Two sources are said to be coherent if they produce waves

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

In an interference experiment,the phase difference for points where the intensity is minimum is $(n=1, 2, 3, \ldots)$

Coherent sources are those sources for which

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.

If the wavelength of light is $4000 \text{ Å}$, then the number of waves in $1 \text{ mm}$ length will be . . . . . . -