What Happens if You Focus a 5W Laser With a Giant Magnifying Glass? Negative Kelvin Temperature!

In this video I show you what it means to have negative temperature by focusing a laser beam down to a single point. I show you what happens if you try to focus a light down to a single point, then I show you how a laser is different due to population inversion.

My Other Channel:

For more awesome videos checkout:
Darker Than Vantablack—Absorbs 99.9923% of Light

Amazing experiment actually makes black fire

Crushing My Own Hand In a Hydraulic Press—Crazy Experiment on My Brain

What Does a 4D Ball Look Like in Real Life? Amazing Experiment Shows Spherical Version of Tesseract

How I Made an Ant Think It Was Dead—The Zombie Ant Experiment

What Happens if You Open a Vacuum Chamber Under Water? And Do Vacuums Float?

Can Light be Black? Mind-Blowing Dark Light Experiments!

Mirror-Polished Japanese Foil Ball Challenge Crushed in a Hydraulic Press-What's Inside?

Mixing the World's Blackest Paint With the World's Brightest Paint (Black 2.0 vs LIT)

Is it Possible to Unboil an Egg? The Amazing Uncooking Experiment!

What if You Try To Lift a Negative Mass? Mind-Blowing Physical Impossibility!

What Does a Giant Monster Neodymium Magnet do to a Mouse?

The Worlds Blackest Black vs The Worlds Brightest Flashlight (32,000 lumen)—Which Will Win?

How Much Weight Can a Fly Actually Lift? Experiment—I Lassoed a Fly!

DISCLAIMER: Any experiment you try is at your own risk

[TheActionLab]

I see a lot of people are having trouble with this video. First, I am very much aware that the reason the laser it getting hotter when it is magnified is due to the reduced area. That isn't the point of this video. The point is to try to explain why it doesn't break the laws of thermodynamics! Now for the negative kelvin explanation, statistical mechanics tells us that at infinite temperature all atomic states will be populated equally. The Kelvin scale was built upon classical mechanics where it would be impossible to achieve a state in which there are more atoms in a higher state than a lower state. However due to quantum mechanical effects, we know that we can stimulate atoms to be in a higher energy state simply by shining light near them that is at the same wavelength as the light it would emit at that state (stimulated emission). So in a laser, the stimulated atoms actually achieve a population inversion where there are more atoms in a higher energy state than a lower one. This is where the negative temperature comes from. In this case we have to define temperature as negative or else we get into problems that break the second law of thermodynamics. It doesn’t matter that my laser has poor optics. What’s important is that lasers can break the conservation of etendue due to the fact that they have light that doesn’t spread, the reason they have light that doesn’t spread is because of population inversion, and this is why we have to say they have negative temperatures (or they behave as if they have negative kelvin). We can never achieve negative temperature in a non-quantum mechanical system thus anything the laser shines on is always at a positive temperate no matter how hot you get.

Of course the reason the laser gets hotter when it’s focused is due to the reduced surface area of the light. That was not my point though. The point of the video was to explain why it doesn’t break the 2nd law of thermodynamics. Please research “conservation of etendue” to understand why you can’t focus a flashlight down to a point that is hotter/brighter than the flashlight surface. This is a very good example of how the second law of thermodynamics can never be broken no matter how hard you try.

[jlco]

If you break thermodynamics, I'm not buying you a new one.

[ssjMaximumGoku]

Sweet, now we can finally build that Predator shoulder cannon.

[wargrasa]

I google searched "What happens when you point a laser at a crystal ball". I found this and learned so much. Awesome video.

[slingblade]

Why do I get the feeling the anhilation of this universe is going to be caused by someone making a YouTube video.

[papadave]

I never thought Kelvin could be negative. I mean, he has such a good outlook on life.

[alyssavonxylander]

A flash light emits defused light and so does the sun...

\|/ defused light
• dot is source

A laser is concentrated light. Ideally, we want all the laser rays to be parallel (direct light)

||| direct light
• dot is source

however when I look at a laser dot ( just how you showed us with laser close and far away) I see the dot get smaller at a distance... That means, the rays aren’t emitting perfectly parallel from the source(laser) but most-likely they are converging a bit.

/|\ converging light rays
• dot is source

now a magnifying glass also converges the light (but much more drastically) depending obviously by the type of lens. so laser plus magnifying glass will look something like this.

. -small dot is where the light focuses
/|\ -converging lines due to refraction
— -horizontal line is magnifying glass
||| -parallel light is laser rays
• -big dot is source


now we do the same with a diffused light source.

o - o is light on wall
\ | | | / -some lines refract to parallel
— -horizontal line is magnifying glass
\|/ -defused light rays from source.
• -big dot is source (sun or flashlight)

a ‘theatrical spot light’ is kinda like a laser but even though it emits rays more parallel than the flash light, it still does not converge the light rays all on one very small spot like a laser does.

now you must also understand what happens when light goes past its focus point
i’ll copy the laser diagram and extend the light past the wall.


