The essential condition for obtaining a sustained interference pattern is that the two sources must . . . . . .

The distance between a point source of light and a screen is $60 \ cm$. If this distance is increased to $180 \ cm$,what will be the intensity on the screen compared to the original intensity?

The light waves from two coherent sources have same intensity $I_{1} = I_{2} = I_{0}$. In the interference pattern,the intensity of light at minima is zero. What will be the intensity of light at maxima?

Two point sources separated by $d = 5\, \mu m$ emit light of wavelength $\lambda = 2\, \mu m$ in phase. $A$ circular wire of radius $R = 20\, \mu m$ is placed around the sources as shown in the figure. Determine the nature of interference at points $A, B, C,$ and $D$.

The frequency of a light wave with a wavelength of $500 \, \mathring A$ is $....... \, Hz$.

For the constructive interference,the path difference between the two interfering waves must be equal to: