Ultrasound

Think about what happens when light hits a window. Some of it goes through and some is reflected. You can choose to focus on what is reflected or on what is on the other side of the window. We call this partial reflection.

In the same way, ultrasound is partially reflected when it goes from one medium into another medium. The ultrasound changes speed when it goes from one medium to another. The bigger the change in speed, the more is reflected. It is the reflection that allows us to make up a picture using ultrasound scanning for eg foetuses.

There is a big change in speed between air and muscle, so if you fire ultrasound towards a woman's stomach, it would mostly reflect and not get into her. For this reason, they smear saline gel on first because there is less of a reflection between saline gel and muscle.