Can you Focus Regular White Light into a LASER?

Added: 2026-05-15

[00:00:00]Here's a question for you. Can you take regular light and focus it somehow and so that it becomes laser light into a tight beam like a laser? Well, the answer is no. And that's because laser light, even though it's made of photons, is very different than regular light that comes out of a light bulb. Let's talk about why. So, laser light can be focused into a very narrow beam. And we've all seen pictures of lasers burning through objects, right? And you can also take sunlight with a magnifying glass and you can focus it down to a very small point and it can also burn objects. So that's why people think you can sort of focus regular light and it somehow becomes laser light. That's not the case. And I'd like to talk about the differences. Now laser stands for light amplification by the stimulated emission of radiation. L stimulated emission of radiation.

[00:00:50]Radiation being photons. So the main thing here is that laser light is from the stimulated or the emission from electrons bouncing around in the atoms there in a stimulated way. In other words, they're sort of like a chain reaction of photons triggering other electrons to decay back down and release photons and so they can be amplified and focused. And that's what a laser is. But the number one thing about laser light that really distinguishes it from regular light like a light bulb or from the sun is that laser light is what we say is coherent. That's the big difference. Whereas light from a light bulb or from the sun or anything else is what's called non-coherent. It's a random wavelengths all mixed together.

[00:01:33]That's like the colors of the rainbow for instance. And the crest and the troughs of the electric and magnetic fields inside of the light itself, they're not lined up at all. And so it's all random directions, random wavelengths. Nothing's really aligned.

[00:01:47]That's what regular light is. But in a laser, it's very highly coherent where the peaks and the troughs of the waves are lined up. Everything is very aligned and focused. And that's why it's able to do the things that lasers can do. The first kind of coherence is called temporal coherence. And that just means that the wavelengths emitted by the laser itself are very, very narrow. It's not a bunch of different wavelengths.

[00:02:10]Ideally, it would be a single wavelength. In reality, lasers produce wavelengths that are very, very narrow bandwidth. In other words, very slight deviations in the wavelength, which means all the same color. There's also spatial coherence and electric polarization coherence. That means that the electric fields of the waves themselves are lined up across the beam and the crest and the troughs are all lined up in the resonant cavity of the laser before it's emitted. So, these are the main reasons why you can't focus regular light into a laser beam, no matter how hard you try, because regular light is not coherent and lasers are.

[00:02:46]One amazing fact is that the Milky Way galaxy is actually on a collision course with the Andromeda galaxy. The Andromeda galaxy being about 2 and a half million light years away. Our galaxy and that galaxy will eventually collide and merge together. You know, as we've used the Hubble telescope and the James Webb Space Telescope to peer out and look at distant galaxies, we see galactic mergers happening all over the sky. For instance, these are two galaxies that have kind of merged into one. It doesn't look like a spiral, but it's two distinct galaxies, and you can see the dust lane in the center obscuring the view from all of the stars. Here's another example, an actual picture of two different galaxies merging and colliding together. But the word collision is a little bit weird. It implies things are smacking into each other. But actually, space is really big and contains a lot of well, space.

[00:03:42]Here's yet another example taken by the James Webb telescope of two different galaxies in the process of merging or colliding. Now, in our in our neighborhood, most stars are about four to five light years apart. They're closer together when you get closer to the center of the galaxy, but four to five light years apart out here. Even when Andromeda merges with the Milky Way galaxy, the stars on average are not going to get any closer together than about 0.1 lighty years. So even though we're going to collide and gravitationally merge together, it'll be very, very rare for two stars to actually smack into each other. Now, here's a composite image of what it might look like. This is not an actual picture. You can see a horizon here from Earth. This is the Milky Way, which you can see when you go into a dark sky and look up at the sky. And here's Andromeda heading our way, merging with the plane of our Milky Way galaxy. Now, just for reference, Andromeda is about 2 and a half million lighty years away from the Earth. And that's really hard to put your brain around, but you can think of it as about 150,000 solar system diameters between here and Andromeda. And Andromeda is heading our way at about 110 kilometers per second.

[00:04:58]Now, the collision itself will actually happen in about 4 and a half billion years from now. And it'll actually take a whopping 2 billion years for it to finally finish and and finish the collision and the gravitational interaction. Now, during that time, stars will be flung from their current orbits. The sun will actually still be around. The planets will all be around, but the inner planets will be much hotter than they are today. and it'll actually probably trigger a lot of new star formation from all of the mergers and interactions. So, in our night sky in the future will look a lot different.

[00:05:30]Now, here's a question for you. We've all seen little insects like that that can walk on the surface of water. It seems to defy gravity. Now, how does that happen? Why does that happen? If you answered surface tension, you're correct. That's the reason. But what exactly is surface tension? And why does it allow insects to walk on water? A little bit closer up, you can really see that there's a skin effect on the surface of water. And you can see that in water droplets, in rain droplets.

[00:06:00]Now, this surface tension, this skin that we like to talk about it is really a surface cohesion because of the air above the water and the property of the water molecule. So, let's dive in a little bit deeper. The executive summary here is that water droplets like this form this boundary which is kind of like a curved boundary at the interface of the air to the liquid because the water molecules inside the bulk liquid are actually attracting each other. I'm going to explain exactly why they do that in a second. But the air above, they're not really attracted to the air above. So what that does is it causes the water molecules since they're attracted to each other, they end up pulling each other inward and forming this curved boundary. when we have air above. Now, this is your handydandy water molecule. You have H2, these are the hydrogens's down here and oxygen up above. Now, because oxygen because of the way the nucleus is, it's attracting the electrons which are being shared here in the bond strongly, more strongly than the hydrogen is. And that means the oxygen is pulling these electrons which are shared closer to the oxygen atom.

