If the wavelength of light is $4000 \mathring A$,then the number of waves in $1 \ mm$ length will be

Two coherent sources of different intensities send waves which interfere. The ratio of maximum intensity to the minimum intensity is $25$. The intensities of the sources are in the ratio:

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

Radio waves coming at an angle $\alpha$ to the vertical are received by a radar after reflection from a nearby water surface and directly. What should be the height of the antenna from the water surface so that it records a maximum intensity? (wavelength $= \lambda$)

When a light wave undergoes reflection at the interface from air to glass,the change in phase of the reflected wave is equal to

Two waves having same intensity $I_{0}$ and originated from two non-coherent sources superpose at a point. The average intensity at that point is . . . . . . .