4 ms·

Coherence is defined as a correlation of the light with itself. In simpler terms "how similar it is to itself after letting it evolve for a bit". Sunlight has a

by roesel 8y ago

Coherence is defined as a correlation of the light with itself. In simpler terms "how similar it is to itself after letting it evolve for a bit". Sunlight has a lot of different wavelengths evolving in many different ways, therefore it is not too coherent compared to an ideal single wavelength laser which will be identical even after "evolving" for a while. However, if you look at sunlight at a certain time and then look at it one femtosecond later, the second look will still be similar to the first look to some extent, therefore you would have to say that the light is somewhat coherent. This is why sunlight is definitely NOT "completely incoherent".

kurthr 8y ago
In fact any thermal radiator (e.g. black body) will be coherent over short distances proportional to the temperature of the body. Effectively the light's phase doesn't (on average) change faster than the quantum of energy (h-Plank's constant) divided by the thermal energy of the body (kT) so the power of a particular wavelength averaged in your eye can still have significant interference effects for a high temperature radiator. The distance of coherence is then related to that coherence time times the speed of light (so redder-longer wavelengths are coherent for fewer wavelenghths). That leads to an equation of L= c*h / 4kT where the Sun can be treated roughly as 6000K. This gives a coherence length measured in microns, which is conveniently several times the half wave is necessary for interference for visible light (0.4-0.7um full wave). It's pretty cool, because I think it means that if you evolved eyes to see light near the peak of your black body radiator then you will be able to see Newton Rings for internally reflective thin membranes sized close to that peak due to the coherence length (due to Wien's law). A nice review paper: http://hank.uoregon.edu/teaching-modules/Broadband-Interferometer/BBcoherence.pdf http://hank.uoregon.edu/teaching-modules/Broadband-Interfero...

by roesel 8y ago

kurthr 8y ago

In fact any thermal radiator (e.g. black body) will be coherent over short distances proportional to the temperature of the body. Effectively the light's phase doesn't (on average) change faster than the quantum of energy (h-Plank's constant) divided by the thermal energy of the body (kT) so the power of a particular wavelength averaged in your eye can still have significant interference effects for a high temperature radiator. The distance of coherence is then related to that coherence time times the speed of light (so redder-longer wavelengths are coherent for fewer wavelenghths). That leads to an equation of L= c*h / 4kT where the Sun can be treated roughly as 6000K. This gives a coherence length measured in microns, which is conveniently several times the half wave is necessary for interference for visible light (0.4-0.7um full wave). It's pretty cool, because I think it means that if you evolved eyes to see light near the peak of your black body radiator then you will be able to see Newton Rings for internally reflective thin membranes sized close to that peak due to the coherence length (due to Wien's law). A nice review paper: http://hank.uoregon.edu/teaching-modules/Broadband-Interferometer/BBcoherence.pdf http://hank.uoregon.edu/teaching-modules/Broadband-Interfero...