[00:07:05]The end result of that is that in every single water molecule, the oxygen, which is up here, is slightly more negative on average and the hydrogens's down here is slightly more positive on average. The end result of that is that even though every water molecule from a distance is neutral, equal number of positive and negative charges, if you zoom in on top, the oxygen is slightly more negative than the hydrogen sides of the molecules. And what that means is if you have another water molecule, this hydrogen is positive and it's going to be attracted to the negative oxygen on an adjacent molecule. And likewise, this oxygen is going to be attracted to the hydrogen side of the other molecules.

[00:07:44]That's called hydrogen bonding. That's the intermolecular attraction between water molecules. And because every water molecule inside is attracted to each other, that gives cohesion being pulled toward the center. and they're not attracted to the air above that causes a curved boundary to form. And if an insect which is very very light is pushing on the surface of that surface tension with a less force than the cohesion pulling from below, it won't break through and it can walk on water.

[00:08:13]Give me two minutes and I will explain exactly why we care about what calculus is, what it's used for, and the core concept of calculus which we call the derivative. First of all, why do we care? Here behind me, I have a rocket launch. So, when a rocket launches, the trajectory is always changing with time.

[00:08:34]The speed is always changing with time.

[00:08:37]The acceleration is always changing with time. The fuel burn is always changing with time. Do you see a pattern here?

[00:08:44]Everything is always changing with time.

[00:08:46]If you're studying magnetism or electricity or pressure disturbances or building almost anything with fluid dynamics and engineering or any other field in general things change with time. So what we really want to know is how do we study things that are always changing in time. Now if you remember back from school you learned about a line specifically the slope of a line.

[00:09:12]But little did you know that the slope of a line is actually the foundation of calculus. So you learned that you have a line and you have a point on a line here and a point on a line here. And you can get a number called the slope of this line by subtracting the y values. That's the rise. And then subtracting the x values, that's the run. And you called it dividing rise over run. And you had this equation. All it's measuring is how tilted is that line. If the slope is like this, it's like a medium uh a medium slope. If the line is steeper, the slope is a bigger number. And if the line is shallower, that is a lower slope. So the number represents the slope. But the nice thing about lines is that the slope is always the same because the line is not changing over time unlike the real world where everything is always changing over time.

[00:10:05]So in a real trajectory like this, what we want to know is the slope of the curve right here where it's tilted here.

[00:10:11]But at a different spot on the curve, the slope of the line tangent might be a little different. Here it might little be be a little bit different. The slope of the tangent line to some real thing changing is always changing itself. So we need more advanced tools. So let's say we have a curve like this. What we do is we draw a straight line through the curve and we calculate the slope of that straight line. We get a number.

[00:10:36]Then we bring the right hand point closer and closer and closer to the left-h hand point calculating the slope of the line that goes through those two points. Eventually we get to the slope of the line tangent. That is what we call the derivative. So the definition of the derivative in calculus is the rise over the run. And the limit is taking those two points closer and closer together. And that's how we analyze things that are changing. You know, I find it really amazing that the ocean is really, really salty, like 350 times more salty than the lakes, the rivers, and the streams that feed into the ocean. You know, it's really crazy when you think about it that the Earth is covered in water. something like 70% of the whole surface area of this planet is actually H2O water. But it's very different. The salinity in the ocean is extreme. If you ever tasted ocean water, whereas all of the tributaries, all of the lakes and rivers and streams that ultimately feed into the ocean don't have very much salt at all. Why is it?

[00:11:40]And it's 350 times more salty. Believe it or not, the ocean actually gets its salinity from the land. And so what's going on is we have the water cycle. We have precipitation coming down all over the land masses. And when it hits the rocks and the mountain ranges and all of the weather all over the planet, that water is draining down. As it does that, it begins to dissolve uh minerals and ions that are on the land predominantly uh magnesium, calcium, uh sodium, chloride, and all of these ions. And these are carried down into the basins that feed into the ocean. The ocean is something like 35 parts per thousand salinity. That works out to about 3.5%.

[00:12:24]Whereas the rivers and the lakes and the streams feeding into the ocean is something like 0.1 parts per thousand.

[00:12:31]So around 300 times or 350 times the salinity. But the question is why is the ocean salty but the rivers not salty if all of the actual salinity is coming from dissolved minerals feeding in from the rivers in the first place. Now the answer is ultimately that the rivers and streams are always flowing. So as the minerals are dissolved from the rainwater hitting the limestone and other rocks and feeding into the tributaries, they're constantly being flushed out. It's like flushing over and over constantly. So the salinity never builds up in the rivers and the streams but instead gets dumped into the ocean which is a giant reservoir for it. Now what happens is the ions get deposited into the ocean over time. They do bond with the clays in the bottom of the ocean floor and get taken out of the ocean water and then the geologic processes folding the ocean crust underneath over time does slowly remove the salinity from the ocean. So it reaches kind of a steady state where it is today about 3 1/2%. Now the main salt is regular sodium chloride. That's just table salt. But there are others like calcium bicarbonate that gets dissolved when the rain water hits limestone, dissolves those ions and takes them into the rivers into the ocean. So ocean salinity comes from land. Learn anything at math and science.com.