The colour of light is characterised by which of the following characteristics?

What is the path difference for destructive interference?

Two light beams having intensities $I$ and $9I$ interfere to produce a fringe pattern on a screen. The phase difference between the beams is $\frac{\pi}{2}$ at point $P$ and $\pi$ at point $Q$. The difference between the resultant intensities at $P$ and $Q$ will be ....... $I$.

When a light wave undergoes reflection at the interface from air to glass,the change in phase of the reflected wave is equal to

Among the two interfering monochromatic sources $A$ and $B$; $A$ is ahead of $B$ in phase by $66^\circ$. If the observation is taken from point $P$,such that $PB - PA = \lambda / 4$. Then the phase difference between the waves from $A$ and $B$ reaching $P$ is.....$^\circ$.

Two sources are said to be coherent if they produce waves