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Lecture Comments (2)

1 answer

Last reply by: Professor Selhorst-Jones
Thu Dec 3, 2015 10:05 PM

Post by Gurwinder Chana on December 3, 2015

You never talked about color which quite important, or you think that information is enough for me in light unit


  • Light is extremely fast. In a vacuum, light travels at
    c=299   792   458   m

    ≈ 3·108   m

  • Depending on the material light is traveling through, its speed will change. Light travels through air close enough to c that we can use that value when working on problems.
  • Since light moves so fast, we can talk about very large distances using its speed as a reference point:
    Lightyear = c ·(1 year).
  • Normally waves need a medium to propagate through. This is not true of light, though. Light is able to travel without any medium (which is why the light of the sun can reach us, even though it's a wave).
  • The word "light" is sort of a misnomer. The visible spectrum we are used to seeing is only part of the much larger electromagnetic spectrum that makes up light.
  • Different kinds of electromagnetic waves have different frequencies. Higher frequencies carry more energy, lower frequencies carry less.


Lecture Slides are screen-captured images of important points in the lecture. Students can download and print out these lecture slide images to do practice problems as well as take notes while watching the lecture.

  • Intro 0:00
  • The Speed of Light 0:31
    • Speed of Light in a Vacuum
    • Unique Properties of Light
  • Lightspeed! 3:24
    • Lightyear
  • Medium 4:34
    • Light & Medium
  • Electromagnetic Spectrum 5:49
    • Electromagnetic Spectrum Overview
  • Electromagnetic Wave Classifications 7:05
    • Long Radio Waves & Radio Waves
    • Microwave
    • Infrared and Visible Spectrum
    • Ultraviolet, X-rays, and Gamma Rays
  • So Much Left to Explore 11:07
    • So Much Left to Explore
  • Example 1: How Much Distance is in a Light-year? 13:16
  • Example 2: Electromagnetic Wave 16:50
  • Example 3: Radio Station & Wavelength 17:55

Transcription: Light

Hi, welcome back to Today we’re going to be talking about light.0000

Light is a complex phenomenon that is extremely important to a huge variety of aspects in physics and we’re not going to have enough time to fully explore it in this course.0005

Instead we’re going to be able to limit our discussion of light to ideas we can easily pull from understanding waves.0014

That’s still going to give us a much better understanding of what’s going on with light.0019

Keep in mind there is a lot of stuff to talk about in light.0023

We’ll talk about a little bit of that at the end of this. We’re still going to get some idea of what’s going on here.0027

First off, light is fast, really, really fast. In a vacuum, light travels at a rate of c equals 299,792,458 meters per second.0033

Which is approximately to equal to 3x10^8 meters per second and for the most part, all the problems that we’re going to be working on and all the problems you’ll ever have to work on, unless you get into serious, serious theoretical physics.0048

You’re going to be enough with 3x10^8 meter per second, that’s pretty much what all physicists wind up memorizing.0060

So 3x10^8 meters per second equals c, the speed of light. This value is so important it’s given its own constant, once again that’s c.0067

Speed of light, 3x10^8 meters per second and that is fast.0074

Light also has this unique property that stationary and moving observers all measure the same speed for light.0081

That’s not true for all other types of waves. Consider if we had a wave on the ocean moving at 3 meters per second this way.0088

If we were in a boat that was going at 2 meters per second, we’d wind up seeing that wave at only moving at 1 meters per second, right?0097

From our point of view, if we can’t see any stationary objects around us, we’re moving because we can’t see that fact that we’re moving if we aren’t experiencing acceleration.0105

We don’t have any reference points. So if we’re in the middle of the sea and we’re moving at a certain speed and we look down at the waves in the water underneath us.0115

It’s going to seem like its moving depending on our speed. But if on the other hand, we were going in the opposite direction.0123

We’d wind up seeing it moving at 5 meters per second, right? So the experience would be 1 meter per seconds if we were going with it, 5 meters per second if we were going against it.0132

If we’re sitting still in the water, it’d be 3 meters per second.0141

For light, it has the phenomenon that for every direction you’re moving, you record the exact same speed.0144

That is fantastically interesting. That’s so different from all other types of waves, it’s just…it’s really interesting, really important.0151

Also what we’re going to wind up…what we’re not where going to wind up looking at, but one of the cornerstones of relativity is this fact.0160

This doesn’t mean that light travels through all materials at equals rates though.0170

In a vacuum, light travels at c and it travels very close to sea through air, like we talked about before, c for a vacuum.0174

In water, it travels at 0.75 c. Through glass, 0.67 c. Through diamond at 0.4 c.0180

Still really, really fast but it does mean when its moving through another material, it winds up being slowed down.0188

