Hey welcome back, it's good to be here, I'm Mark Rover. This is science class. So today, why is the sky blue and maybe in your head, you're like like blue light scatters, you just memorize that fact, but you should know by now we're not about facts here. This is about mental models.

Let's get a good mental model of why it's blue, because that's more useful, you could extrapolate and you just have a deeper level of understanding. So, as usual, we have a question. We have three clues by the end of this. We should understand it better and we use a few equations as possible.

So why am I doing this? Because yeah my review of the coronavirus is zero out of five stars. This is kind of a bummer. You guys are home from school. It didn't really feel right to just not you know, do monthly videos, so I'm just trying to do something.

So we connect a little more give you guys a little sense of normalcy in your life. Now I have to start with something I'm very ashamed of. As you might know, my last video was about washing your hands and how you can transmit viruses by touching things and, most importantly, the important why you got ta wash your hands. Is we tend to touch our face? So you shouldn't touch your face.

This is the exhaust port on the Death Star, your eyes, your nose, your mouth. Having said that, well, let's just roll the tape, I'm not proud of this out of the gate. I'm not proud of this beans cucumber mango avocado regards to that density equates start to sink four. So just a little bit the gravity.

You go your ears, there's this that's a verse and it floats leave classes. This isn't that easy, yeah, so less than twenty seven is my goal today. You guys can keep me honest. Alright, I'm really gon na do a better job.

I picked this topic because, to be honest, I didn't have a good mental model of it, and so I basically give myself a day to learn something really deeply so then I could turn and explain it simply. You cannot explain something simply until you understand it. Well so this turns out is a little bit more complex than last week, but I don't think it's too complex. I think you guys can get this and we're gon na make it happen today.

The last thing I'll say is: I could mess up here. This is live. We built this set. It's a miracle, we're here right now I won't get into it.

It's been a crazy morning. Things could come crashing down if they do we roll with it. I might miss, speak and say something a little bit off. If I do, I mean I decided I'm gon na in the video description, I'll put any corrections afterwards, but I appreciate your patience.

I basically have a day to come up with these things, but the lesson plans are, and it's fast and loose and we're live. So as an example last I forgot to state the assumption on the fart one that you know your intestines aren't under pressure. That's like was one of the premises I wanted to state. That makes you that's.

The reason makes you heavier so things like that check back to video description in a few days and if you're like, I don't think that was right, I'll, try and tweak it in general. You can trust me, but things just might be a little off cuz. It's fast and loose okay, why is the sky blue? We're just get right into it? Clue number one electro magnetic waves, you're like what's that electromagnetic waves are radio waves, Wi-Fi your microwave, your cell phone, that you talk on these? Are electromagnetic waves they're all around us, but we can't see most of them right, so what's really cool and crazy about these, though, is the only difference between like x-rays and microwaves and radio waves is the wavelength so picture. I have a slinky here, the more I stretch out the slinky, the greater the wavelength right.

If I count the little rings, the Rings they're less frequent. So it's a it's a lower frequency right, lower pitch like with sound there's a lot of analogies here. If I stretch this pretty close slinky closer together, the Rings happen more frequently higher frequency right and the wavelength is smaller. So what if I told you that there was a specialized instrument that you can use to actually sense electromagnetic waves and it's within, like a foot of you right now, there's a specialized instrument for sensing electromagnetic waves and maybe you're like no there's, not I'm on the Couch the TV's way over there, I'm not by anything electronic, it's not electronic, it's biological and you have two of them and they're in your head is your eyes.

These are electromagnetic wave, sensors and you're like what are you talking about and I'm telling you like these eyes, you look around it's like that is 400 nanometers. It's telling you the wavelength and you're like that is 700 nanometers and you're like I don't get that feedback and you do as a lot of you probably know now in colors. Colors are your brains way of telling you different electromagnetic waves that are coming off things and hitting your eyes, so here's a chart that will come in on and here's you know big wavelengths radio waves. These can be like as big as cities in their wavelength.

That's a stretched out spring slinky all the way to really really tiny gamma rays. X-Rays. Now, there's this little barrel band here, the visible spectrum is what we call it from about 700 to 400 nanometers red orange, yellow, green blue purple. It's the rainbow! What's interesting about this is that if you go infrared, we can't see infrared, but some animals can like snakes right or mosquitoes or bedbugs.

Now, why would blood sucking insects? Why is it helpful for them to see infrared well if you've ever seen him for red, you could see body heat, so they find their victims right on this end of the spectrum ultraviolet ultraviolet, like extra violet, infra red, because Reds over here ultraviolet, you have like Butterflies and elk, they can actually see this. They use our reindeer. They use it to navigate bees as well, so our eyes a specific range, but other animals can see more okay. So that's clue.

