AP Physics 1 & 2 Light As a Wave

Hi everyone and welcome back to Educator.com.0000

Todays lesson is going to be on light as a wave, so we are going to start transitioning from waves into a specific type of wave -- electromagnetic waves or light and optics.0003

Our objectives are going to be to recognize that light is an electromagnetic wave and shares characteristics with the entire electromagnetic spectrum and explain the concept of polarization and how polarization can be used in applications, such as video displays and low reflection sunglasses.0014

Let us start by talking about EM waves or electromagnetic waves.0031

EM waves do not require a medium in which to propagate; they can move through a vacuum.0035

Light is an electromagnetic wave, which is visible to the human eye, but there are lots of other types of electromagnetic waves that we cannot see as well.0041

The speed of all electromagnetic waves in a vacuum is approximately 3 × 10⁸ m/s, probably the most important constant in this entire course.0049

Now electromagnetic waves are transverse waves due to the modulation -- the variation and the electromagnetic field's perpendicular to the wave velocity.0060

I have a diagram down here, so if we have the wave traveling with a velocity to the right, the electric field of the EM wave is going to vary in a plane perpendicular to that velocity and the magnetic field is going to vibrate or oscillate.0069

It is going to modulate in a direction perpendicular to both the electric field and to the wave velocity.0084

That is what makes it a transverse wave, even though there is no medium required.0091

Let us talk about characteristics of electromagnetic waves and the electromagnetic spectrum.0097

The product of an electromagnetic waves frequency in wave length must be constant in a vacuum.0103

If V = F(λ) and in a vacuum, V = C or 3 × 10⁸, frequency times wavelength must be constant.0108

Higher frequencies then, must have shorter wavelengths and lower frequencies must have longer or bigger wavelengths.0117

The relationship between frequency and wavelength for various types of electromagnetic waves is shown in the electromagnetic spectrum.0124

I have a diagram here, although you can find tons of different diagrams of the electromagnetic spectrum.0130

Here is just one version of it -- as we look, light is just this little bit here in the middle.0135

Now it has expanded out so you can see the different colors here, but all of these other waves are electromagnetic waves.0142

Now we start over here on the left and these are the high energy waves; they have the highest energy and they also have the highest frequency.0149

The energy of an electromagnetic wave is related to its frequency -- more frequency, more energy -- so if things like gamma rays have a ton of energy, they have a very high frequency and therefore a very short wavelength.0158

So we move to the right along the scales and we hit x-rays -- not quite as much energy as gamma rays, but still pretty energetic -- a little bit lower frequency, a little bit longer wavelength, but still pretty short wavelength.0171

We get into UV, ultraviolet radiation.0182

We get into the visible spectrum, which starts ultraviolent, near violet and has more energy and as we go the right we get to the lower and lower frequencies -- the longer wavelengths and less energy.0188

Infrared, microwaves, radio waves, of which TV, FM, and AM are different types and long radio waves and so on.0201

A very, very wide variety of types of electromagnetic radiation.0210

Now when we talk about EM waves, let us talk for a minute about polarization.0217

The direction of the electromagnetic wave -- electric field and magnetic field modulation varies.0221

What is called un-polarized electromagnetic waves exhibit modulation in all directions.0227

Here in this diagram, imagine the wave is coming toward you out of the screen.0231

If that is the case, the electric field could be vibrating this way; it could be vibrating horizontally or anything in between.0236

That is called un-polarized light.0242

Polarized light, however, consists of light where we have the oscillations -- the modulations of the electric and magnetic fields in a single direction only.0246

Here, think of this as the electric field vibrating in one direction and we are simplifying this a little bit just until we get the concept across; there is certainly some more complexity to it as we get into more depth.0254

What does that allow us to do? We could talk about things like polarizers.0267

Polarizers are materials which act like filters to allow only specific polarizations of light to pass.0273

You could almost think of it as like a picket fence.0278

If you have all these different polarizations of light at different angles coming toward you -- if you had a picket fence where all of the fence rows are lined up, only certain polarizations that line up with the polarization of your picket fence are going to make it through.0280

You are blocking the light that does not fit the polarization of what we will call your polarizer, your picket fence.0296

So you have taken un-polarized light and only let one polarization of light get through, so you have now made polarized light.0301

Polarizers are typically sheets of material in which you have a bunch of long molecules lined up, kind of like a picket fence.0310

Of course they have to be a lot smaller because the wavelength of light is so much smaller.0316

But what can you do with these? Tons of cool things.0320

Sunglasses -- if you have heard of polarizing sunglasses, they are often sold with a polarizing filter to reduce reflections.0325

