Consider the following statements about interference of light A....

In interference, the fringes are equally bright and equally spaced and intensity of the bright fringe is four times the intensity of each incident wave. Resultant intensity at a point is given by $I=I_1+I_2+2 \sqrt{I_1} \sqrt{I_2} \cos \delta$ where, $I_1$ is the intensity of the wave from source $1, I_2$ is the intensity of the wave from source 2 . No two sources in nature can be coherent. So, sources are made from one source by either splitting the amplitude or the wave fronts, by means of double-slit or Fresnel's prism or Lloyd's mirror etc. Further, to get a high contrast $I_1=I_2=I$. So, for constructive interference and at the central maxima $\delta=0$, $I_{\max }=2 I+2 I \cos \delta=2 I(1+\cos \delta)=4 I \cos ^2\left(\frac{\delta}{2}\right)=4 I$ So at central maxima intensity is four times the incident wave.