or
  1. Because, radio waves in the shortwave band can be reflected or refracted from the ionosphere. Therefore, short waves directed at an angle into the sky can be reflected back to Earth at great distances, beyond the horizon.

  2. Due to the refraction of sound, we can hear sounds easily at night than during clear days.

    On a clear day, the lower layer of the atmosphere is hotter than the layers above. Since sound travels faster in a hotter medium, its speed is greater near the surface. As a result, the waves are bent away from the surface. Thus, the intensity of the sound waves diminishes and does not seem to travel a long distance. On a clear night, the lower layer of the atmosphere is colder than the air above. Now, the sound waves travel faster at the higher layers than at the lower layers. Thus, the waves are bent towards the earth's surface. The intensity of the sound increases and seems to travel a greater distance.

  3. The differences between transverse and longitudinal waves are as listed below:

    Transverse waves
    Longitudinal waves
    The particles of the medium vibrate in the direction perpendicular to

    the direction of wave propagation.

    The particles of the medium vibrate parallel to the direction of wave propagation.
    Alternate crests and trough are formed.
    Alternate compression and rarefaction of the medium are formed.
    These are formed in solids and over liquid surfaces.
    These are formed in solids, liquids and gases.
    These waves produce the variations in the density of medium.
    These waves do not produce variations in the density of the medium.
    Transverse waves can be polarized.
    Longitudinal waves cannot be polarized.

  4. The differences between progressive waves and stationary waves are as listed below:

    Progressive waves
    Stationary waves
    The disturbance travels forward.

    The disturbances are confined to a particular region.

    The amplitude of vibration of each particle is same.
    The amplitude is zero at nodes and goes on increasing to become maximum at antinodes.
    Energy is transferred forward along the waves.
    No transfer of energy is in the medium.
    No particles in the medium are at rest.

    Particles at the nodes are permanently at rest.


  5. The Planck constant, or Planck's constant, is a fundamental physical constant which is denoted by h. When a photon's frequency is multiplied by the Planck constant, it gives the energy contained in the photon. Due to mass–energy equivalence, the Planck constant also relates mass to frequency.

    E=hf

    Where

    E is energy of photon

    h is Planck's constant

    f is frequency of the photon