Question

When a sound wave goes from one medium to another , the quantity that remains unchanged is

• A. Frequency
• B. amplitude
• C. wavelength
• D. speed

Answer 1

The correct option is A Frequency

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 $E^{\prime }=h{\nu }^{\prime }$ inside the glass must be equal to the energy outside, so
$E=E^{\prime }$ gives
$h\nu =h{\nu }^{\prime }$
so $\nu ={\nu }^{\prime }$
the frequency in the medium is equal to that of outside.
Ans: A