Hey, I'm Dianna.
You're watching Physics Girl, and I
want to show you something that I made.
It's a video with actually a strong warning.
I don't know what legal language I'm supposed to use. --ilepsy.
Warning this video may potentially
trigger seizures for people with photosensitive epilepsy.
Viewer discretion is advised.
This video is uploaded at 60 frames per second.
So in order to see what I'm trying to show you,
you have to watch it at 60 frames per second.
OK, here it goes.
Exciting.
OK.
What you should have seen was a rainbow of colors.
But if you go back and pause at any point--
I'm going to play it again--
you pause at any point during this video,
you'll only see red, green, or blue colors any time you
pause the video.
And if you don't believe me, try it.
Rewind back and see if there's anything but red, green,
or blue if you pause the video.
And if you didn't see the colors,
then it's probably because there's like a mobile problem
with displaying 60 Hertz.
This is so cool to me.
In fact, it's related to a phenomenon I've been noticing
and had some trouble explaining for a while.
And that phenomenon is--
how do I describe it?
OK, so imagine you're watching a white screen.
Say, it's white paint drying.
But if you look quickly to the side,
sometimes you'll see, like, these stripes
of color on the side of the screen despite the fact
that the only thing that you're seeing on screen is white.
How does it work?
It's an everyday mystery.
I'm going to turn it over to Diana--
that's me-- in the studio.
She's going to help us figure out what's going on.
So this color thing is something that I've
noticed with low-quality projector screens or some LED
displays or particularly, looking
at the projector light-- from the side, guys,
not directly at it.
That stuff will blind you.
And I know it's not just me that's seen this because I
asked my friends on Twitter.
But if you've never seen this, try this.
Move your eyes back and forth across the source of light,
or look away really quickly, or sort of blink
and look away, or try this.
Try blinking really fast and look at the Physics Girl logo,
and you'll notice like a spread of color.
Why?
I shall take us back to the scene for some clues.
Clue number 1.
It happens when you move your eyes quickly.
I've seen this while looking away or while blinking
and looking away or looking past something.
Clue number 2.
It happens when there's some kind of lighted screen--
projector, LED displays, et cetera.
Those are the clues we're working with
to try and figure out what's going on.
Adults will call them observations.
I call them clues.
So now we science.
From clue number 1, we see there must
be something about the motion of your eyeballs making
you see things.
And I wondered, are there other situations
where your eyes moving makes you see something different?
Yeah.
Turns out there are.
Look at my eyes as I move them back and forth, back and forth.
Do you notice anything weird?
No.
You should just see my eyes moving.
But try it for yourself.
Look in a mirror, and look at your left eye and then
you're right eye--
left, right, left, right.
You will never see your own eyes moving.
Why?
There's a common myth that when you move your eyes,
you don't see anything during that period of motion.
And on a normal day, your eyes are constantly moving around
like the pilgrims-- over here, over there, little corrections,
and jerks--
movements called saccades.
Now, it would be very disruptive if you
saw all of these movements, so it
would be useful if you could suppress
your perception of them.
But a study in 2015 showed that, in fact, you don't go blind
when your eyes move.
The researchers showed super fast
flickering vertical and horizontal lines that
were moving across the screen and found
that the lines usually blurred together,
but you could make them out when your eyes were moving
and matched up with the speed of the lines
moving across the screen.
So you're still seeing when your eyes move,
but it turns out the visual system is very complicated
and involves your brain.
And studies have shown that the brain's
response to what you see changes during periods
of psychotic motion.
And in fact, that change starts before your eyes
even start moving and lasts longer than
the period of motion.
Which means your brain is discarding tons of information
about what you're seeing, particularly
when your eyes are moving.
You ever look at a clock in a classroom, and you're, like,
is that clock stopped?
Because the second hand doesn't appear to be moving.
And then suddenly, it does.
You were probably moving your eyes
as the second hand was moving, and your brain
processed it as being still before your eyes settled on it.
So by the time it moved again, you perceived a longer second.
If you've ever seen this before, you're not alone.
It has a name.
It's called the stopped-clock illusion.
The point is, seeing is not perceiving.
Let's move on to the second clue which
is that you see colors despite the fact
that the screen looks white.
But is it white?
No, definitely not.
And your screen is not white either.
If I may magnify your screen, oh, look.
It's just red, green, and blue pixels.
You are only seeing red, green, and blue light right now,
but you are perceiving white.
Same with projectors.
They are, in fact, flashing red, green, and blue light--
and sometimes other colors--
whenever it appears that there's white on screen.
Here's some photos I took of the same white paint
on the projector screen at 960 frames per second,
although you'll notice that the flashes aren't perfectly
colored because interestingly, the shutter of the camera
is matching up with the refresh rate of the projector,
and we're getting this blending of colors.
It's unfortunate.
But if it was a faster camera or a slower projector,
you just see red, the green, then blue.
And there might actually be yellow in this projector that
would help the image look a little bit brighter.
So our visual system adds colors of light
when we see them in the same place at the same time
or in the same place right after one another
but flashing too fast for us to perceive the changes.
What's happening to make you see colors that aren't there
is called persistence of vision.
Your visual system holds on to something
that you saw for longer than it's there,
like the stopped clock, or in this case, the colors.
And then your brain blends together the colors
even though they're showing one by one.
And that's what makes projectors work.
Projectors use multiple colors and make
use of your mind blending to make you think that it's white.
And then the reason that you can see the colors separately,
sometimes, is because you move your eyes
so that each of the colors of light
hits different places on your visual field.
So this everyday mystery relies on the motion
of your eyes and the fact that projectors
are flashing different colors and persistence of vision.
Mystery solved.
That wasn't even physics.
It's eye perception.
It's colors that aren't there.
Ow.
This is an ice cube that I got from Comic-Con when I was there
with Kari Byron and Simone Giertz--
my girls.
Huh.
This plastic ice cube was in my drink.
And I was, like, I wonder how that works?
And then I saw two screws that connect
when you put it in the water.
And I was, like, huh, you know what that means?
When you put it on your tongue, it'll light up.
Hey, Thank you for watching this video.
If there's any everyday mystery that you're
dying to know the answer to, leave a suggestion
in the comments with the hashtag everyday mysteries.
Go forth into the world--
interruption.
I am soliciting opinions.
PBS Digital Studios channels, including this channel,
are hosting a survey about what content you like to consume
and stuff about you.
There's a link in the description or on screen
if you're one of those people that likes to type in URLs.
They ask which channel is your favorite.
So you better get over there because last time PBS Space
Time beat me out.
So, yeah.
You could have your say in future content for PBS.
And they're also giving away shirts, so get to it.
We will now return to our regularly scheduled
programming.
Happy physicsing.
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