In the phenomenon of interference,energy.......

To produce the phenomenon of interference,we require two sources that emit radiation of:

Two light beams of intensities $4\,I$ and $9\,I$ interfere on a screen. The phase difference between these beams on the screen at point $A$ is $0$ and at point $B$ is $\pi$. The difference of resultant intensities,at the points $A$ and $B$,will be $....I$.

Two beams of light having intensities $I$ and $4I$ interfere to produce fringes on a screen. The phase difference between the beams at point $A$ is $\pi/2$ and at point $B$ is $2\pi$. Find the resultant intensities at points $A$ and $B$.

The total energy carried by a light wave when it travels from a rarer to a non-reflecting and non-absorbing medium:

If the wavelength of light is $4000 \mathring A$,then the number of waves in $1 \ mm$ length will be