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Electromagnetic Spectrum 34,317 views

Teacher/Instructor Jonathan Osbourne
Jonathan Osbourne

PhD., University of Maryland
Published author

Jonathan is a published author and recently completed a book on physics and applied mathematics.

The electromagnetic spectrum depicts different frequencies of electromagnetic radiations shown from blue to red. On the electromagnetic spectrum, wavelength is shown to be increasing to the right and frequency is shown to be increasing to the left. The radiations in the order from blue to red on the electromagnetic spectrum are gamma, X-ray, UV, visible, IR, microwaves and radio waves with gamma being the smallest and radio waves being the biggest.

So let's talk about the electromagnetic spectrum, the big thing about the electromagnetic spectrum is trying to distinguish the differences between all the different wavelengths that are possible for electromagnetic waves, we can have lengths as small as 100 the size of an atomic nuclears and as big as the whole solar system or even bigger. And these are not going to behave the same way, so all these different characteristics give us the need to write down a spectrum like this. So we have wavelength increasing to the right frequency increasing to the left. Remember that we always have to have wavelength times frequency equals the speed of light which is a constant. Now we're going to be looking at this in terms of waves propagating in the vacuum now it's true light waves can also propagate through glass, through air, through water, through all kinds of things. Now when they do that their frequency will stay the same but their wavelengths will change.

Despite that fact it's customary to talk in terms of wavelength so we'll be doing that today but just remember that if you're looking at this in water the wavelengths are going to be different. Okay so the smallest wavelength electromagnetic waves are called Gamma rays, these gamma rays come as the result of nuclear reactions. They're rare but very, very dangerous. They'll come out of for example a hydrogen bomb that goes off there'll be gamma radiation that comes out and you don't want to be standing there. X-rays of course also come out of nuclear decays but they're not quite as dangerous. We use x-rays of course to take pictures of our bones. The reason why that works is that x-rays will penetrate through our skin but they'll be stopped by our bones so when we try to expose something the rays will all go through our skin they won't go through the bones and so that's what allows us to take a picture of the bones. Alright then we get into ultraviolet light ultraviolet light is what leads to sunburns when it comes from the sun thankfully most of that is stopped by the o-zone, that's because o-zone O3 will absorb these ultraviolet rays and turn into the more innocuous oxygen O2.

Alright then we get into visible light, visible light of course is associated with many different colors running the whole rainbow pretty much by definitions the smallest frequencies are the violets and the blues and then we run on through to the I'm sorry the smallest wavelengths so the blues the largest frequencies or the blues and then we run on into the largest wavelengths in the reds. So that's visible and of course we get that from the sun then we go into infrared. Infrared radiation is really something that you think about when you think about temperature thermo radiation. Where all our little light bulbs and the infrared basically because we've got temperature. Then we get into microwaves which are larger wavelengths and then we go up into the radio waves. Radio waves are useful because the atmosphere does not absorb them like it does microwaves and infrareds. Radio waves we can use to transmit information like radio signals, alright do let's look at this a little bit more specifically.

I've got them all written down here and so we'll just go through them again, gamma rays are the smallest wavelengths, highest frequency very, very dangerous basically if you got gamma rays going in there're just going to kick off electrons off of your DNA molecules and mess everything up that's where radiation sickness comes from partly. So that's gamma radiation, wavelengths less than about 0.1 nanometers. So these are wavelengths that are smaller than an atom very, very small wavelength, very, very high frequency. Alright what about x-rays, x-rays also come from radiation but these are usually secondary radiation they don't come directly from the nuclears. Their wavelengths are around the size of an atom from about 0.1 nanometers to about 50 nanometers, they're also dangerous but not quite as dangerous. These were very important because they're what led to the discovery of atomic radiation in the 1890s they also led to a lot of very, very important discoveries about the atom around the same time.

Then we've got ultraviolet radiation, ultraviolet radiation comes from fluorescents so we can get mercury for example to radiate in the ultraviolet. It's also what gives us sunburn, it's also these interesting little stamps that you can put on to the ultraviolet and they can tell that you've paid your way in whatever. So these guys have wavelengths that are just shorter than the visible that's why it's called ultraviolet because it's got shorter wavelengths than violet light which is the shortest visible. So we've got 15 nanometers to about 400 nanometers because that's violet light. Alright then we go into visible light, visible light comes from the sun. The interesting thing about visible light is that it's a little window in which the atmosphere does not absorb. So the atmosphere absorbs ultraviolet and it absorbs infrared fairly strongly but in the middle there we've got this little non-absorption window and that's this visible light and that's pretty much the reason why life developed on this planet able to see light in that little window because it comes in so it was useful for us to evolve in such a way that we could see it.

Okay visible light of course is represented by the rainbow given by ROYGBIV red, orange, yellow, green, blue, indigo, violet. Red light is around 700 nanometers, violet light is around 400 nanometers again this is in vacuum they have different wavelengths in water but it'll have the same frequency. So we're from 400 to 700 nanometers, alright then we go to infrared. Infrared is thermo radiation basically anything with a temperature ranging from around I don't know 100 Kelvin to around room temperature or a bit higher than that will be like a light bulb in the infrared. So if we, and that's how infrared goggles work. We can look at things thermal imaging. So we look and we can see the infrared radiation coming through the walls of a house and we can see anything moving around inside that has a temperature around room temperature higher our body temperature, we're all little flash lights in this infrared. And so this is from about 700 nanometers to about 500 micrometers which is about a half of a millimeter. Alright then we get into microwaves, microwaves are released in chemical processes and we also use them in microwave ovens. The reason why we use radiation with this wavelength in a microwave oven is because it naturally excites water molecules into moving around and gaining energy. So they absorb that energy resonantly, so that means that they'll absorb almost all the energy that comes in and we tune it to what water will absorb because essentially anything we're trying to heat up in a microwave oven is mainly water.

Alright these guys have wavelengths that range between about 500 micrometers to about 50 centimeters and again these transition regions are strict it's just kind of about. Alright so those are microwaves and then we have radio waves and these are the longest wavelength waves. We use these mainly to transmit information, we'll do it through radio waves which are the largest wavelengths that we'll use maybe around 3 meters in wavelength and these also are not absorbed strongly by the atmosphere so that means that we can send radio waves through clouds and through the atmosphere without really worrying too much about power being absorbed. So that's radio waves, then we go down to television waves and the cell phone waves those guys have a higher frequency, shorter wavelength maybe down to about 50 centimeters or even less. And these guys are actually generated by those radio towers which are just big long metal rods where we just ask the electrons to go back and forth like this. Alright and the physical size of those metal rods coincides about with the wavelength sometimes there's a factor of 4 or so just depends on how we're going to generate the waves. But anyway that's the electromagnetic spectrum; radio waves will get to sizes, the size of the earth now we can't measure those waves but they certainly exist and they're radio waves even larger than that anyway that's the electromagnetic spectrum.