It’s able to move at its full speed in a perfect vacuum, but as it winds up having to encounter other objects to pass through, it’s not able to keep up its speed quite as much.0195

Light speed. Since light moves so fast and it’s so important to the nature of the universe, occasionally we talk distance using the speed of light.0205

If we’re talking about astronomical stuff, it’s a really great thing to be able to talk about, because we’re talking about really, really large distances in space.0212

So being able to talk about how far light manages to travel in year, we’re able to talk the speed of light times the time of one year.0219

Multiply it by the time of one year, we’re able to talk about a light year.0226

A light year is a measurement of distance because it’s how far light could travel in on year.0229

For large, large distances in space being able to talk about the distance in terms of light years is a really handy thing to be able to talk about.0236

It also tells us how long it would take for information from that galaxy to make it to us, because of its sun, its sun for example goes supernova and explodes but its 50 light years away from us.0243

We’re not going to be able to get the information of it exploding, the light of its explosion isn’t going to get to us until 50 years after it explodes.0255

Some of the information we’re getting, some of the stars we see in the sky could actually wind up being dead stars, we just don’t know they’re dead yet because we haven’t seen the information of their explosion come to us yet or their nullification in some way.0261

Medium. Normally a wave needs a medium to propagate through. If we have waves and water, we need the water to be able to move up and down.0275

If we have a wave in a string, we need the string to whip up and down.0283

Me speaking needs this air for it to be able to bounce against and have those differentials.0286

Light on the other hand needs no such medium. Light is able to move through a vacuum without issue.0290

Nobody else can do that. If there’s nothing there, other waves can’t transmit themselves because they don’t have anything to vibrate against, they don’t have anything to move.0296

Light is in itself its own motion. It’s its own medium. We’re starting to get into a complicated thing here about the dual particle wave nature of light, so we’re not going to talk too much about this.0306

But light suffices to say doesn’t need a medium, it’s able to just go on its own.0319

Unlike all the other kinds of waves that we learned about. Now notice the speed can still be affected by what it travels through.0324

Remember if you’re traveling through diamond, you go slower. You travel through water, you go slower.0329

It’s not the same as that material being what’s transmitting the wave.0333

It’s traveling through the water doesn’t mean that the water’s motion is what moves the wave along.0337

The wave is able to go through the water but then also hop out to going through just pure raw vacuum.0342

It’s a really big difference.0346

Electromagnetic spectrum. Light is kind of a misnomer and me using the word light; we want to expand that to more than just the light.0350

Just the light that you’re seeing me with. There is way more quote on quote light than just the light that you’re seeing me with.0358

The light that we see, visual light is only one of many forms of light.0364

Light is really just part of the electromagnetic spectrum.0369

The EM’s spectrum is a really wide set of possibilities. It’s going to go over a huge amount and we’ll talk about some of those.0373

Visible light, the light that we see, is just a small fraction of the EM spectrum.0380

There is many more possibilities and it’s those other possibilities that allow us to transmit other kinds of information.0385

We’ll talk about that in a little bit. Any electromagnetic wave, they share many similarities for every electromagnetic wave.0390

Such as their speed and their ability to move without a medium. Different frequencies and wave lengths give some different properties though, such as the amount of energy that the wave carries.0397

A higher frequency has more energy because that means that it’s vibrating more.0404

If we had a string for example and I whipped it up and down only once a second, I’d have way more energy in that string if I was whipping it up and down a hundred times per second right?0408

There’d be more energy being put into it and a similar idea is happening with waves.0416

More energy is in the higher frequencies because they’re vibrating more.0420

The different classifications for electromagnetic waves.0427

So the long radio wave has a frequency of anywhere from 10^0, so just 1 hertz to 10^5 hertz.0430

Radio waves, the kind of stuff we use to send television or cellphones or…cellphones actually start to verge into microwaves depending…well anyway, there’s a lot of different possibilities in different sections of the spectrum.0439

Radio waves, we use to send radio as you might guess. We used to send, I believe radar, I’m not actually sure about that, don’t hold me to it.0455

We definitely use to send television. A lot of information gets transmitted from place to place.0464

Because by the way that we move the wave by taking slight variations off that frequency we can send that light, quote on quote light, we can send those electronic, those electromagnetic waves through a space.0469

Say from the top of a mountain with a transmitter to a city that has a radio in it.0482

We’re able to put out a certain kind of electromagnetic spectrum and by vibrating slightly different than the expected frequency, that radio on the other end is able to pick up those slightly different vibrations and turn that into some sort of information.0486

Like say sound information that it then puts out through a speaker.0502

Those slight variations off of the starting base level put out information, right?0505

