Question about observed shadows.

[Dhevix]

This might sound absolutely redundant and obscure to you, but I've been trawling the internet over and I just can't find anything at all! I get really hung up over these kinds of things and it has been bugging me for days.

 

Hopefully someone's actually got an answer and I'm just looking in the wrong places. Or at very least you'll understand my perhaps poorly-explained question

 

 

"If there are three lights casting separate shadows from a pole in the very centre of them, then why does the shadow which is closest to you at the time appear the darkest?"

 

I'm actually referring to a typical ceiling light here, with three bulbs set around a central pole coming down from a circular fixture on the ceiling. I've noticed that whilst the long shadows extending out across the ceiling of course look the same no matter where you stand, the ones in the very centre of the three bulbs being cast onto the fixture seem to change. Basically, the one closest to you appears darkest whilst the other two seem to disappear/fade based on which side of the room you so happen to be at the time.

(Edit: If you stand underneath it however, all three shadows are visible)

 

It's so silly when I consider it but I can't figure it out! Is this an optical illusion or something to do with the way the light is reflecting off that particular material/surface?

Thanks so very much if anyone can offer an answer.

 

 

Edit: I'm also a little unsure of whether this is in the right place now. Oh dear.

Edited March 12, 2012 by Dhevix

[CaptainPanic]

I would guess that all shadows are equally dark. It's the light around the shadows which isn't equally bright.

 

The reason the light is more/less bright is that I guess the light rays hit the ground at a wider/smaller angle. Or, you could say that the light intensity is less when you are further away from the lightsource. Both factors affect the light intensity.

 

You need light to get a shadow.

 

Btw, You put it in the right place, and it's not a silly/obscure question at all!

[Dhevix]

I considered that, but then surely where you stand to look at the shadow would have no effect? Given there's three shadows and darkest shadow always appears to be the one being cast towards you, depending on which side of the room you so happen to be. Unless, of course, you are to stand directly underneath the three lights in which case all three to be appear the same.

 

I am only assume it's got something to do with the way the light rays reflect but... I just don't see how even that works at all. I can't believe this is actually bugging me so much aha, but what I'm seeing just seems totally impossible to me.

[CaptainPanic]

What matters is the contrast between the light and the shadow.

 

I would expect that the contrast is the largest from the light that is the closest to you, regardless of where you are standing. This is also the shortest shadow in many cases (if the lights are on the ceiling).

[Dhevix]

What matters is the contrast between the light and the shadow.

 

I would expect that the contrast is the largest from the light that is the closest to you, regardless of where you are standing. This is also the shortest shadow in many cases (if the lights are on the ceiling).

I'm not sure I understand; could you possibly clarify?

[Schrödinger's hat]

A greater proportion of the light in the region of the shadow comes from the closest light. You can think of it this way:

Any given spot has [math]l_1+l_2+l_3[/math] total light where the l are the contributions from the different lights.

If you block out a light it becomes [math]l_2+l_3[/math]

Blocking out the brightest one will reduce the light by more.

[Dhevix]

A greater proportion of the light in the region of the shadow comes from the closest light. You can think of it this way:

Any given spot has [math]l_1+l_2+l_3[/math] total light where the l are the contributions from the different lights.

If you block out a light it becomes [math]l_2+l_3[/math]

Blocking out the brightest one will reduce the light by more.

Alright, that method is a good way to put it

 

So how come this only seems to happen to those particular shadows which occur in the middle of the three lights, but the larger ones which extend out across the ceiling don't appear to 'change' based on where you stand at all?

[Schrödinger's hat]

Alright, that method is a good way to put it

 

So how come this only seems to happen to those particular shadows which occur in the middle of the three lights, but the larger ones which extend out across the ceiling don't appear to 'change' based on where you stand at all?

 

You're going to have to give me a picture or describe it more clearly. Can you take a photo?

[Dhevix]

Certainly. Lighting fixtures aren't a usual subject of photographs for me, but I'll give it a go Hopefully this will better demonstrate what I'm talking about than my (limited) explanation. I'm referring to this part of the images:

 

Directly from one side of the light:

 

Move a little to the left...

 

Halfway between the two shots, it's possible to see both shadows nearest you, but not the far one. Here's what becomes visible standing beneath the light fixture; in case it's a little unclear on your monitor (it's fairly difficult to make out on mine) you can see all three shadows overlapping each-other and nowhere near as dark as when you can only see one of them.

 

I apologise for being so much trouble with this one! The photos are larger than I thought I'd resized them to, I'm happy to shirk them if they're a little big for the forum.

Edited March 13, 2012 by Dhevix

[Schrödinger's hat]

Not entirely sure what you mean. If the paint reflects diffusely there shouldn't be any real effect (other than light reflecting off of you) that depends on your position.

From the photos, it looks like one of the shadows is darker than the others, I cannot tell if this is position based or from reflections in the room (if there is more light colored stuff in one direction, the shadow won't be as dark). There are also extra-dark spots where two shadows overlap.

It could also be some trick of your eyes/visual processing. When there is a light near the path between you and the shadow you are less able to perceive the contrast in that region.

The final option is that the reflection off of the paint is specular or otherwise directional (is it shiny or pearly looking?), you could check this by shining a smallish light from one angle at it and moving around the room (with the light fixed).

[CaptainPanic]

Dhevix, it's an interesting question. You can stop apologising for the way you post, btw. It's all quite ok. Your post is eloquent, and polite. We don't ask for more from new members. Also, the pictures are absolutely essential for understanding the question in this case, so well done for posting them.

 

So, you are suggesting that the shadows on the ceiling (or rather, in the middle of the light fixture) change? They really don't... or at least, they shouldn't. But I see the same thing you see on the pictures, so that's quite interesting.

 

The shadows on the ceiling do not move or change in any way. The reason for that is:

- the lights don't move.

- the ceiling does not move.

- the light fixture does not move.

- logically, the shadows do not move.

 

However, the perception of the light (and shadow) that enters your eye, or indeed the camera's optical chip, does change when you move around. That much is clear from the pictures.

 

My first, and so far only, guess is that the surface of your light fixture is not giving a perfectly diffuse reflection. If you like, you can test this by covering the circle of the fixture with a piece of ordinary paper (not glossy!), because normal paper should create a nearly perfectly diffuse reflection (although, be warned - white paper will reflect so well, that it might create its own illusions).

 

But please note that I emphasized the word 'guess'. I am not sure about this (which is why it's interesting!).

[Dhevix]

Tried your suggestion and it worked, the shadows intersect as would be expected, so that would indeed tell us it's to do with the texture (of the surface off which the light is being reflected

 

Thank you so very much for your help, anyone who has contributed or tried to help in some way couldn't have been more lovely.

Edited March 13, 2012 by Dhevix

[CaptainPanic]

Hah. Case closed.