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The Magnetic Shadow Effect
The Magnetic Shadow Effect
minutephysics
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5:55 · Jun 26, 2025
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A
friend
recently
sent
me
a
video
where
the
shadow
of
a
post
seemed
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0:01
A friend recently sent me a video where the shadow of a post seemed
0:04
to grow outwards towards them
0:05
when their shadow got close to
0:06
that of the post.
0:08
Their question for me, what's going on?
0:10
I had noticed this previously myself
0:11
and immediately tried to replicate it in a controlled setting.
0:14
It's a bit trippy.
0:14
What's weird about this touching shadow phenomenon is
0:17
that it feels a little like one shadow is a magnet pulling the other
0:20
one towards it or like there's surface tension.
0:22
Like you know how when you touch water,
0:23
it stays stuck up
0:24
and attached to your finger for a bit
0:25
when you pull it away from the water.
0:27
Except shadows don't have surface tension and they aren't magnetic.
0:30
Before I get to the explanation,
0:31
here are a few more cool examples where the same phenomenon happens,
0:34
except instead of shadows, it's out of focus objects in an image.
0:38
You can literally see this with your own eyes.
0:39
If you focus on something far away
0:41
and hold a finger in front of your hand,
0:42
the hand seems to grow outwards towards the finger.
0:45
But if I hold my finger behind my hand,
0:47
it's the finger that seems to grow outwards.
0:49
At first, I wondered
0:49
if perhaps this phenomenon has something to do with the wave nature of light,
0:52
defraction, interference, and all that.
0:54
But it's actually much more straightforward.
0:56
It's just simple, if slightly counterintuitive, geometry.
0:59
Here's the thing.
1:00
This touching shadow phenomenon, which is also called the shadow blister effect,
1:04
happens when you have a light source shining on two objects
1:06
that are different distances away from the light.
1:08
The farther one on its own casts one shadow,
1:10
and the near one casts another.
1:12
And when those shadows get close,
1:13
the near object's shadow blocks the far object's shadow,
1:16
and the shadows appear to merge.
1:18
But it gets interesting
1:19
if the light source isn't a point
1:20
but has actual size
1:21
because then the different parts of the light source cast slightly different shadows
1:24
which overlap and combine to create a blurred shadow.
1:27
You can see this
1:28
if I turn on first one light
1:29
then another then another.
1:30
Multiple shadows blur together into one blurry shadow.
1:33
The core dark part of a shadow is where light from everywhere on the
1:36
light source is being blocked.
1:37
The blurry edge of the shadow is where some parts of the light source
1:39
are being blocked while others get passed.
1:41
And the brightness outside the shadow is where all parts of the light source
1:44
reach without being blocked.
1:45
The geometry of the touching shadow phenomenon means
1:47
that as the object nearer the light moves to shade the far object,
1:50
the first shadows that it intercepts are the ones
1:53
that land on the inside dark edge of the blur.
1:55
Then it intercepts shadows farther out and farther out and so on,
1:58
growing the shadow of the farther object until the shadows fully merge.
2:02
It's kind of hard to describe in words,
2:03
but easier to intuitively see in a visualization or with real lights and shadows.
2:07
The point is that the shadow of the farther away object seems to grow
2:10
from the inside out until the shadows merge.
2:12
And this is purely due to the fact
2:14
that the light source has some area
2:15
and the objects are at different distances from the light.
2:17
In particular, the fact
2:18
that you see this effect with shadows outside is due to the fact
2:21
that the sun is not a point source of light
2:23
and outdoor shadows are blurry.
2:24
Another nice way to see why the blisters appear is to replace the farther
2:28
away object with a pinhole.
2:29
This makes it obvious
2:30
that the rays of light get flipped
2:31
when they pass the object.
2:32
And I'm showing the rays in different colors just
2:34
so you can track
2:34
which one is which.
2:35
So when a nearer object moves to block the light,
2:37
the shadow is blocked in reverse.
2:39
This is the blister on the opposite shadow.
2:41
Essentially, the same blistering happens with a camera or your eye,
2:44
except that the effect is due to the size of the lens opening rather
2:47
than the size of the light source.
