focus

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



Sharing buttons:

okay today I have a giant magnifying

glass and my five watt burning laser and

I'm going to be testing what happens if

you try to focus the point of a laser

pointer down to an even smaller point

will it get even hotter and how small

can you get it can you just get an

infinitely hot temperature with lasers

so first let's see what happens when I

try to focus down my 30 mm lemon

flashlight into one single point is it

possible so in order to do that I want

to gather all of this light that's

coming off and focus it down to one

single point but because the light

spreads out like this I have to put my

magnifying glass really close to it to

capture all of the light but you can see

what happens when I put the magnifying

glass really close to it it spreads out

the light instead of focuses it to a

point so in order to focus it to a point

I have to move it further away from the

lens but as I move it further away from

the lens you can see now I'm losing

light on the wall on the background wall

so I've lost a lot of the light in order

to focus it down to a smaller point and

you can see what this starts to look

like actually it's just a miniature

version of the LED lights on the light

itself so this is what the LED looks

like and that's the picture you saw

there so in order to focus this down to

a very small point that just means I

need to move my flashlight further and

further and further away so I'm gonna

move it all the way across the room and

now you can see I can focus it down to a

tiny little spot but now because the

flashlights clear across the room I've

actually lost most of the light on the

wall and only a tiny little bit is

making it to the lens and it's focusing

it down to a small point and because

I've lost so much light that small point

is now not brighter than the light

itself so the magnifying glass did

nothing to brighten the light the dot is

never gonna be brighter than the

original flashlight itself so this is an

important point for a flashlight you can

never use a lens to focus the flashlight

down to a brighter point than the

original source itself so you can never

do anything with any combination of

lenses you could never make a brighter

point then the LED lights on the

flashlight itself and the same thing

applies when you use them

magnifying glass to focus down sunlight

to burn things that means that that

point of light that you get from a

magnifying glass can never make a point

that's brighter than the surface of the

Sun so no matter how big of lens we ever

had on earth no matter what we did we

can never focus the sunlight down to get

something hotter than the surface of the

Sun if I were able to make a point on

the ground that's hotter than this

source that would mean that I was

violating the second law of

thermodynamics because that would mean

that I'm flowing heat because this is

hotter I'm flowing heat from cold to hot

so the energy is going from cold to hot

but we know that that can't happen it

always goes from hot to cold

but does that apply to lasers as well

well let's check ok so in order to see

this I've turned my exposure way down or

else you're just gonna see a really

bright ball of light and I have it on a

black surface here to absorb even more

light so we can actually see how big the

point is so I have my laser across the

room let's turn it on and see how big

the point is now okay so you can see

it's kind of a line of light now and

it's kind of burning the background here

now let's see what happens if I stick my

lens in front of it whoa I can make a

tiny point and it easily burns the

background now so you can see the smoke

coming off of it it's slightly burning

the background here but then as soon as

I put my lens in there I can focus it

down to a tiny little point and it

immediately burns whatever's around it

so you can see that it kinda starts to

burn the wood here it takes a while for

it to burn have to leave it there for a

bit because it's far away across the

room but now watch what happens when I

focus it down it pretty much immediately

burns and starts on fire it's awesome it

essentially just itches into the wood

immediately and you can see that this

happens even more powerful than if I had

the laser just right up next to it you

can see that I still have to leave it

there for a bit to even start burning so

it seems as though I get my highest

temperature when I put my laser really

far away essentially so that I get that

little tiny laser slit to a single point

because I place it far away and so that

all of the light coming from the laser

is now focused into a very very tiny

point and so all of the energy from the

laser can be focused to a small point

due to the magnifying glass and so you

can even get a higher temperature than

if you just place your laser directly on

the thing you were trying to burn so

what's going on here I just told you

that it violates the second law of

thermodynamics if you are able to focus

light down to a point hotter than the

source itself but I just did it with a

laser so why does that happen but

actually with lasers this actually

doesn't violate the second law of

thermodynamics and the reason may sound

a little weird it's because flashlights

and the Sun those all have positive

temperatures they have very high

positive temperatures but lasers

actually have a negative temperature let

me show you what I mean so negative

temperature can only exist in something

that has a limited number of states so

when something is really cold it means

that all of the atoms are in a low

energy State

if it were absolute zero that would mean

every atom is in its lowest energy state

and as you heat it up these atoms can

pop up to a higher energy state and the

hottest you can possibly get is for an

equal number of atoms to be in the high

and low energy state and when you have

something the hottest you can possibly

get that means you have an equal number

of atoms in the high energy state and

the low energy State so the temperature

scale actually goes like this

it starts at zero Kelvin and then you

can get up to some in-between

temperature like 500 degrees Kelvin and

then as you keep going you can actually

get to an infinite temperature as high

temperatures you can possibly get where

there's an equal number of atoms in the

high energy State in the low energy

State but you can actually keep going

from there and you can actually get more

atoms to pop up and be in the high

energy State and the only way you can do

that is in quantum mechanical based

systems like lasers and in that case it

goes past positive infinity and then

actually the next step is to go to

negative infinity Kelvin and then if you

keep going from there you could get to

negative 500 Kelvin and then if the

highest temperature you could possibly

get is negative zero Kelvin now this

sounds crypt now this sounds crazy to

say that the highest temperature you

could get is negative zero Kelvin but

what this means is actually that

negative temperatures are hotter than

positive temperatures so focusing any

light source that doesn't have a

population inversion like a laser so

basically anything that's not laser

light you can never get a hotter point

than the source or you can never get a

brighter point than the source itself

but for lasers you can get to any

temperature you want by focusing it to a

smaller and smaller point which is

actually pretty cool

and because lasers actually have a

negative temperature it means that

you're still not breaking the second law

of thermodynamics that was close hey

everybody thanks for watching another

episode of the action lab I hope you

liked it if you have any questions let

me know in the comments section if you

haven't subscribed the action lab yet

remember to hit the subscribe button and

hit the bell to be notified with my

latest videos out and head over to the

action lab com if you haven't checked it

out yet check out my new act

lab subscription box thanks for watching

and I'll see you next time