\|/ -rays diffuse past the focus point.
x - x is the focus point
/|\ -converging rays due to refraction
— -horizontal line is magnifying glass
||| -parallel light is laser rays
• -big dot is laser source

as we see, rays past the focus point will start to diffuse out. I have seen this happen with cheap lasers where the rays aren’t perfectly parallel (the rays converge a bit)

when i point the laser at certain distances it it will make different size dots.

at point blank the dot is source size

at a bit more far, the dot is smaller because the rays are converging closer a bit /|\

if we can find the right distance to find the focus point, thats were will get the smallest spot of light.

if we give even more distance and so the light can pass its focus point, then we will see that the laser light will start to diffuse out just like flashlight rays \|/. Becoming practically a red spotlight (if it’s red laser)
you will notice this by seeing bigger less intense spots on walls(i did this outside from balcony to distant buildings)

at a very far distance the laser light gets so diffused out that the spot totally disappears giving this cheap laser a limited range. good quality lasers will try to emit as best they can (perfectly) parallel lines so that their focus point can be as far away as possible to give them a much better range(not the only reason).

lasers aren’t just perfectly parallel rays, they are also very many rays in a very small area(intensity aka concentrated light)
and usually the laser has a color because its mostly just one type of light that the laser is shooting. white light is all colors of visible light.

on this video i don’t get what you are going on about with negative kelvin etc
lasers just focus and concentrate light.
A laser ‘BEAM’ is focused and concentrated light!
light from the sun or a flash light is drastically dispersed(not concentrated and focused)! that means there are “MORE RAYS” of light hitting a “SINGLE SPOT” with a laser source(i’m ignoring the frequency) than with a flash light or sun source

also if what you say is true, then why isn’t your laser hot enough to burn right through your wall in less than a nanosecond, since you say its ‘beyond’ infinite kelvin..
why does it take time to burn the wood? maybe because it not as hot as you claim..

its just many concentrated rays on a smaller spot area than the source area.

lets say the source rays is 3 dots

••• (front view of the laser hole)

if we focus those rays in a single spot

• (view of spot on the wall)
thats 3 rays overlapping the ‘same amount of area of just one ray source area.

so technically that spot is 3 times hotter
than the one spot from the source, but has the same energy of all the 3 source dots added together.

now the light coming from the sun is just like many dots emitting light ••• but every dot emits diffused(scattered light)


sun surface is made of many ‘dot light sources’ that emit diffused light.

like this:

\ | / \ | /
— • — — • —
/ | \ / | \

\ | / \ | /
— • — — • —
/ | \ / | \

and the sun at a distance is just considered a dot light source as well.

sun:

\ | /
— O —
/ | \

like we see stars

so the only reason why we see stars even though they are so very far away and emit defused light, is because they are so VERY VERY‘BIG’( the stars)!!
the surface of the stars that point at us is SOOOO ‘vast’ that we can consider that light source area to be a flat area light(background from CG 3d lighting), therefore its emitting many (nearly) parallel rays towards us (but not converging rays. they are still diffusing a little) and since none of those rays are focusing, it wont burn anything and even if u did focused those rays that do reach our planet, the amount of rays(intensity)wouldn’t be much because most the rays from that star are lost and dispersed in different direction and so we are only receiving a very very small percentage of that light sources rays.

another thing to consider is the angle of attack of the rays with the lens of magnifying glass.

[GT-Person]

Raise your hand if your waching in 2024 ✋️

[friendgaigthemostepicguest]

this man's bravery is so high that he does not even fear an actual fire in his house

[elenab.]

He:
"negative temperatures are hotter than positive temperatures"
My brain:
*Exploding*

[MartinSanchez-emji]

*”Hotter than infinity”*

Other side of the world:
Where is this laser coming from?

[allenreeder]

When I have a question about something I tend to lean towards your videos! I praise the fact you're smart enough to do these things about by yourself:) 5hank you always!! And the talk of temperatures will throw a lot of people off, haha but, trust me you made perfects sense to me! Can't wait to watch all the other vids! :) keep up your good work!

[SoirEkim]

I bought one with the black coating. I found I could adjust the lens at its tip with a flat head screw driver. This proved useful as mine, like yours, was emitting a tiny line instead of a dot. Now I love it even more.

[Duck-qcie]

Mom: *eat the food its not that hot*
The food: *hotter than infinity*

[imbouttashowyoumycaillou-k]

Everyone gangsta until 0 has positive and negative forms

[AMaass-bhzd]

This is an awesome experiment I wish you had an infrared thermometer where you could measure the heat with a infrared temperature gauge gun or whatever and get it focused on after you put the magnifying glass in front.

[TheZotmeister]

As a math guy, it's _fascinating_ to see a physical representation of a number ring, where negative numbers become greater than infinity. I'm reminded of how complex algebra (imaginary numbers) was adopted by electrical physics. It's great to see these abstract concepts mathematicians dream up to be shown to have practical real-world applications.

[spamtongspamton]

“-0 Kelvin”

understandable, have a great day

[zikemdg]

I was going to say something stupid like I didn't come here for a history lesson, I just came to see something blow up or burn, but I actually learned something so thank you very much.

[darmok]

This is a super cool demonstration, and you've provided an excellent explanation as to why you can't get something hotter than the source with magnifying glasses. Thank you!