2:48
The rays from a point source of light get focused by a lens down
2:51
to a point and
2:52
then diverge again.
2:53
So, if you put the image sensor
2:54
or your retina just in front of
2:55
or behind that point,
2:56
the image of the point light source becomes spread out into a circle.
2:59
This is the source of bokeh in a blurry image.
3:02
When you put another object in between the light source and the lens,
3:05
you block the outermost light rays generating the bokeh circle,
3:08
which shrinks the bokeh from the opposite side.
3:10
Well, it looks like the same side,
3:11
but we have to remember that lenses flip images.
3:13
So, the image of the blocking object will be up here,
3:15
and the bokeh shrinks from the opposite side relative to it.
3:18
Here's what the result looks like for a real blurry image of a point
3:20
source of light.
3:21
The net result for an extended blurry object like your hand is
3:23
that the shrinking of all the combined bokeh of the light shining through between
3:27
the hand and the finger makes the hand appear to blister outwards towards the
3:30
finger,
3:30
but it's really the bokeh that's shrinking.
3:32
Let's look at an object lit from behind by a grid of point lights.
3:35
You can see that the bokeh circles around the object appear cropped
3:38
or cut off on the sides of the circles farthest away from the object
3:41
and are more cut off the closer they are to the object.
3:44
As we increase the number of points on the grid,
3:46
the cutff bokeh overlap
3:47
and combine to create the illusion
3:48
that the far object is growing outwards.
3:51
But there's another side to the lens blister effect.
3:53
So far, we've just been looking at the situation where the image sensor
3:56
or retina is too close to the lens,
3:57
which happens when you've focused the lens too far away.
4:00
If the image sensor is too far from the lens,
4:02
which happens if you focus the lens too close,
4:04
then a point source still spreads out into a circular bokeh.
4:07
But now when you block the light,
4:08
the bokeh shrinks in the same direction as the blocking,
4:10
which you can see in this blurry image.
4:12
In fact, if you put a funny shape in front of the lens,
4:14
you can see the shape normally in the far bokeh
4:16
and inverted in the near bokeh.
4:18
The net result for a normal extended blurry object like a hand is
4:21
that the shrinking bokeh makes your closer finger seem to blister outwards towards the
4:25
hand.
4:26
Except how can the finger be growing?
4:27
The farther away hand can't do anything to block
4:29
or affect the light from your finger reaching the lens,
4:32
it's your finger that's blocking the light.
4:33
Another demo with colored textbooks reveals the explanation.
4:36
Even though the nearer green textbook appears to blister outwards,
4:39
the blister has the orange color of the more distant textbook.
4:42
It's a somewhat bizarre mirage-like vision of the more distant textbook that's revealed by
4:47
the shrinking bokeh between the books.
4:48
The near object blisters or grows when you focus too near,
4:51
while if you focus too far, then the far object blisters.
4:54
Either way, the blister has the same color you'd see in
4:57
that location if the background were dark.
4:59
That light is always reaching the lens.
5:01
It just normally gets overwhelmed by the bright bokeh of the bright background.
5:05
Put another way, when blurry fingers overlap against a bright background, there's a blister.
5:09
But against a dark background,
5:10
you just see the fingers overlapping more or less normally.
5:12
And when you focus a lens on the far side of a bright grid,
5:15
a blurry finger appears to attract the lines.
5:18
While when you focus on the close side of the grid,
5:20
the finger appears to repel the lines.
5:22
But there's no repelling or attraction happening.
5:24
What you see here is explained by the geometry of light rays passing through
5:27
a lens.
5:28
In summary, blurry fingers and blurry shadows don't have surface tension and aren't magnetic.
5:33
It's just that the geometry of shadows and lenses is a little weird.
5:40
I want to say a big thank you to everyone who supports Minute Physics
5:42
on Patreon.
5:43
It really helps keep things going around here.
5:45
And everyone else, if you like this video,
5:47
please consider supporting us at patreon.com/minutphysics.
5:51
The more Patreon support we have, the fewer sponsorships we have to do.
5:52
It really helps.
5:53
Thank you.
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