Electromagnetic Waves

For sky wave propagation of 10 MHz signal, what should be the minimum electron density in ionosphere?

The equation of a simple harmonic wave is given by , where and are in meter and time is in second. The period of the wave in second 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

Two trains are moving towards each other with speeds of 20 m/s and 15 m/s relative to the ground. The first train sounds a whistle of frequency 600 Hz, the frequency of the whistle heard by a passenger in the second train before the train meeets is (the speed of sound in air is 340 m/s)