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

Two independent monochromatic light sources produce waves given by $y_1 = 2 \sin \omega t$ and $y_2 = 3 \cos \omega t$. Which of the following statements is true?

$A$ beam with wavelength $\lambda$ falls on a stack of partially reflecting planes with separation $d$. The angle $\theta$ that the beam should make with the planes so that the beams reflected from successive planes may interfere constructively is (where $n = 1, 2, \dots$)

Two waves of same intensity $I_0$ emitted from two sources having same phase difference $(\phi)$. Due to superposition of two waves, the intensity of resultant wave is directly proportional to . . . . . . .

In the phenomenon of interference:

Two coherent sources produce waves of different intensities which interfere. After interference,the ratio of the maximum intensity to the minimum intensity is $16$. The ratio of the intensities of the waves is: