# The Naked Scientists Forum

### Author Topic: Wave length, amplitude, frequency?  (Read 4063 times)

#### Europan Ocean

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##### Wave length, amplitude, frequency?
« on: 05/02/2015 11:15:21 »
I am studying transmissions and reception, radio waves, microwaves... I find it difficult to understand that to make it simple, the wave length and frequency of blue light can have a different amplitude. Wave height. How does this work? What difference would there be in the blue light with great amplitude compared to small amplitude?

Also some waves are not received under bridges, others are, how does that work?

#### Colin2B

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##### Re: Wave length, amplitude, frequency?
« Reply #1 on: 05/02/2015 12:08:35 »
I am studying transmissions and reception, radio waves, microwaves... I find it difficult to understand that to make it simple, the wave length and frequency of blue light can have a different amplitude. Wave height. How does this work? What difference would there be in the blue light with great amplitude compared to small amplitude?

Also some waves are not received under bridges, others are, how does that work?
There are some different things here. Keeping it simple:
Frequency and wavelength of electromagnetic waves describes the same thing they are related by the speed of light.
Speed of light = wavelength x frequency. In light the wavelength is equivalent to colour, longer wavelengths towards red, shorter towards blue.

Amplitude. With water waves the amplitude is the height, with sound waves the equivalent is the peak pressure, Radio it's the intensity of the electric and magnetic components. We don't usually refer to amplitude of light, but intensity. That is brightness. You could think number of photons per second, but to really understand you'll have to get to the point of doing the maths of EM waves before this will click properly.

Wave can be affected by objects eg water waves by bridges, to create shadows. Water waves can also be refracted, reflected and diffracted just like radio or light waves, also affected by depth of water. This is a whole fascinating subject of its own. Look around for some surf science sites, or how waves on a beach are refracted as the water shallows.

So, in the case of EM radiation, The wavelength of the radio wave and size of object will affect how they respond to the object, creating either shadows or interference patterns. One example of this is higher frequency (shorter wavelength) radio waves which can't go beyond the horizon, but lower frequencies (longer wavelengths) can 'bend' over the horizon. Waves can also be reflected off surfaces or atmospheric layers.

Keep up the studies, you'll soon get there.

« Last Edit: 05/02/2015 13:27:21 by Colin2B »

#### evan_au

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##### Re: Wave length, amplitude, frequency?
« Reply #2 on: 05/02/2015 20:40:04 »
In the case of sunlight and indoor lighting, the light consists of many different frequencies, with random phase. (Early radio transmitters also had many different frequencies, but this was banned soon after Titanic...)

Modern radio transmitters, microwaves and lasers are "coherent": the waves have a single frequency and are all in phase (well, pretty much). This can produce some interesting interference effects if you shine a laser pointer at a wall or a sheet of paper or a bubble - you see speckles and bands of light, where the reflection is stronger in some parts, and almost cancelled at other locations.

When you drive under a bridge, the radio waves reflect off the metal structure of the bridge, causing interference patterns which cancel out in some lanes (while getting stronger in others), at some positions under the bridge (but not others), and for some radio stations (but not others). This is why the effect appears so random.

So if you have a 1mW red laser (at around 430 THz), it has almost twice the wavelength of a 1mW blue laser (at around 790 THz), because the frequency has a ratio of around 2:1; this means that the interference rings are spaced much more closely with blue light than with red light. Now compare that to an AM radio station with a frequency of 1MHz - the wavelength is far longer, enough for you to hear distinct periods of loud and quiet as you drive towards and under a bridge. (It is less pronounced with FM radio at frequencies around 100MHz because this form of modulation does not convey information on the amplitude of the signal, and the interference bands are much closer.)

You will notice that wireless microphone receivers (and modern WiFi receivers) have 2 or 3 antennas, so that if one antenna is receiving no signal due to reflections off the building, the other antenna (just a few inches away) is likely to be receiving a good signal.

