Now waves are a very special thing and what i'm saying now is probably easiest to imagine to to visualize um if you're thinking of waves that you can make in water you can drop a pebble into water you can put uh piece of wood into water and make waves in a controlled way waves don't move in straight lines they're not just a line a beam a straight connection they move in wave fronts and in particular at each and every point of such a wave front we have a new wave originating so if we looked at the wave in water and we'd be looking at each and every point there we would see that new circular waves start there and altogether they make up that wave front like a wave coming into a beach for example now when we put obstacles in the way like little barriers little walls and we leave one or two open holes in them we can see the effect of this wave waves starting from each and every point because behind these little barrier openings we will not just see a wave in a straight line we will see it spreading out to the sides it will sort of go around corners in a way if it was a straight line it would not be able to reach a place that's hidden from it but since it is a wave it can do that it can sort of go around things all waves do that but it scales and that's sort of easy to see when you do the water experiment it scales with the wavelength the longer the wavelength the more it can sort of go to the side and fill the dark spaces so to say um where it actually wouldn't be able to reach if it was just a straight line so everything scales with wavelength and that's the reason why in light we can't see this effect very strongly because light has just a wavelength which is tiny tiny tiny we cannot our eyes can typically not see this if we could see the waves of wi-fi we would see them doing exactly this quite strongly because the wavelength i mentioned it 12 centimeter it's a lot lot bigger than that of visible light so that means waves do strange things radio propagation does strange things the easiest to understand is probably absorption radio waves get absorbed in matter in material and the two materials that are strongest in interacting that are most absorbing those are metal and water water in all its forms fog rain also the water in the human body if you put a human body in the directly in the way of a radio wave in front of your access point for example you can measure that difference you can measure the absorption in the human body because the human body is mostly water 70 80 percent of us human beings is water and then you have materials that are sort of in between walls wood it depends a lot on how humid is it how much water is in that material and i'm not giving any precise values here because end of the day you'll have to find out case by case as a general rule just water and metal will always be in the way of radio waves then the second effect diffraction is what we talked about earlier this going around corners in the picture on the right you can see a mountain top and the waves that the signal actually shouldn't be able to reach the point down on the other side however because radio behaves like a wave it can do some of this it can go around corners a little bit and again how well it can do that depends on the wavelength radio waves get reflected just like light in a mirror and the laws are the same an incidence angle and an angle out it does that and then lastly waves can have interference and this in the strict sense of the term only appears if the frequency of two waves is the same or at least very close to one another and they have a fixed phase relation to one another like fixed place where the wavelength has its ups and downs where the wave has its tops and its lowest points when you have this same frequency some form of fixed phase relation you get strict interference and you can have the effect that one plus one doesn't make two but in fact zero waves can cancel each other out we're also using the term interference in a wider way where we're saying you know this frequency is close to one another and we'll have interference between them they'll disturb one another in some way even though it might not be 100 they might not cancel out each other completely but interference is definitely something to look at in the behavior of radio waves.

© Produced by Philip Smith and the Network Startup Resource Center, through the University of Oregon.

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