Well, how does that work?0333

When light reflects off of non-metallic surfaces, it is partially polarized parallel to the surface, such as things off of water or off of the street on a sunny day.0334

That is going to come back partially polarized to that surface, so your sunglasses have a polarizing filter where your picket fence of the molecules in there is kind of like this.0345

So the reflected rays that are all coming back parallel to the surface they reflected off of are blocked; you have cut down on the reflections that make it through the sunglasses into your eyes.0354

That is what those polarizing filters do.0364

We could also talk about liquid crystal displays and they are everywhere these days.0368

The liquid crystals are held in a suspension and they align themselves in a specific orientation when you apply a voltage, so what you are doing is you are shifting polarization.0372

As the liquid crystals align, they take on a specific polarizing orientation and if you cross-orient one polarizer in the matrix of liquid crystals...0382

...if you turn that polarizer on and off, you can either allow light through or block all light -- light getting through, light getting turned off.0390

And that you can do on a pixel by pixel basis to allow light through or not.0399

That is how you get your screens on an LCD display.0403

If you want to have some fun, find a polarizer, or a pair of polarizing sunglasses and go hold it in front of an LCD screen -- LCD TV, computer monitor, or even a liquid crystal watch.0406

Go try that and as you hold the sunglasses in front of it, start turning it and turn it the other way.0417

You will see that the light getting through is modulated considerably as you go from very dark to very light as you turn things.0423

Even the displays in cars that are LCDs are typically oriented at 45 degrees so that even when you have your sunglasses on, however you are looking at it, some of the light is getting through.0429

Try that with your car's display; take the polarizing sunglasses and turn them in front of it and watch what happens to the light coming through from that display.0439

Let us talk about an example with the color of light.0451

What color of light has a wavelength of 5 × 10^-7 m in air?0454

Well first thing we need to know is that frequency determines the type of EM waves -- that is our key.0460

Frequency determines type and energy of the wave.0469

Although we could look up 500 nanometers here, it is going to be a lot more accurate if we start with frequency, so let us find the frequency of that wave.0472

If V = F(λ), our wave equation, and we know that frequency must equal V/λ (wavelength) and the velocity of light in air -- similar to light in vacuum, which is almost the exact same -- is 3 × 10⁸.0479

So frequency is 3 × 10⁸ m/s divided by our wavelength, 5 × 10^-7 m in air is going to give us a frequency of about 6 × 10¹⁴ Hz.0494

Where does that fall in our scale? Right around there -- green light is starting to get a little bit toward the bluish end.0510

So what color is it? Green -- 6 × 10¹⁴ Hz, gives us green light.0520

Let us analyze this EM wave.0529

A 1.5 micron long segment of an electromagnetic wave having a frequency of 6 × 10¹⁴ Hz -- that sounds familiar as we just did 6 × 10¹⁴ Hz as represented below.0532

First off, mark two points on the wave that are in phase with each other and label them with the letter (P).0544

Well, in phase is the same point on consecutive wave, so let us pick a point maybe right there (P) and on the next wave, another wave there, 2 points in phase with each other -- easy.0550

Which type of electromagnetic wave does the segment in the diagram represent?0565

What type of electromagnetic wave -- the frequency up here of 6 × 10¹⁴ Hz -- we already said that must be green from the previous problem. Excellent.0570

Moving on. A television remote control is used to direct pulses of electromagnetic radiation to a receiver on a television.0583

This communication from the remote control to the television illustrated that electromagnetic radiation is a longitudinal wave? No, that is not right. EM waves are transverse.0592

It possesses energy inversely proportional to its frequency? Now that is not true either.0605

As frequency increases, so does energy, so that is a direct relationship, not an inverse.0610

It diffracts and accelerates in air -- now that is just silly.0616

Finally, it transfers energy without transferring mass? Yes, there is no mass moving, but we are transferring energy.0621

That is what is really cool about electromagnetic waves.0627

Let us take a look here. A microwave and an x-ray are traveling in a vacuum.0633

Compared to the wavelength and period of the microwave, the x-ray has a wavelength that is...?0637

Well, let us see. If we go back to our electromagnetic spectrum or even think about energy, the x-ray had a higher frequency; it had more energy.0642

So the frequency of the x-ray was higher and we know that if its frequency is higher, then its period is 1 over frequency, so that has to be shorter.0651

So frequency is higher, the period is shorter, and if V = F(λ) -- V is constant and if (F) is higher, then wavelength must be smaller, so we must have a wavelength that is shorter and a period that is shorter. Answer D.0663

Hopefully that gets you a good start on light as a wave.0683

We will move further ahead in our next lessons as we talk about reflection, refraction, and lenses and mirrors.0686

Thanks so much for your time everyone. Make it a great day!0692