Number one electromagnetic waves just know that your eyes are eyes. Allow us to see a certain spectrum right here and we see those as colors. Alright number two clue: number two: the Sun, okay, so the sun's. Just like amazing.

I give the Sun a five out of five stars. Unlike coronavirus and the Sun blasts, this whole range of electromagnetic waves. It's not just a certain way, everything at all times. It's like just blasting us all across the spectrum.

It's like one trillion nuclear bombs. A second is the energy coming from the Sun. It's 93 million miles away, which means it takes 8 minutes and 20 seconds for the Sun to reach us. So I am setting a timer for 8 minutes and 15 seconds, because that means right now a photon just left the Sun.

It's on its way here. So in 8 minutes we're all gon na celebrate and be like yo welcome to earth photon like welcome what a journey you just made 93 million miles away. It's on its way, we're gon na come back to that in a second, that's gon na be exciting. It's Naruto running its way right here, real fast, alright, so we're gon na check in with that, the Sun is all of the colors, so we see it as white right.

The Sun kind of looks up. It looks white basically, but that means that's because when you combine all the colors it's white, then you may have seen this I'll come over here with the prism. Maybe you've seen like a chandelier in your room where it hits some glass and it splits it out. That's when it's giving you the clue that you know it actually has all those colors in there at when it, when you see that it's kind of unpacking them for you, so the other thing to point out is the color you see is the color.

That's actually reflected so it's saying that certain wavelength red is about 700 nanometers. That's the wavelength! That's hitting my eyeball, not the color! That's absorbed! If I see green, that's closer to 500, that's the wavelength of electromagnetic! That's hitting my eyeball! So here's a question: if you have a plant, you love this point. It's your favorite plant. You want it to live.

I give you one choice of color to shine on it day and night. What color do you choose? Remember? Green plants are green. If you chose green your plants dead, you killed your plan. That's really sad.

I put an F in the comments in the chat for your plant. It's dead! Why? Because remember! The color you see is what's reflected so green is the one wavelength that plants like don't need right, they're, sending that back any other color. It would get more nutrients and be able to do photosynthesis, but Green is the one it doesn't need. So that's kind of a little helpful sort of mental model, so you should know also black absorbs all the colors white reflects all the colors.

This is why, unlike a hot day, if you're wearing a t-shirt, a black shirt gets hotter than a white shirt, because whites like reflecting other colors black is absorbing you want further proof. I will I've got this flashlight here right. So let's pretend this was a red flashlight. We have to use our coronavirus imaginations because I wasn't able to actually get a red flashlight in time.

If I shine a red flashlight on this. What color is it the paper's black if I shine a green flashlight on this? What colors the paper become? It's black. It sucks all those colors, it's like whatever you send at me, I'm sucking it in I'm. Staying black now, let's say white.

If I had a red flashlight, it was a dark room and I shined it on this. What color does this paper look? It looks perfectly red if I had a green flashlight and I shine on this. What color does this paper look? It looks perfectly green, so white is like yo, I'm a color mirror, whatever you throw at me, I'm just gon na balance back exactly what you sent at me. So that's.

Why we're in right now in this room, all the colors of the spectrum. You hear it hitting it, it's like I'll, send them all back at you baby and it's gon na look white because of that, because that's the color when you combine it together. So one thing, that's kind of interesting to think of is a blue marker. Isn't really like? Oh, a blue marker, it's kind of like a blue blocking marker right.

I just put some liquid on this paper. That's really good at blocking the blue electromagnetic wave, so it's like I'm gon na block these and send them back at your eyeball. So it's not really a blue marker, actually, all right, so that brings us to clue them through the sky. Okay, what is the sky when we say the sky we're kind of talking about the atmosphere? What is the atmosphere? Here's the globe there's like a layer around here, that's like 80 %, nitrogen 20 % oxygen right and you might say: well what would it look like if we didn't have an atmosphere and there are exactly 12 humans who have a first-hand account and only 12 of What it looks like with their own eyes to have be on a planet or a celestial body with no atmosphere, you guess who that is the astronauts who have walked on the moon, because on the moon, you can see it's daylight there on the moon, but you Could still see the stars at night, the sun's shining, it's just there's no atmosphere on the moon.

The other benefit to that you come back. Is that with the atmosphere? It creates friction so in asteroids, come in! That's why the moon is so cratered with our with our atmosphere. The asteroids come in you're really hot and they burn up, so they don't hit us usually on this, a really big RIT, the dinosaurs, all right, so that is the sky. Now, let's talk about we're, things are about to get really crazy, so my favorite physics, teacher Paul Hewitt, gave an analogy if we can I'll bring this down all right here.

We go watch this okay, high tech on our set today, so you could kind of make an analogy of colors and the frequency in the electromagnetic spectrum to sound right, see if we can see this so something that has a higher frequency is going to sound. It's small, its tiny right and purple and blue are just the end of the spectrum where they have a higher frequency. If something has a lower frequency which is red on the other end, which means a longer wavelength, lower frequency, less frequent, lower pitch. You would expect this to be really high or really low.

