# Behaviour of White light?

1. Sep 24, 2015

### 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???

2. Sep 24, 2015

### blue_leaf77

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

3. Sep 24, 2015

### mathman

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

4. Sep 24, 2015

### Andy Resnick

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

5. Sep 25, 2015

### 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.

6. Sep 25, 2015

### lightarrow

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

7. Sep 25, 2015

### Andy Resnick

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".

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