How Does a Double Slit Create Coherent Light from an Incoherent Source?

[Freyth]

Young's double slit experiment produces regular intervals of bright fringes and dark fringes. The bright fringes happen when the 2 sources of wave are in phase and superimpose while the dark fringes are produced when the 2 waves are in antiphase and vectorially cancel each other out.

However, I've a little confused. It is said that for the experiment to work, the sources of light must be coherent. Meaning that they have constant phase difference; always in phase/ antiphase, having a fixed phase difference.

The fact that there are dark and bright fringes indicate that the phase difference is not constant meaning that they are not always in phase/ antiphase.

Am I missing something here or am I interpreting it wrongly?

 

[Freyth]

After further reading. I realized that the term "constant phase difference" actually means that the 2 waves at the point, no matter the period, will always be in phase/ antiphase. Unlike incoherent light sources which has random emission of light waves, thus constant phase difference is not present.

But I'm still unsure of something. If a sodium lamp emit light waves randomly, why will putting a narrow slit in front of the light source produce a coherent one?

 

[rl.bhat]

After further reading. I realized that the term "constant phase difference" actually means that the 2 waves at the point, no matter the period, will always be in phase/ antiphase. Unlike incoherent light sources which has random emission of light waves, thus constant phase difference is not present.

But I'm still unsure of something. If a sodium lamp emit light waves randomly, why will putting a narrow slit in front of the light source produce a coherent one?

By putting a narrow single slit in front of a sodium lamp does not produce coherent light. If you keep a narrow double slit in front of the single slit, it produces the coherent light.