Ultrasound can be used to determine the properties of materials, and to find the shape of objects and the distance between them. (a) how do ultrasound systems determine the density of a material? what would make it difficult to "see" a marble hidden inside an air-filled hollow black box using ultrasound? explain using physics concepts. (2 marks) (b) a sound wave generated by an ultrasound "wand" reflects off a bone that is situated 3cm inside a water sample. water has an acoustic impedance of 1.48 mpa s / m and bone has an acoustic impedance of 7.75 mpa s / m. the output of the wand is perfectly matched with water (with gel) so that ultrasonic waves enter the medium without reflection and travel at 1 484 m/s. the amount of reflection off the boundary between two objects can be found with the following equation 2 z2 - z2 r2 \z, +z2. (i) how much of the ultrasound is reflected back to the receiver? (1.5 marks) (i) how long does it take for the sound to return to the wand after it is emitted?