Okay today, I'm going to be doing an experiment that proves that light is a wave
And then I'm going to do an experiment that shows you that light is a particle and then after you're thoroughly confused
I'll explain what light really is okay, so first
I'm going to be proving to you that light acts as a wave and to do that
I'm just going to be using a standard laser pointer
So this isn't anything fancy. It cost like five dollars or something so the experiment
I'm going to be doing is called the double slit experiment and what it does is it uses the
interference of light waves to show light and dark patches of the original source
So when any wave strikes a barrier that has two tiny little slits that are close together?
You can see an interference pattern on the backdrop here
so basically if waves come in so this represents the peaks of the waves and
It hits this wall here. What happens
Is these small little slits here act as point sources for the wave so it starts the wave over again?
And so the peaks and valleys of these new wave sources interfere with each other so that when the waves hit a
Backdrop here you see areas that have high peaks and low peaks and it looks like a striped pattern
But in order for this to work
You need light that is coming in in parallel lines like this so light from a flashlight
Won't work because because any light coming from a bulb source like this is
Coming from all directions, so it's curved
But there is a source of light you can use that's not curved and that's laser light and so laser light is mostly
Coherent light and so the peaks and the valleys are all lined up
So essentially it's coming in in parallel lines like we saw in my drawing there so basically if I can just pass this laser light
Through some tiny little slits, then we should be able to see an interference pattern
If these light rays are really waves
So it's actually quite easy to do the double slit experiment with a handheld laser like this
All you do is put a very thin wire across the opening of the laser
And then you place tape on either side of it so that basically when you're finished you have a wire in the middle
With two tiny gaps on either side of it, and so now if I shine my laser light, and I see interference patterns
That's proof right there that light is actually wave-like and has wave-like properties
So let's see if I can see an interference pattern with my laser. Now. Okay, the double slit experiment
Let's turn on the laser and see if we see any interference
Whoa it worked
Look at that you can totally see the pattern there
That's so cool, you can easily see the bands of interference here
Wow just by sticking a wire in front of the laser
Okay, so I just proved to you that light is actually made of waves or has wave-like properties
But now I'm going to show you that light is actually made of particles and has particle-like properties and in order to do that
I'm going to use some glow-in-the-dark
Material this specific one is called lit. It's actually a really cool. Glow in the dark pigment
So how cool in the dark materials work is they have some electrons?
This is my electron here, and they're at their base state here
But when a specific kind of light hits it and knocks it up to a higher energy level, and it stays there for a while
Normal materials as soon as they get knocked up to a higher energy level
They just fall back down, and they re-emit the light in a matter of nanoseconds, but for glow-in-the-dark materials
They get knocked to a higher energy state and they stay there and the reason they stay there is
Because in order to fall back down. They have to go a different way
they have to go through a different transition state called a forbidden transition and
the forbidden transition means that it's very
Statistically unlikely to happen so it takes a while for it to happen
But eventually the electrons will fall back down and when they do they emit some light
But here's the weird thing about glow-in-the-dark materials
Is that in order to knock this electron to a higher energy level all types of light don't do it
But it takes a specific
Color of light to do it so for example if I shine red light on this glow in the dark material
No matter how strong of light, I shine on it the electron won't get knocked up to the higher energy level
Nothing no charge whatsoever
But if I shine just a very dim blue light on it
The electrons will get knocked up to the higher energy level, and you can tell they did because they eventually glow and re-emit the light
So what's weird about this experiment?
