Given scientific fact as input, generate the question from this fact such that it can be answered from the input.

Ultrasound, a diagnostic technology, uses high-frequency vibrations transmitted into any tissue in contact with the transducer.
What diagnostic technology uses high-frequency vibrations transmitted into any tissue in contact with the transducer?

Fermat’s principle states that light will always take the path of the least amount of time.
Fermat’s principle states that light will always take the path of least amount of?

Light falls roughly in the middle of the electromagnetic spectrum, having shorter wavelengths and higher frequencies than microwaves, but not as short and high as x rays.

What falls roughly in the middle of the electromagnetic spectrum, having shorter wavelengths and higher frequencies than microwaves, but not as short and high as x rays?