Young's Double slits

Main Question or Discussion Point

I would like to ask why a single slit is needed.
I have done some research and find that it makes sure the light goes to the double slits is coherent.

How can a single slits produce coherent light ?
and why the light comes from a natural source (e.g. lamp) is not coherent ?
In my knowledge, light is conherent within the coherent length..

thank you.

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Coherent light means light of same wavelength and frequency, also it has to be in phase

A standard bulb emits light of lots of frequencies and wavelength. Its not necessarily in phase either....

This will just result in lots of destructive interference and you probably wont get a pattern with any symmetry.

If you want to carry out this experiment just use candle light, thats how it was first done i believe.

I'm sure a candle's light is no more coherent than an electric bulb's. Also, I understand collimating a light source will basically always increase the coherence length. Kind of like saying the light that passes through the first slit must be coherent with itself to some degree, but probably wouldn't of been as coherent with the light that struck a centimeter left of that slit.

thank you for the explanation.
But how can the light for the light bulb becomes coherent after a single slit ?
How can the slit change their wavelength and frequency and make them the same ?