Microwave, the same sort of thing that you in a microwave is 10^9 to 10^11 hertz.0510

The reason why that works even though they’re lower energy waves than the visual spectrum.0517

We’re able to have it putting out energy because it’s able to sync to the appropriate frequency that water and sugars and many kinds of fats, that they’re on so that it’s able to vibrate them and cause them to pick it up and have a resonate frequency as we talked about before.0521

And able to increase the vibrations in those atoms, thus increasing the energy, the heat in it.0537

Infrared, 10^12, 10^14. Visual spectrum, see how small it is, it’s just from 4x10^14 to 7.9x10^14 hertz.0542

ROYGBIV. Red, orange, yellow, green, blue, indigo, violet.0551

Haven’t put down the specific times, if you want to know more, easy to look up.0557

As it cycles up, we go from red up to violet as we go through higher and higher frequencies.0560

That’s why it’s called infrared, below red and for the next energy above it, ultraviolet, above violet.0567

More than that we get ultraviolet 10^15, 10^16.0574

X-rays, even more and finally gamma rays covers the really extreme high, high, high frequency. Really high energy things.0579

Now the reason that ultraviolet rays, x-rays, gamma rays, though you’re really unlikely to be exposed to those.0585

The reason why these can be potentially damaging to us is there is such high energy things and they have such a small wave length.0594

They’re able to pierce through our body and they’re able to hit a cell and they’ve got enough energy in them to be able to potentially cut off a piece of DNA.0598

Cut DNA in a certain way. Potentially, if you’re really unlikely, that DNA will be cut in just the wrong way, and this happening to you lots of times.0607

You get exposed to a lot of ultraviolet light, suddenly you’re exposed to more possibility of skin cancer.0615

You get exposed x-rays, suddenly you’re exposed to the possibility of maybe getting some kind of cancer inside of yourself.0620

There is a lot of other things that cause other carcinogens out there, but this one possible way to get cancer is because DNA winds up getting split.0625

So suddenly the code that tells the cell how to behave, how to multiply, goes haywire and most of the time the cell fails to work at all and just dies.0633

Sometimes it manages to get cut in the quote on quote right way, not right at all, very bad from our point of view.0643

Cut in just the wrong way and suddenly it goes haywire, goes out of control and it starts to produce many, many of itself and we get a cancer.0649

It does something bad to the body. The reason why that’s possible is because we’ve got such high energy in them that they’re able to actually effect ourselves and they’re able to potentially do things to our DNA.0656

Now there’s huge amounts left to explore. We could talk more, I mean that idea that I was just talking about; x-rays and ultraviolet and gamma rays being able to cause cancer damage to cells.0668

So much we could talk about just in that tiny little idea, but there is way more things here.0679

We’ve just begun to talk about the tip of the iceberg in terms of light.0682

There is so much more. We could fill many college physics’ courses and then more onto a lifetime of research after this.0686

Huge amount of things. Some of the topics you might one day encounter if you’re interested in light and there is lots to be interested in about.0693

Just go ahead, search for yourself. You find out all sorts cool ideas or go ahead and take more courses.0698

This is…I mean there’s all sorts of cool things in physics and this is one of them.0703

First off, optics, how light behaves with various materials.0705

The way light is going to move through them, pass through them. Change, be reflected.0709

The energy in light, we started to talk about there is more energy in light and less energy in different things but we didn’t get into any specific numbers.0713

Way more to be talked about there. The fact that energy in light, not energy in light.0720

Light can both be treated simultaneous as wave and as partially.0725

You’ve probably heard of photons, that’s a single packet of light.0728

Light behaves, light has some of the effects of a wave. It has those interference effects that we’ve talked about previously with waves.0732

The same time it also can be broken down into discretized quantities of single chunks.0739

It’s got a really strange thing going on there. We don’t normally thing of a wave as something that can broken up into a single piece.0743

Light able to do both at once. Once again, really unique phenomenon.0749

Finally, all of relativity. Everything in relativity is based on this fact that light is this unique thing and that light is top speed limit of the universe.0753

And why that is a whole kettle of fish to get into. There is huge amounts of stuff here.0765

Light is interesting. Still at least we’ve managed to crack up an amazing new vista to be interested in and we’ve dipped our toes.0769

We’ve got some new ideas and it helped us understand our universe just a little bit more and it’s a really cool thing.0777

I’d really encourage you, go ahead, just do some research. Get yourself exposed to a whole bunch of ideas.0783

You’ll get the chance if you want to, to start taking more courses or just do some personal reading and you can learn a lot about what’s going on in the world and the universe.0788

I mean everything at once. Alright, ready for some examples.0794

How much distance is in a light year?0798

We start off, we know the speed of light, right? C is equal to 3x10^8 meters per second.0800

