Diffraction and interference of light demonstrate the ....... of light.

What is the path difference for destructive interference?

In an interference pattern obtained by two coherent sources,the variation in intensity is $5\%$ of the average intensity. Find the ratio of the intensities of the two sources.

Given below are two statements: one is labelled as Assertion $A$ and the other is labelled as Reason $R$.
Assertion $A$: The phase difference of two light waves changes if they travel through different media having the same thickness,but different indices of refraction.
Reason $R$: The wavelengths of waves are different in different media.
In the light of the above statements,choose the most appropriate answer from the options given below.

Two coherent sources $S_1$ and $S_2$ are separated by a distance four times the wavelength $\lambda$ of the source. The sources lie along the $y$-axis,whereas a detector moves along the $+x$-axis. Leaving the origin and far-off points,the number of points where maxima are observed is

"If you illuminate two pinholes using two lamps, the interference pattern will not be observed" - Explain.