Is that it disproves that light is a wave because if light were a wave all you would need is very high
amplitudes of a specific light and
Eventually you'd be able to knock electrons up to the higher energy level
But what I'm telling you that even if you had enormous amplitudes
Extremely high intensity of red light you still couldn't knock this electron up to a higher energy
State but even with a very very very dim blue light
you would be able to knock it up to the higher energy state so an experiment similar to this one called the
Photoelectric effect is what led scientists to develop the field of quantum
mechanics and what they said is that light actually isn't a continuous wave like this
But it's actually made of discrete particles
And those discrete particles have specific
Momentum's and in order to knock an electron up to a higher energy state it has to be hit with a particle that has enough
Momentum to knock it up to that higher energy level and that makes sense right not just any particle can hit it
but it has to have one that has enough momentum to knock it up there and
So it turns out that red light doesn't have enough momentum to knock electrons up to the higher energy level
But blue light does and so just this simple experiment of trying to charge a glow-in-the-dark material with red light
Versus blue light shows you that light is actually not made of waves, but it's made of particles, but in the previous example
I just showed you that light is made of waves so now for my final really weird experiment
What if I shine the double slit experiment red laser light on this glow-in-the-dark pigment will it charge it?
Okay now for the experiment that puts the cherry on top
I'm going to be shining my double slit experiment laser light onto
The lit and showing you that it can't charge the lit
and if it's not
Registering how crazy this is I'm going to be showing you that I'm shining waves on it because you can see the wave
Interference of the light, but then the intensity is 24 times greater than sunlight
But it still can't charge the lit
Because the particles of red light don't have enough energy to knock the lit up to the higher energy state
Okay, so here. We go. You can clearly see the slits on there that show you the wave interference of light
But it looks like it's not charging the lit
But if I just shine an ordinary flashlight, that's orders of magnitude lower in intensity onto the lit
It easily charges it
Crazy
Okay, so what I just showed you is extremely weird
Basically, I showed you that when the light was coming out of the laser
It was a wave
But then when it hit the glow-in-the-dark
Material it turned into a particle that didn't have enough momentum to knock the electrons to a higher energy state
But it was a wave when it left the laser, and then it turned into a particle right about here
So what is it is light a wave or a particle it can't be both right? I mean a wave has no specific location
It's spread out in space you can't really say a wave is here or there
It's kind of everywhere, but a particle has a specific location you can say a particle is right here or right there
But you can't say something is both a wave and a particle
So for example in this double slit experiment if I say that light is a particle
Then does that mean that that specific particle splits into two goes down both sides of the slit here and then recombines later
What we say is that light has a wave particle duality
Meaning it kind of takes on properties of both
But what's weirder than that it's not only light that takes on a wave particle duality
But in fact all known particles in the universe
Have a wave particle duality, so if I were to do this double slit experiment, but instead of using light
I was shooting electrons through you would actually still get an interference pattern I
Mean these are electrons right you're talking about single electrons, and you can even shoot them out one at a time
And you'll still get an interference pattern on the back stop, so a wave particle duality
Basically means that everything in the universe doesn't have a specific location or momentum until you measure it
So here's what I mean
Let's take a hydrogen atom so in the center is the nucleus
But we want to know where the electron is so let's say we measure the electron and we find it here
but then next time we measure it we find it here and
Then next time we measure it we find it here and then next time here here here here
and if we do a lots of times
Then we can get this probability distribution of where we're likely to find an electron
So what I'm telling you is that before you measure something you cannot know with certainty?
What its property will be you can only know the probability that it will have some specific property
Or have some specific location, and if you've taken any chemistry courses you actually already know this
But it was told to you in a different way so for example you may have seen a hydrogen atom with an S. Orbital
so when you're saying an orbital of an
Electron you're basically just saying that here's the location that you're likely to find an electron when you measure it
So that's why in chemistry when you see s orbitals or P orbitals. It's talked about as a cloud
It's actually not a cloud of electrons because whenever you measure an electron it's at a single point
But what electron orbitals actually represent is the?
probability that you'll find the electron in that spot when you measure it and
What I've shown here is the basics of quantum mechanics
Quantum mechanics is actually the most proven theory in all of science. Hey everyone
Thanks for watching this episode of the action lab. I hope you enjoyed it
Hope you learned something or if you have any suggestions or comments. Let me know in the comments section
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