I mean low compared to this right here, the difference so here's what's really cool. This is this: is the cool mental model here, I'm getting this globe out of the way the globe just broke. We've got Ken, we brought him back, duct tape, kilt ken! That's you on the surface of the earth. This is the sky.

Okay and what you'll notice here is most of the little pieces up here in the sky are shorter, chimes closer to this, because nitrogen and oxygen are really lightweight right, really lightweight, so they're gon na resonate with something that has a much higher frequency. So here's the Sun when the Sun shines it sends all of these colors. At the same time, pretend this is above it kind of goes off camera, so pretend I'm perfectly above I send the Sun all the colors. What happens? Is it's the higher it's those little nitrogen and oxygen they're lightweight that kind of resonate with of all the colors? They resonate most with the higher frequency ones, the blue and then the violets, especially the violets, and they scatter that light, because heavier molecules are much more less calm and you see a long one here.

It's like it's like ten times fewer heavier ones than the light weight runs most of what gets scattered is the blue light. So that's why the sky is blue at noon. When you're looking up, you hit this, it's because all the particles up there high in the sky are really lightweight and they resonate with the higher frequencies. Okay, so that kind of helps with our mental model, but now that we do have a mental model, the question is what happens at sunset, so now duct-tape kilt ken, is over here at sunset right.

The reason why that is if I come to the board, this is the surface of the earth right. This is kind of the the atmosphere here. It's thin layer, Oh welcome to earth. There we go, there's the timer, it just came, it just arrived.

So those are the photons we talked about earlier they're here. Look outside your window! Welcome them what a glorious moment! Okay, so the Sun right at noon. It comes straight down. It doesn't have to go through my that much atmosphere, but at sunset, what happens when the sun's over here I've already got it drawn here, look how much atmosphere it has to come.

You're over here, duct-tape kilt ken is over here. It has to go through a lot more atmosphere. So now this is the situation. Here's Ken okay, 10 G's up a little higher here's, the Sun Sun.

Does its thing sends all the frequencies coming. But what happens is that there's so many more little of these chimes coming that they run out of energy at first, it's like oh they're, shining, real, bright, really loud the high-pitched, but by the time it gets over here. There's nothing left! So all the blue and the violet - all that's left are the lower frequencies, the Reds and the oranges right. It's the leftovers, basically, which is really interesting to think about.

So when you're standing here at sunset, the reason that you see that is red and not blue anymore - is you have to go through so many and all of the the higher frequency there's no more energy left at those wavelengths and so you're left you get the Leftovers of the longer ones, and now this mental model then helps us think about why, let's say in Arizona you really beautiful, blue skies or right after it rains, the sky looks really deep, blue. Well, there's no pollution, there's no call it water moisture in the air. It's just those really lightweight little chimes so resonate really loudly at those and it's very vibrant, blue. If there's a fire, a lot of pollution, that's fine little particles in the air, and so the sunsets are especially beautiful because they'll blue light dude by the time it gets you get out of town like there's no blue light left at all.

It's all been totally scattered because there's so many little particles in the air right, if there actually is a fire going on, you can almost look at the Sun at noonday and it looks Orange the reason you could do. That is because that blue light - and it's really dim - don't look the Sun by the way, but I'm just saying those blue thing: those blue particles, they've been they've, been scattered right so and then I'd say one more thing about this: that's even you could expend this Model even more, if you're in your bathroom and the window fogs up right, what happens? Is you can't see it becomes opaque? That's scattering all the light instead of it, because you have heavy particles, light particles, you got the whole range right, so a cloud or when it's foggy or hazy, those are like water droplets and because you have all sorts of different sizes in there, not just a Higher concentration of smaller ones, it's just scattering everything and when you scatter everything it's white, okay, all right! So I'll end with this cool little experiment. This was a happy accident last night it we checked. It doesn't look as good on this camera as it did for me last night, but this is basically a sunset in a bottle up top, I'm going to show you now what I got at home because it just shows that better than these cameras, but you can Do this experiment yourself? All that's in here is just a little bit of milk, so that looks like the noonday Sun right.

It's blue and white. Look at that white, sun, blue sky! When I move it down, there's just a little bit of milk in this beaker. You get a sunset, so what's happening there well, look at that! That's beautiful come back here, Alex what you're seeing is that up here, the the the beaker is more narrow, so there's fewer particles to go through as you go down and expands out. There's more little particles for the blue to be scattered off and all you're left with is the is the longer wavelengths, the Reds and the oranges.

From my fly. How cool is that you can do that yourself, just a little bit of milk, maybe in different glasses. Alright, so I think that should be kind of powered through that the wireless knowledge of transfer from my brain - hopefully some of the stuff came through, but hopefully some stuff made sense there and you have a better mental model of what's going on. So I want to show you guys some of the great video submissions.

