Define ‘coherence’ for light waves.

The frequencies of two wave-sources $A$ and $B$ shown in the figure are equal. What type of interference will occur at the points $P$, $Q$ and $R$? The distance $AB=\raisebox{1ex}{${\displaystyle \lambda }$}\!\left/ \!\raisebox{-1ex}{$2$}\right.$ (assume that the waves from $A$ and $B$ start in the same phase).

Light from two coherent sources is reaching on a screen. If the path difference at a point on the screen for the yellow light be $\raisebox{1ex}{${\displaystyle 3\lambda }$}\!\left/ \!\raisebox{-1ex}{$2$}\right.$, then what will be the colour of the fringe at that point? Explain with necessary argument.

Why is no interference pattern observed when two coherent sources are infinitely close to each other?

Why is no interference pattern observed when two coherent sources are far apart from each other?

When a low-flying aircraft passes overhead, we notice a slight shaking of the picture on our TV screen. Why?