2 00:00:02,680 --> 00:00:03,552 Hi, I'm Diana.

You're watching "Physics Girl," and in that apartment right there is my friend Nick Uhas from Nickipedia, and he's setting up the scene of a crime.

All right, so I'm going to try to cover my tracks by taking this dry ice and breaking it up.

Pick up the pieces later.

OK, too hot.

It's going in the fridge.

Too hot.

Mm.

No one will ever find that.

Solid.

All right, well it's time to hide.

17 00:00:33,280 --> 00:00:34,780 So I'm going to go break in there, and I'm gonna use this infrared camera here to see what clues we can find.

So let's go check it out.

[INAUDIBLE] Whoa, there is all kinds of cold on the floor here, and I see dry ice on the floor, so-- OK, there's something really hot on the floor over here.

Looks like something was spilled.

There is a giant hot spot on the couch right here.

So you can tell someone was sitting on the couch and just put some tea in the refrigerator because that is the only hot thing.

Everything else in this refrigerator is about the same temperature.

Whoa!

What is this?

Whoa!

Nick, did you steal my pen?

I did.

You stole my Guinness pen.

39 00:01:28,190 --> 00:01:30,960 That experience was incredible.

Having the ability to look at a scene through a totally different set of wavelengths of light, at least, right after the crime went down, it's eerie.

It's like a sixth sense.

Yes, it is super cool.

This fancy toy we're using right here can actually turn your phone into an infrared camera.

That's pretty amazing.

It's awesome.

It takes invisible infrared light and turns it into colors we can see on the screen.

But just a real quick recap here.

What is infrared?

Good question.

The prefix infra means below, and red means red.

So it's the range of colors of light with less energy than red light, which is the least energetic light in the visible spectrum.

So infrared light, red light, and visible light are all types of electromagnetic radiation.

So it's all the same spectrum as microwaves and radio waves and X-rays.

It's kind of all in that same sphere, right?

It's all light, It's all the electromagnetic radiation.

So why does infrared allow you to see such different things?

Like, think about it, we were able to see the aftermath of motion.

We were looking into the past to see what happened before we were there.

Like, I could tell where you walked just by where your shoes rubbed on the floor.

Why are we capturing these unusual aspects of the world with infrared?

Why aren't we using X-rays or something?

Great question.

With infrared, you can see temperature.

So you can look at an object, right?

You can see it's glowing red hot.

We can also look at infrared and see that it's hot without it actually looking like it's hot.

Like, steam's not coming off of it.

Or like, there's no, like, waves of heat.

So you can actually see the temperature.

Yeah.

I think that's a good place to start in this discussion because infrared is often thought of as the way to see heat.

But why do we think of infrared as the heat detector?

Well, hot things are emitting infrared light.

Good point.

The key is that everything is glowing, right?

Yes.

I mean, even like me.

I'm super glowing.

You're so glowing.

That means everything is emitting light.

It's not just the sun and bonfires.

It's also ice cubes, liquid nitrogen, dry ice.

But we can't see the light that we give off.

The color of that light depends on the object's temperature.

Right and that's what this graph is all about.

It's called the spectrum of emission.

And right now, it's showing us how much light is given off by, say, a frog warmed up to human body temperature.

So down here on this axis is wavelength or color of light and on this axis, we have basically how much of each wavelength is emitted.

So for body temperature, you can see most of the light is emitted here in the infrared.

Visible light is somewhere way over here.

Now, the shape of this graph and where it peaks depends on temperature of the frog.

If we started heating the frog up, the emission spectrum would change until you get more visible light emitted, which is when you start to see the frog visibly glowing.

Luckily for the frog, room temperature graphs look much more like this.

You see there's mostly infrared and almost no visible light emitted.

In fact, everything that we see on a daily basis is glowing in infrared.

It just might not be glowing in the visible spectrum, unless it's super hot and glowing red like a fire poker.

Pause there.

You've almost touched on the key here, which is that the temperature of most things you find here on Earth, causes them to glow in infrared.

That's why we think of infrared as the way of seeing heat because we don't get up higher than a peak in infrared wavelengths that often with Earth temperatures.

You need to get up to about 800 degrees Celsius to start glowing in red.

Like lava, right?

Exactly.

Ah, that makes so much sense.

So that's what the camera is picking up, the light the objects are giving off based off of their temperature.

When you touch something colder than your hand, you transfer heat to that object, raising its temperature a little and changing the color of the light that it gives off.

Yeah, that's so crazy.

Like, if you touch something like this shoe and it suddenly became a different color, that would be weird.

Is that your shoe?

This is my shoe.

OK.

But that happens in infrared, not because you've changed what colors of infrared are reflected but because you've changed which colors of infrared are emitted from the shoe.

Ah, it makes sense.

OK.

So shall we tour the YouTube space LA with our infrared camera and see what cool stuff we can shoot with it?

Yes.

All right.

Yes.

Let's do it.

Go-- go breathe on the wall.

Go breathe on the wall.

Whoa!

169 00:06:07,280 --> 00:06:09,867 This is hot water.

Oh, wow, that's amazing.

Wow.

Now, will you turn on the cold?

Yeah, that's cool.

That is super cool.

Back to hot.

Yeah.

Whoa!

It's like paint mixing.

Yeah.

Whoa!

Dude, you can see through walls.

You really can.

That's crazy.

Yeah, you're seeing the-- what is that?

The plywood?

No, not plywood.

The two-by-fours.

Two-by-four structure.

The structural component of the actual building itself.

You can't even see anything on the wall.

And here on the camera, it's like this bright orange spot.

You can see through walls.

Oh, wow.

Oh, yeah.

It's actually warm.

Whoa!

Look how inefficient the actual electrical circuit is because it's giving off so much-- I would not have thought that.

I would not have guessed that.

Electricity is something that you can't see.

And there is some current actually flowing in this outlet and heating it up, which you can see losses right there.

Go ahead and walk around there and I'll see if I can see you.

206 00:07:15,730 --> 00:07:16,550 You're gone.

208 00:07:19,100 --> 00:07:20,338 I can only see myself.

What?

211 00:07:23,970 --> 00:07:25,647 Nothing.

OK, can you just stand there for a second?

Yeah.

See, now, oh, that is amazing.

And walk away.

That is awesome.

Oh, my god.

Look, it's still there on the wall.

We're picking that up on the infrared.

Now, I'm going to go ahead and turn this off, all right?

Yeah.

Whoa!

OK, so the lights are off.

It's completely dark, and I can still see-- I can see all those same lights.

They don't look any different.

Nick, I think it's amazing how infrared light just reveals a completely different landscape from what we're used to.

It's like having like two different worlds.

Yeah, it enhances a totally different part of the scene compared to visible light, and that's why people use infrared cameras, to see what they normally can't see.

People use these things to check for leaky windows and pests like termites because termites like to hang out in moist places that look different than the surrounding walls when you look in infrared.

Fun fact.

So over on Nick's channel, Nickipedia, we've got another video with some fascinating ways to use an infrared camera, some surprising things that you would see in the world of infrared.

Yeah, yeah, we're really going to get into like some old adages about what your dad used to tell you in the summer.

You don't heat the whole neighborhood.

And so we're going to bust that myth about are we actually heating the whole neighborhood or cooling the whole neighborhood?

Yeah.

Check that video out.

I'm going to put a link in the description.

And thanks for watching and happy physicsing.