An ultrasound pulse is sent from the bottom of the vessel. The pulse reflects off the lake bed and returns to the vessel.

• The total time for the pulse to travel to the lake bed and back to the vessel is 0.24 s.
• The speed of ultrasound in the lake water is 1480 m/s.

Calculate the depth of the lake (the distance from the vessel to the lake bed).

Use the equation:

$$distance\; travelled=speed\times time$$

The sonar transducer emits an ultrasound wave with a frequency of $40000Hz$.

• The speed of ultrasound in the lake water is 1480 m/s.

Calculate the wavelength of this ultrasound wave in the lake water.

Use the equation:

$$wave\; speed=frequency\times wavelength$$

As the ultrasound wave travels from the lake water into a thick layer of soft mud on the lake bed, its speed decreases.

Complete the following sentences to describe what happens to the properties of the ultrasound wave as it enters the mud. Choose your words from: decreases, increases, or stays the same.

• The frequency of the ultrasound wave _______________
• The wavelength of the ultrasound wave _______________