double slits exp.

A simple explanation is as follows:

Any light source (e.g. a sodium light bulb) has a dimension. If the light source is placed behind two double slits (i.e. without the single slit in between), light from different parts of the source could reach the two slits. In other words, light passing through one slit may not come from the same part of the source as light passing through the other slit.

In such situation, it could not be sure that light from different parts of the source are always emitted coherently. In fact, in most light sources, light emitted from different parts of the source are incoherent.

In order to improve such situation to obtain a coherent light from an incoherent light source,. the most simple method is to reduce the dimension of the light source to infinitely small, such to ensure that light passing through the two slits must come from the same part of the light source.

This is the reason why a single slit is placed in between. After the slit is inserted, light passing through the "double slits" is for sure to come from this "single slit" which, because of its small width, plays the role of reducing the dimension of the actual light source.