I got last time with the neutral buoyancy thing before I do. I just want to say both saw what I was doing and they said that they will give away about eight of these seven hundreds. These are like their dopest headphones. They make.

You know they've worked with me before I'm not getting paid for this, but they just said: hey, we'll give them away so they're, going to give four away to anyone who tells people about this class either on Twitter or Instagram or Facebook or tic-tock tic-tock. Is that the ticket? I will randomly pick someone who's telling people about this class, so we can get more people here for people get that and then for people who do who accept the challenges or answer these questions and do a good job. You got to see some of these responses we got here. We go so both will be reaching out to you if you're one of the eight people they selected for this week's challenge, we'll come back over here.

I love this quote. Your shadow was confirmation that light has traveled nearly 93 million miles, unobstructed only to be deprived from reaching the ground in the final few feet. Thanks to you so with this in mind, the challenge this time for next Monday is to create some shadow art. Okay.

This is where you find everyday objects around your house, and you kind of turn off all of your lights in the room, put a single point of light on it and make something cool now these are really advanced, so don't feel like you got to get this Crazy but get creative and you know maybe spell something out see if you can make a shape, something cool that you wouldn't see it looking at the objects. But when you shine it from a certain angle, it looks awesome. So if you want a Google to get some inspiration, it's called shadow art all right, so I'm gon na end with the question, but first this is my opportunity to say something and for me the thing I want to talk about this time. Real quickly is this element of curiosity, the most important thing to say in science: isn't Eureka.

I have discovered this it's so last night I had a moment like that with the speaker. I actually wanted a perfect cylinder and then, as I was like playing around with this and seeing where we find the best I realize I could like actually create a the microwave was discovered when someone was standing next to a radar and a friggin. Mr. Goodbar melted and this guy's pocket and he's like that's interesting, what's going on there so that he observed that and did more so don't be afraid to be curious and to ask questions and observe first step in the scientific method is observation.

So that's how all great discoveries are made by just observing so pay attention to the world around you touch my face and then good things will happen. So, let's talk I'm going to show you the question for next time from my friend Sally, oh hi, mom. It's doc decided a patient and I was wondering if you could help me answer a question. I've been struggling with.

How can you put your hand in water without it getting wet drink? Okay, so put your guests in the comments we're through the comments below and by the way the answer isn't like where a glove. What are some other creative ways? You can keep your hand by the way sally has a PhD in evolutionary biology from Oxford and she's. Doing a live stream once a week I'll put a link to her channel in the video description. She started from the basics and going up so it's really interesting uh all right! Well with that say what a hot mess this was, and I love it.

This is beautiful, we're going to class is officially dismissed, as you know, we're gon na go to some questions and then I'll answer these for like three minutes, and then you have your assignments. The challenge I'll see you guys next Monday. Okay, so here we go. Did you consider other engineering paths, because I enjoy lecture cool engineering and I want to know what you think you chose wrong? I mean it's as simple as that's a fact.

Mechanical engineering is the best I'm just kidding. I don't. Electrical engineering is like in chemistry you're, like I can't see those things right. Mechanical engineering is always just really resonated with me.

It's like oh there's, there's this piece of chalk. I could tell you exactly how long till it hits the ground right. I could see the things I like the interactions, the collisions, so it's kind of whatever clicks for you. So for me, that's what clicked, but you should do what you like, even if you choose the wrong one all right.

What's the difference between normal people are normal and colorblind people, so I should be careful on this because I don't want to go too far out of like what my expertise is. Colorblind people don't see, the world is like black and white and, like oh, you get. No information of electromagnetic waves - they just see it off a little bit and it's harder for them to distinguish, has to do with, like the cones in our eyes, so like certain colors are harder to tell the difference, but it's not that they see the world in. Like black and white, why do shadows appear darker if it doesn't affect wavelength all right? Well, I have to okay.

I've got two more questions. I love the next one after this. Okay, I just got excited. Why do shadows with your darker eye? Shadow is just a lack of light right.

If you have a white light, you know in a dark room, so shadows are dark because there's just there's no waves hitting that spot. If you turn off all the lights in room, it's plee dark. That means there's! No electromagnetic waves in that visible spectrum in that room all right, so a shadow is dark because it's just like an absence of electromagnetic waves hitting that spot last question: why is water blue from far away and transparent close up? Basically, why is water blue and this one's cool, because water actually absorbs there's something about? It were absorbs infrared and wavelengths kind of more in this spectrum. So the reason water looks blue is because it's like sucking up and absorbing some of the energy.

That's like more red and infrared, and so what happens if all that goes away, all that's left to bounce back is the bluish greenish stuff. So there we go. That's the lesson for this week, thanks for joining we'll see you on Monday.