/05/2011 19:06:35

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 :)

RD · **Reply #6 on:** 11/05/2011 20:03:25

This sort of thing ? ...

nikkig · **Reply #7 on:** 11/05/2011 22:02:00

yes correct..
if i knew how to post the picture i would")

RD · **Reply #8 on:** 12/05/2011 04:07:28

Quote from: nikkig on 11/05/2011 22:02:00

if i knew how to post the picture i would

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

http://www.thenakedscientists.com/forum/index.php?topic=2893.0

imatfaal · **Reply #9 on:** 12/05/2011 10:21:40

RD - surely those footprints are raised rather than sunken - or is the optical illusion that good.

RD · **Reply #10 on:** 12/05/2011 18:52:56

Quote from: imatfaal on 12/05/2011 10:21:40

... the optical illusion that good.

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.

imatfaal · **Reply #11 on:** 13/05/2011 13:39:16

Quote from: RD on 12/05/2011 18:52:56

Quote from: imatfaal on 12/05/2011 10:21:40
... the optical illusion that good.

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.

nikkig · **Reply #12 on:** 14/05/2011 08:53:38

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 · **Reply #13 on:** 14/05/2011 11:58:13

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 · **Reply #14 on:** 14/05/2011 11:59:52

thanks RD !!

imatfaal · **Reply #15 on:** 14/05/2011 12:13:50

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.

nikkig · **Reply #16 on:** 14/05/2011 13:57:15

THANKS

still I am going with the obvious stated! I am a witch

RD · **Reply #17 on:** 14/05/2011 14:29:45

Quote from: nikkig on 14/05/2011 11:58:13

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

RD · **Reply #18 on:** 14/05/2011 15:00:47

Quote from: nikkig on 14/05/2011 08:53:38

... I dont know how to reduce the file size

http://luci.criosweb.ro/riot/ [many other tools are available to resize images].

nikkig · **Reply #19 on:** 15/05/2011 10:30:48

again thanks RD!

CliffordK · **Reply #20 on:** 16/05/2011 19:54:10

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 · **Reply #21 on:** 17/05/2011 04:53:05

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 · **Reply #22 on:** 11/07/2011 12:11:23

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

rhade · **Reply #23 on:** 13/07/2011 11:45:04

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 · **Reply #24 on:** 13/07/2011 11:56:36

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

Naked Science Forum

Question of the Week Podcast

Author Topic: QotW - 11.08.07 - Why would footprints in sand appear raised? (Read 12469 times)

thedoc

RD

MikeS

imatfaal

CliffordK

nikkig

RD

nikkig

RD

imatfaal

RD

imatfaal

nikkig

nikkig

nikkig

imatfaal

nikkig

RD

RD

nikkig

CliffordK

RD

syhprum

rhade

imatfaal