A stone is dropped into water from a bridge. Fig. 3.1 shows water waves hitting a wall as plane wavefronts.

a) A wavefront is a line or surface connecting points of constant phase on a wave, such as the crests or troughs of a wave.

b) When water waves hit a wall, they undergo reflection.

• The waves change their direction of motion, bouncing off the wall.
• The speed, wavelength, and frequency of the waves remain unchanged.
• The amplitude of the reflected waves may decrease due to some energy being absorbed by the wall.

c) Sound travels faster in water than in air. Therefore, the sound waves from the splash reached the girls underwater more quickly than they reached the girls above the water.