Extending the photon approach: Radio waves and light can be thought of as a stream of photons, the "particle" of light (which also has some wave-like properties). The photon Energy is proportional to frequency. The red laser puts out almost twice as many photons per second as the blue blue laser because of the frequency difference. However, they both produce the same power (amplitude). Then compare this to a wireless microphone producing 1mW of radio power at around 100MHz, which puts out far more photons than the red or blue laser, because the frequency ratio is quite extreme, but it also has the same power.  But photon properties do not become obvious until you are talking about very low intensities or very high frequencies, where events occur 1 photon at a time. Otherwise, it is better to use the ocean wave analogy.

[Where I could use some help: In electrical circuits, power is proportional to the voltage squared; in a radio transmitter, power is proportional to the voltage squared; in ocean waves, power increases more than linearly with wave height. Is the power of a free-space electromagnetic wave proportional to the square of the electric field strength?]
« Last Edit: 06/02/2015 17:10:53 by evan_au »

#### Colin2B

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##### Re: Wave length, amplitude, frequency?
« Reply #3 on: 05/02/2015 23:27:11 »
[Where I could use some help: In electrical circuits, power is proportional to the voltage squared; in a radio transmitter, power is proportional to the voltage squared; in ocean waves, power increases more than linearly with wave height. Is the power of a free-space electromagnetic wave proportional to the square of the electric field strength?]
Spot on.
Although the electric and magnetic fields are perpendicular vectors  they can be resolved with rate of energy transfer perpendicular to both, in the direction of the wave propagation and proportional to the product of the two. Using E= B x c you can substitute B and end up with E squared.
« Last Edit: 06/02/2015 05:46:31 by Colin2B »

#### UltimateTheory

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##### Re: Wave length, amplitude, frequency?
« Reply #4 on: 06/02/2015 01:00:11 »
Quote
the wave length and frequency of blue light can have a different amplitude.

Amplitude of light in classic physics, is replaced by quantity of photons in quantum physics..
« Last Edit: 06/02/2015 01:02:42 by UltimateTheory »

#### evan_au

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##### Re: Wave length, amplitude, frequency?
« Reply #5 on: 06/02/2015 17:20:19 »
I spoke with an amateur radio enthusiast about interference effects on FM radio. He described it as a "picket fence" effect.

Imagine a fence whose fenceposts are separated by gaps of equal width. If you drive past this fence in your car (with the window down), you will hear a rapidly changing echo of your car.

Driving under a bridge, or near the limits of an FM transmitter's coverage, you will hear this rapid variation of the signal, which sounds quite different from the slow fading of AM radio (at the same speed).

#### alancalverd

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##### Re: Wave length, amplitude, frequency?
« Reply #6 on: 06/02/2015 17:33:55 »
Frequency is the inverse of wavelength, and describes the "quality" of electromagnetic radiation. At very high frequencies (very short wavelengths), from infrared to cosmic gamma radiation, where it is useful to consider individual photons, frequency is directly related to photon energy.

Amplitude describes the "intensity" of radiation: the strength of a radio signal or the brightness of a light.

Propagation of radio waves through tunnels is a bigger subject than can be accommodated in a chatroom! At some wavelengths, commensurate with the tunnel dimenskions, it may act as a waveguide, producing a fairly unattenuated signal at any point inside. At very long wavelengths the electric field may simply oscillate between the ends of the tunnel, again producing a strong signal throughout. Diffraction may propagate short wave (e.g. mobile phone) signals some way into the tunnel but many long railway and road tunnels use "leaky feeders" or repeater stations to actively generate FM (usually public service stations) and phone signals inside the tunnel.

#### UltimateTheory

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##### Re: Wave length, amplitude, frequency?
« Reply #7 on: 07/02/2015 14:14:15 »
Frequency is the inverse of wavelength,

Not exactly.

f = c/wavelength, when we're talking about light.

Inverse of frequency is period.
T=1/f

#### alancalverd

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##### Re: Wave length, amplitude, frequency?
« Reply #8 on: 07/02/2015 16:57:05 »
True. Frequency is (a constant multiplied by) the inverse of wavelength where the constant is the propagation speed of whatever in whichever. I admit to being outpedanted.

#### The Naked Scientists Forum

##### Re: Wave length, amplitude, frequency?
« Reply #8 on: 07/02/2015 16:57:05 »