Applications of focusing fall into two main categories, one having to do with imaging, the other with energy concentration. A lens used as a reading glass is an example of imaging, while a lens used as a burning glass is an example of energy concentration. The applications of optical imaging are numerous. Imaging with sound waves is used in a variety of applications, such as acoustical microscopy, non-destructive testing, medical diagnosis, underwater viewing and seismic investigations. The use of focusing to obtain energy concentration has many applications. In medicine, focused laser beams are used in eye operations, and focused beams of ultrasound and microwaves are used in the treatment of cancer. In optical imaging, lenses and mirrors are used to introduce appropriate phase delays so that a diverging spherical wave emitted by a point object is transformed into a converging spherical wave centred at the geometrical image point.