Intro to Electromagnetic Waves

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In this last MCAT Physics 1 lecture, we will discuss light and optics. Now, light is composed of electromagnetic waves. When we draw an electromagnetic wave, we can see that it has both an electric component and a magnetic component and these are 90 degrees from each other. So as the electric component moves, it creates a magnetic wave that is 90 degrees from it.

Now the velocity of an electromagnetic wave is given by c = f * lambda. Where f is frequency and lambda is wavelength. Now, c is a constant, 3.0 times 10 to the eighth m/s. This is one of the few constants you wanna have memorized for the MCAT. Now a velocity of light in any other substance besides the vacuum is given by v = f * lambda, where v just indicates that the speed is slower than the speed of light in a vacuum.

Now, the electromagnetic spectrum is just a way to display all of the possible wavelengths of electromagnetic radiation starting with the shortest, going to the longest. So if we draw it, we would have down at this end where the wavelength is very short, things like gamma rays and X-rays. And then, around here, we would have our visible spectrum and all the way at the other end we get things like radio waves, television waves and so on, and so forth.

Now, which has a higher frequency, violet or red? If c = f * wavelength, a high frequency means a short wavelength. So this is the same thing as asking which has a shorter wavelength. Now, violet has a wavelength of about 400 nm. And red has a wavelength of 700 nm, which means that violet has the higher frequency.

If we are going to write the visible spectrum on the electromagnetic spectrum, we would then put violet with the shortest wavelength and go up V, indigo, blue, green, yellow, orange, red or ROYGBIV backwards. Now energy for electromagnetic radiation is given by E equals hf, where h is Planck's constant and f is the frequency.

So if the frequency goes up, the energy goes up, which means that violet has more energy than red. Now just to the left of violet we have our ultraviolet, and just to the right of red we have infrared. Since ultraviolet has a even higher frequency than violate, it's more energetic.

Consequently, you where sunblock to stop UV rays. We don't really worry about IR rays as much, because the UV rays have more energy. High frequency means high energy. Dispersion is a way to separate an electromagnetic wave into its various components that have different frequencies, because these different components will see a different effective index of refraction.

So if we took white light and we shot it at this prism, it would break into the various colors of the visible spectrum that we can see. Diffraction is often thought of as light bending, because when light hits something like a slit, it appears to bend. Now, if we have two slits and we do two light rays passing through here, the same thing happens where the rays will appear to bend.

But what happens is we can see interference. Now in interference, we have constructive and destructive interference. Now if we go along the points where the waves add together, that's constructive. If two waves are negative, or out of phase, they'll create destructive interference. So constructive interference, one wave plus another wave that are in phase add together to give a much bigger wave.

In destructive interference, one wave plus another wave that's out of phase add destructively and essentially neutralize the wave. When we have these two slits, we get both constructive and destructive interference where along the arrows we get constructive interference. And in between them we have destructive interference. Polarization refers to the orientation of the wave's oscillations.

And we can polarize a wave by having a series of light rays pass through a narrow slit and only a certain orientation will actually pass through. So if we had our slit here, and we had a mix of rays, only those with an orientation that's parallel to the slit would pass through. Things that are perpendicular, or some angle in between, would get blocked. And if we added a second polarizing field, and this one was at 90 degrees to the first, we can block out all light using two polarized films.

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