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what is coherent light

jamilm9

jamilm9

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I think some one here should explain what is the difference between a led and a coherent laser.(Yeah this is the first post in this section)
 





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NovoRei

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Re: coherent

Seeing the light as a wave with eletrical and magnetic field you have A*cos(wt+p) where p is the phase of light.

In a laser diode with 4cm, E field, for the p at time 0:
at 0cm p is 0;
at 1cm is 90;
at 2cm is 180;
at 3cm is 270;
at 4cm is 360/0;

The laser emission is phase aligned so you always see the intensity at 100%. Considering it have the same frequency and go at the same direction, you have a coherent light.

In a LED you dont have the phase alignment. The light is a superposition of different phases, leading to a mean of intensity. Besides, the propagation is "isotropic".
 
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nikokapo

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Re: coherent

well, has anyone requested this explanation?

i'm gonna ask you a favour, please write correct titles on the threads you create, nobody gets what you're tryin to say if you just write COHERENT as a thread title..






Coherent light is light which only works on a single wavelength (ideally) and its power is concentrated into one single spot; it differs from LED light because these produce light in a bandwidth of a section of the visible spectrum (if it's not an UV or IR led) and its power is not concentrated but irradiated through space from the diode's end.



i hope i explained it right.
 
T

toked323

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Also the human eye doesn't dialate to coherent light, but they do for incoherent light (from a standard light bulb as if no one knows that)
 
GooeyGus

GooeyGus

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So if I pointed a laser straight into my eye, you're saying that my pupils wont constrict? :-?
 
C

caleb

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interseting.

furthermore, can i columate a LED into a laser like beam.

proably wont work. but tell me info anyway, thanks man.
 
GooeyGus

GooeyGus

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caleb said:
interseting.

furthermore, can i columate a LED into a laser like beam.

proably wont work. but tell me info anyway, thanks man.
Click to expand...

Nope. LED light is not coherent (meaning the photons do not radiate in the same direction, in line with each other)
 
daguin

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hydrogenman15 said:
I always thought that coherent means it's the exact same color like 650 nm's. :-?
--hydro15
Click to expand...

When light waves are all the same length, they are the same color. They can come out in all directions and still be the same wavelength. Coherent light has its photons in "synch" with each other. They are not only all in the same uniform (the same color/wavelength), but they are marching in step with each other and all together.

Peace,
dave
 
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pseudonomen137

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hydrogenman15 said:
I always thought that coherent means it's the exact same color like 650 nm's. :-?

--hydro15
Click to expand...

No, you're thinking of 'monochromaticity' - the other big special property of lasers.

Sorry nikokapo, but I think you're also confused in the same way as your definition seemed to be for monochromaticity and not coherence.

Coherence can be a very in depth topic, and for more info you can look up for temporal and spatial coherence. Long story short though, imagine a bunch of photons demonstrated by a bunch of sine waves. With regular light, these waves would be pretty much random. With coherent light, the waves will coincide with the same phase (IE, at any point in time, all waves will be at the same amplitude).


EDIT: Dammit, danguin beat me to it and he did it better as usual. Hmph! ;D
 
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nikokapo

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Ohhhh, so photons are in phase with each other from the end of the source to wherever it's directed? Cool, that's what i was missing :)
 
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Ace82

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coherent light
–noun light in which the electromagnetic waves maintain a fixed and predictable phase relationship with each other over a period of time.  

I herd not to believe everything you read on the net, but [url=coherent light
–noun light in which the electromagnetic waves maintain a fixed and predictable phase relationship with each other over a period of time.

I herd not to believe everything you read on the net, but [url=coherent light
–noun light in which the electromagnetic waves maintain a fixed and predictable phase relationship with each other over a period of time.

I herd not to believe everything you read on the net, but I don't think there's a better explanation! :)
http://www.reference.com/browse/wiki/Coherence_(physics)
 
Justin

Justin

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Light needs to be monochromatic in order to stay coherent over a distance, but the property of coherence basically just means that the waves of light are in phase with one another and travelling in the same direction. For this reason the coherence length of a laser is dependent on the spectral linewidth (the amount of variation in wavelength within a laser beam), among other things.
 
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