When a wave enters into a rarer medium from a denser medium, the property of the wave which remains constant is

A car sounding a horn of frequency 1000 Hz passes a stationary observer. The ratio of frequen'ies of the horn noted by the observer before and after passing of the car is $11: 9$. The speed of car is (speed of sound $v=340 \mathrm{~ms}^{-1}$ )

A pipe with 30 cm length is open at both ends. Which harmonic mode of the pipe resonates with the 1.65 kHz source? (Velocity of sound in air $=330 \mathrm{~ms}^{-1}$ )

If the frequency of a wave is increased by $25 \%$, then the change in its wavelength is (medium not changed)