# I Light Coherence

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1. Dec 10, 2017

### dlbi

I've read that using a pinhole aperture and a wavelength filter can turn a "white" incoherent light source like a light bulb into a temporally and spatially coherent light source (albeit at low efficiency).

Can a temporally and spatially coherent light source be made with a monoenergetic (or actually narrow bandwidth) light source like a UV LED and a pinhole aperture alone?

Thanks!

2. Dec 10, 2017

### lekh2003

The reason why this occurs is due to diffraction of light since light is wave. Waves will curve around solids as they move, so you are able to create something like a light bulb. The strength of the diffraction which occurs is completely up to the wavelength of the light.

Light has a relatively small wavelength to other waves. Sound has quite large wavelengths which means it diffracts around basically everything (doorways, objects, etc.). Since light has a smaller wavelength, the diffraction isn't as much as a sound wave, so you need tiny pinholes to create any kind of light bulb effect.

But to answer your question regarding narrow bandwidth light (UV led), you could create a diffraction. You could create a diffraction with any electromagnetic wave, and anything which acts like a wave to be precise. The only thing changing is the effect of the diffraction.

3. Dec 12, 2017

### tech99

When I was at school it was before the invention of the laser, so we used a Sodium flame and a slit to obtain coherent light for Young's Slits.
I think that spatial coherence will be obtained if a source is very distant, because we then have a plane wave having a uniform phase across our area of interest. It also seems that if we are in the Far Zone of a source we will have spatial coherence, because the boundary of the Far and Near Radiation Zones is where all the rays from the source are approximately in phase. This boundary occurs approximately at the Rayleigh Distance, defined as R = D^2/(2 x lambda), where D is the largest dimension across the source. To take a typical case, for orange light with a 1mm pin hole, R = (10^-3)^2 / (2 x 0.6 x 10^-6) = 83 cm.