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. Shortwave radio is used for broadcasting of voice and music, and long-distance communication to ships and aircraft, or to remote areas out of reach of wired communication or other radio services and that’s why it is more energetic than long waves. Shortwave radio is radio communication using the upper MF (medium frequency) and all of the HF (high frequency) portion of the radio spectrum, between 1,800–30,000 kHz. Shortwave radio received its name because the wavelengths in this band are shorter than 200 m (1500 kHz) which marked the original upper limit of the medium frequency band first used for radio communications. The broadcast medium wave band now extends above the 200 m/1500 kHz limit, and the amateur radio 1.8 MHz – 2.0 MHz band (known as the “top band”) is the lowest-frequency band considered to be ‘shortwave’.

  6. A basic approach would be to use the definition of oscillation. One oscillation of Kinetic energy occurs when it reaches minimum(extreme position) from maximum( equilibrium position) and then to maximum. This process occurs twice in one oscillation of the particle, so the frequency at which the kinetic energy would oscillates is twice than that of the particle which is 2f.

  7. In the block-spring system, the inertia is concentrated in a single element of the system which is the block, but in the string the inertia is distributed throughout the system. Similarly, the elasticity of the block-spring system  is lumped in one element ( spring) but in the vibrating spring it is distributed throughout the system. Although there is only one way for the block-spring system to store kinetic and potential energy, the vibrating spring has an infinite number of ways to store its energy. In general, a lumped system of N elements has N different oscillating frequencies, each of which corresponds to a different pattern of oscillation. In the figure in the question, shows an example of a lumped system with one, two and three elements. The limit N tends to infinity leads us to the completely distributed system of the stretched string, with its infinite number of vibrational frequencies.


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