Incoherent Light Sources


by fantispug
Tags: incoherent, light, sources
fantispug
fantispug is offline
#1
Sep18-07, 12:20 AM
P: 105
I was thinking about a collection of monochromatic light sources with a randomly oriented phase - for example in a sodium lamp the atoms will radiate almost independently, so they should act as such a collection of incoherent light sources.

Assume for simplicity each light source is classical and has the same electric field magnitude [tex]E_0[/tex]. The electric field of each of them at a point in space would be [tex]E_0e^{(i\varphi)}[/tex] (for some [tex]\varphi[/tex] dependent on the individual light source and the position in space) and the total electric field would be the sum over all of these electric fields at that point.

Given that the phases of the sources are unrelated it should be expected that any value of [tex]\varphi[/tex] could occur with equal probability, that is the phasor (complex vector) corresponding to the electric field could be oriented in any direction with equal probability. The sum over a large number of these sources should thus give something close to zero.

However this would imply zero intensity and since we can observe the light from a sodium lamp this doesn't make sense.
What am I doing wrong here?
Phys.Org News Partner Physics news on Phys.org
Physicists design quantum switches which can be activated by single photons
'Dressed' laser aimed at clouds may be key to inducing rain, lightning
Higher-order nonlinear optical processes observed using the SACLA X-ray free-electron laser
cesiumfrog
cesiumfrog is offline
#2
Sep18-07, 12:46 AM
P: 2,050
Isn't the intensity related to the average square of the amplitude, rather than the average amplitude?
fantispug
fantispug is offline
#3
Sep18-07, 01:10 AM
P: 105
Yeah, you're completely right. I don't know why I was thinking the square of the average, rather than the average of the square (since the electric field at a point can tell you nothing about the power passing through that point). So the cross-terms from the square - the interference terms - will average to zero, and the intensity will just be the sum of the individual intensities!
Thank you so much, that makes much more sense

cesiumfrog
cesiumfrog is offline
#4
Sep18-07, 01:41 AM
P: 2,050

Incoherent Light Sources


Pleased to help


Register to reply

Related Discussions
Standard light sources Biology, Chemistry & Other Homework 3
sources of light General Physics 3
Are World Counts Incoherent? General Discussion 35
Light sources Atomic, Solid State, Comp. Physics 2
Light point sources General Physics 1