# Understanding the Nature of White Light: Coherency and Mixture Explained

• Raman Choudhary
In summary, white light is a combination or mixture of different wavelengths of light, which do not necessarily need to be coherent. Coherency conditions are only necessary for certain sources of light, and the coherence of light can also be affected by factors such as distance and the size of the source. The spatial coherence of light can change during propagation, and a thorough understanding of coherence can be found in Wolf's book "Introduction to the theory of coherence and polarization of light".
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?

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

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

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.

blue_leaf77 said:
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.

--
lightarrow

blue_leaf77 said:
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 said:
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".

## 1. What is white light?

White light is a combination of all the visible colors of light, which appear to our eyes as white. It is composed of a spectrum of wavelengths ranging from approximately 400 to 700 nanometers.

## 2. How does white light behave?

White light behaves as both a wave and a particle, exhibiting properties such as reflection, refraction, diffraction, and interference. It also follows the principles of optics, such as Snell's law and the law of reflection.

## 3. Can white light be separated into its component colors?

Yes, white light can be separated into its component colors through a process called dispersion. This can be achieved using a prism or a diffraction grating, which causes the different wavelengths of light to bend at different angles.

## 4. Why do objects appear to have different colors under white light?

Objects appear to have different colors under white light because they absorb certain wavelengths of light and reflect others. The color we see is the result of the wavelengths that are reflected back to our eyes.

## 5. How does the behavior of white light differ from other colors of light?

The behavior of white light is similar to other colors of light, but the main difference is that white light contains all the visible wavelengths, while other colors only contain a specific wavelength or combination of wavelengths. This difference affects how the light interacts with objects and its overall appearance to our eyes.

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