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Even with incoherent light from the sun and not having any laser, Young achieves a result by reducing the incident light to a point-like source. This makes sense because a wider beam works like many pinholes. Each hole creates its own intensity distribution behind slits and there are no fringes visible. A further explanation of the pinhole highlights that the pinhole makes incoherent light coherent.

But how a pinhole makes light from a thermic source - for example from a sodium-vapor lamp - coherent?

Furthermore, does the light behind a hole then have the same properties as laser light?

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The pin hole serves to make the light spatially coherent:all wave vectors point in the same direction. To turn it into laser light it must also be temporally coherent : all frequencies are the same. The intrinsic bandwidth of the light coming from a sodium source should be much larger than that of a laser, as a high quality factor resonance cavity is missing.

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  • $\begingroup$ Not sure about the bandwidths. The bandwidth of a laserpointer is around 1 nm or more, the bandwidth of a sodium-vapor lamp has two spectral lines very close together at 589.0 and 589.6 nm, which is less as from the laser. $\endgroup$ – HolgerFiedler Dec 1 '18 at 11:03
  • $\begingroup$ Is that the intrinsic bandwidth? $\endgroup$ – my2cts Dec 1 '18 at 11:42
  • $\begingroup$ That I tried to find out. About the sodium lamp see Wikipedia. About the laser diode what do you know? $\endgroup$ – HolgerFiedler Dec 1 '18 at 12:10
  • $\begingroup$ A German source itwissen.info/Laserdiode-laser-diode-LD.html says, that “Die spektrale Breite mit der Laserdioden ihr Licht aussenden, beträgt nur 1 nm, im Falle des Distributed Feedback Lasers (DFB) ist sie sogar nur 0,1 nm.“ (The spectral width with which laser diodes emit their light is only 1 nm, in the case of the Distributed Feedback Laser (DFB) it is only 0.1 nm.) $\endgroup$ – HolgerFiedler Dec 1 '18 at 12:38
  • $\begingroup$ Following the link to the article on DFB lasers I read "einer geringeren chromatischen Dispersion innerhalb der Monomodefaser nieder". So 1 nm is the inhomogeneous linewidth and the homogeneous linewidth is smaller than 0.1 nm. $\endgroup$ – my2cts Dec 1 '18 at 12:44

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