Understanding the Nature of White Light: Coherency and Mixture Explained

[Raman Choudhary]

Is White light a superposition(Interference) of different wavelengths(V,I,B,G,Y,O,R) ? Or is it a mixture of these wavelengths, i do not even properly know about mixtures but my sir told me that for mixture you do not need sources of these light(V,I,B,G,Y,O,R) to be COHERENT , so we can light up different bulbs(each with different color) and the resultant we would get is white light,but how is mixture possible(scientific explanation) I mean what are the waves doing when we say ther are just mixing up ?Why is there no need of coherency conditions since eventually these are waves and there must be superposition?

 

[blue_leaf77]

Be it coherent or incoherent, a light beam whose spectrum spans the entire visible region will look white to us.

how is mixture possible(scientific explanation) I mean what are the waves doing when we say ther are just mixing up ?

What your teacher referred to when saying mixing, is that the frequency components in the light's spectrum do not have correlation with each other, in other words the phase relations between them is random.

Why is there no need of coherency conditions since eventually these are waves and there must be superposition?

Not every wave present in nature is coherent, in fact almost all of them are incoherent. It's the nature of the source which determines the coherency of the emitted wave.

 

[mathman]

Coherence on a macroscopic level exists only for lasers and similar sources.

 

[Andy Resnick]

Not true- starlight (sunlight exempted) is highly spatially coherent, as is light from short arc sources.

 

[blue_leaf77]

EDIT: In response to the example given by Andy, apart from the nature of source, coherency is also determined by the distance between the observation point and the source. The farther it is, the bigger the coherent area.

 

[lightarrow]

EDIT: In response to the example given by Andy, apart from the nature of source, coherency is also determined by the distance between the observation point and the source. The farther it is, the bigger the coherent area.

Yes, but more than that distance, I would say it's the angle subtended by the source from the observation point: the light from Sun seen from Earth is less spatially coherent than a led's light seen 100 metres apart.

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lightarrow

 

[Andy Resnick]

EDIT: In response to the example given by Andy, apart from the nature of source, coherency is also determined by the distance between the observation point and the source. The farther it is, the bigger the coherent area.

Yes, but more than that distance, I would say it's the angle subtended by the source from the observation point: the light from Sun seen from Earth is less spatially coherent than a led's light seen 100 metres apart.

Yes- the spatial coherence is determined by the (angular) size of the source- as the source approaches a point, the coherence area increases. An aspect of this that I don't fully understand is that the spectrum of partially coherent light can change during propagation- the 'spectral interference law'.

A very good introduction to coherence in all its aspects is Wolf's "Introduction to the theory of coherence and polarization of light".