How is it possible to achieve waves which are spatially, but not temporally, coherent? Can this be done with a bandpass filter?
Conversely, how is it possible to achieve waves which are temporally, but not spatially, coherent? Can this always be achieved with a pinhole?
Best Answer
Yes! Even at home you can see this.
First, a star is spatially coherent and not temporally coherent (not the sun, but any other star). It creates a near plane wave when illuminating Hubble or even many terrestrial telescopes. http://skullsinthestars.com/2008/09/03/optics-basics-coherence/
Second, a distant low-pressure sodium light is temporally incoherent and spatially coherent. Try this at home. At night, stand about 2 - 5 ft from a screen door or window screen. Look at a distance street lamp. You'll see a distinct sinc(x) pattern (repeating dots in the vertical and horizontal direction aligned with the lines on the screen). This can't happen with an extended source. See: http://en.wikipedia.org/wiki/Diffraction and http://en.wikipedia.org/wiki/Fraunhofer_diffraction_%28mathematics%29 for sinc(x) patterns.