Acoustics and Sound Waves

The properties of a medium necessary for wave propagation are its elasticity, low resistance and inertia. 
Ans: D

The electric and magnetic fields have to remain continuous at the refractive index boundary. If the frequency changed, the light at each side of the boundary would be continuously changing it's relative phase and there would be no way to match the fields.
Furthermore, for conservation of energy frequency should remain same.
Energy $E' = h\nu'$ inside the glass must be equal to the energy outside, so
$E = E'$ gives
$h\nu = h\nu'$
so $\nu = \nu'$
the frequency in the medium is equal to that of outside.
Ans: A

In wave motion, particles of the medium vibrate about their mean positions. The particles of the medium do not move from one place to another

Waves produced in a spring by compressing a small portion of it and releasing are longitudinal waves.

Compression and rarefaction are seen in longitudinal waves

$\begin{array}{l} { f _{ 1 } }=3f \ { f _{ 2 } }=5f \ \Rightarrow 5{ f _{ 1 } }=3{ f _{ 2 } } \ Hence, \ option\, \, A\, \, is\, \, correct\, \, answer. \end{array}$

When tuning fork is sounded by striking its one end then the prongs vibrate in and out and stem vibrate up and down. Hence, vibration of prongs are transverse and those of stem are longitudinal. 

Transverse waves cannot propagate in a gas because there is no mechanism for driving motion perpendicular to the propagation of the wave.

When we pluck the wire of a sitar, the wave produced in the wire are transverse wave.

Amplitude and frequency depend on source and as frequency is different, wavelength also has to be different to maintain same speed as speed is constant for a medium in a particular condition.

Ultrasonic wave , Infrasonic wave and Seismic wave are longitudinal waves

In case of light, the waveform changes perpendicular to the relative motion. So, it is a type of transverse wave, which causes the medium to vibrate perpendicular to the wave energy .

As the sensation of sound persists in our brain for about 0.1 s, to hear a distinct echo the time interval between the original sound and the reflected one must be at least 0.1 s. If we take the speed of the sound to be 344 m/s at a given temperature, say at 22 $^0 C $ in air, sound must go to the obstacle and reach back the ear of the listener on reflection after 0.1 s. 

The persistence of audibility is 0.1 s

Bats can produce ultrasonic waves by flapping their wings. They can also detect these waves.these waves produced by the bats after reflection from the obstacles like building guide them to  remain away from the obstacles during night with the help of ultrasonic waves.