We’ve got that down. If we want to know what distance is covered, we need to know how much time is in a year.0805

If time is equal to 1 year. Well 1 year is equal to 365.25 days. We’ve got that leap year every four years.0811

We’ve got a quarter of a year in there, so 365.25 days.0820

If we want to know how many hours are in there, 365.25 times 24 hours in a day.0826

We get that we’ve got 8,766 hours in a year.0839

Which means that we can multiply that by 60 and we’ll get how many minutes are in a year, 525,960 minutes.0846

Which we could turn into seconds. We can multiply that number by another 60 seconds.0863

We’re going to get 3.156 x 10^7 seconds.0872

If we want to know what the distance that it covers, we just put the two together.0879

Distance equals velocity times time. So the velocity of our thing is 3 x 10^8 and we multiply that by the time that it has to travel, 3.156 x 10^7 seconds.0882

We get a distance of 9.467 x 10^15 meters. That is a huge, huge distance.0894

10^6 means that we’re dealing…so 10^3 means we’re dealing with thousands.0908

10^6 means we’re dealing with millions. 10^9 means that we’re dealing with billions.0913

10^12 means that we’re dealing with trillions. 10^16 means that we’re dealing with quadrillions.0918

That’s almost 1 quadrillion meters. That’s a massive amount of distance, this is just a huge amount of distance that we’re able to cover.0925

Now keep in mind that the closest star system to us, Alpha Centauri is approximately 4.3 light years away.0933

We’re dealing with an absolutely massive amount of distance between us and that other system.0942

If we want to get there in any reasonable amount of time, we’re going to have to figure out some way to get a close approximation to the speed of light.0948

Once again to relativity, you start to realize that getting into reasonable amounts of speed like that, really, really difficult and there is even more complex stuff going on there.0954

Just even moving at 1/10th the speed of light, think about how much energy you’d have to put into that.0964

Moving at 1/10th the speed of light, 3 x 10^7. If we’re dealing classical mechanics, that would be 3 x 10^7, the whole thing squared times ½ times the mass of the object.0970

½ mv squared. Huge, huge, huge amount of energy.0983

To be able to get any sort of space ship to any other solar system is going to require some incredible feat of engineering or some incredible feat of scientific process for us to be able to cover these huge distances.0989

Right now we’re basically in the solar system for at least a while longer.1000

Being able to actually touch the other stars is going to take something really, really cool and really, really smart from humanity.1004

Example two. If we perceive an electromagnet wave at the color green it’s going to have something…so we perceive a 525 nanometer wave if it’s moving through the air at the color green.1011

What frequency is that? Remember electromagnetic waves very near to sea and air.1023

Remember, we know that the speed of a wave to equal to its frequency times its wave length.1027

If we’ve got the speed of the wave at 3 x 10^8 and we want to know what the frequency is.1035

Well we know the wave length, 525 nanometers. So 525 x 10^-9 meters.1043

Now we can easily just solve for frequency. So frequency is equal to 3 x 10^8 / 525 x 10^-9 which is equal to 5.71 x 10^14 hertz.1050

Smack dap in the middle of that ROYGBIV spectrum, right in where G is going to be.1065

That’s what the frequency that we’d wind up getting out of that wave.1071

Now say you want to tune into my favorite radio station, KSBC at 88.7 megahertz.1076

If KSBC is at 88.7 megahertz, which it is, what wave length does that mean you’d have to be scanning for?1081

If you’re scanning for KSBC at 88.7 megahertz what wave length would that mean that we’re looking to be able to pick up?1089

Well once again we use the exact same thing just slightly different.1097

Frequency times wave length. Well the velocity we’re dealing with electromagnetic waves is 3 x 10^8 equals whatever frequency it is, so in this case 88.7 megahertz.1100

88.7 x 10^6 hertz because it’s mega. Times the wave length, so the wave length is going to be equal to 88.7…oh whoops, sorry, put that the wrong way on.1111

3 x 10^8 / 88.7 x 10^6. Which means that we’re going to be looking for a wave length that’s 3.38 meters.1125

That’s pretty big. It’s really interesting to compare how much difference there was between the wave length of that green light.1138

Tiny, tiny thing. 525 nanometers to 3.38 meters. That’s practically two of me standing on my shoulders.1145

That’s a really tall wave length. That means if you’re going to want to pick it up, you’re got to have some way of being able to see all that information in that really long wave length passing by you.1152

Which has to do with the way waves work, but we once again aren’t going to quite get into that.1160

3.38 meters, really long wave length because it’s got a really small frequency compared to some of the other ones.1165

Alright, hopefully that gives you some idea of how light works and possibly spark your interest in some of the many, many interesting…1170