r/Physics • u/i-touched-morrissey • 3d ago
How does this happen?
It’s a flashlight shining on fog, pics on iPhone 17. Why are the droplets linear?
139
72
u/aktentasche 3d ago
Actually you can see the PWM of the flashlight in the traces. Really cool.
11
u/i-touched-morrissey 3d ago
So this particular flashlight is the cause of the stripes and lines, not the camera or the drops themselves?
72
u/TheJeeronian 3d ago
No, the flashlight is responsible for the lines being dotted instead of continuous. The camera is responsible for seeing lines instead of individual points.
The fog is responsible for the individual points which the camera and flashlight have made appear as dotted lines.
1
u/i-touched-morrissey 3d ago
Is there any other way to observe the dotted light pattern that the flashlight causes?
19
u/flappity 3d ago
It's basically like if you took a blinking LED that blunk once a second and took a 10 second exposure of it while waving it around. You would see 10 individual tracks of the LED in the exposure. The flashlight is not a continuous light but also a "blinking" light that blinks many times a second. You don't pick it up without looking for it, but each time the flashlight "blinks" it illuminates the droplets in their current position. In between these "blinks" the flashlight is dark and is therefore no longer illuminating the droplets. This results in this dot pattern that the droplets exhibit in the image.
As for the "linear" look, it's simply because there are discrete droplets in the fog. The fog and air are a fluid, and they aren't going to just be moving randomly -- even "calm" air is going to have currents, weak circulations and vortices, calm areas and chaotic areas. You're basically just seeing the ambient (and/or potentially disturbed by you and/or anything nearby) motion of the air visualized.
14
4
u/TheJeeronian 3d ago
Sure. Any time you have a flickering light illuminating an object that moves a significant distance between each flicker you'll see this effect.
Twenty years ago you'd see it all the time by the light of CRT screens. Wave your hand in front of an old TV and it looks like there's multiple copies of your hand.
In your case the stuff you're imaging is tiny flecks of water, so they would just look like dots, but every flicker the dot moves a bit and so you get a dotted line.
It's much easier to replicate with a camera than the naked eye. Our eye exposure time is something like 100 milliseconds, so you need multiple flickers and significant movement to happen during that time. Cameras can have much longer exposures and capture shots like this.
1
u/funkybside 3d ago
just need something moving fast enough, and something able to record it at a sufficiently high frame rate for a reasonable duration...like you have here.
8
u/GianlucaBelgrado 3d ago
The LEDs are flashing really fast you can see it if you point your phone toward the light.
10
u/DocClear Optics and photonics 3d ago
Brownian motion is moving the fog droplets. Slow shutter time makes the moving drops appear as lines. The strobing of the flash makes the lines broken
4
u/Individual-Staff-978 2d ago
Brownian motion is such a funny explanation.
"Why does this happen?"
"Because of random"
2
12
1
1
1
u/NathanArizona 3d ago
Where are they linear?
1
1
u/Individual-Staff-978 2d ago
You can't say for certain the droplets aren't moving along their geodesic path!
1
u/i-touched-morrissey 2d ago
Not in actual lines, but some of them look like little threads.
1
u/ScenicAndrew 2d ago
Your shutter speed is too slow. It's an image over time thing, not a particle thing.
Same reason a fly moving around the room might look like this on camera, or like a blur to your eyes.
1
u/jippiex2k 3d ago
Camera opens to capture light
This takes a little while
During this while the fog particles move
The image captured on the camera is now an average of all the places it moved through
1
u/netsec_burn 3d ago
Is there a rule in this sub about random pictures of light belonging in /r/AskPhysics? 70% of the posts here are "why is light doing this".
2
u/maxh2 3d ago
Id say 70% are about relativity and 50% are about black holes.
3
1
u/Glad_Contest_8014 3d ago
I would say we can’t know what the topic is if we know what the probability of the topic is. Heisenberg something or other…. Blah blah blah.
1
u/i-touched-morrissey 2d ago
When I was looking for a sub to ask, I think r/askphysics said no pictures. Sorry to bother you with my problem.
0
u/Fit-Student464 3d ago
Exposure time. Way it works, you are accumulating photon hits (not actual photons) over time, so the only way your sensor can make sense of it is, well, the weird pictures you have
147
u/Opposite-Cranberry76 3d ago
Could be exposure time, but if the exposure time was less than 1 second, consider testing for radium in your basement [is joke].