Question

what is practical application of optical fibers?

Answer 1

Single Mode fiber optics is used in communication and data transfer.
Multi-mode fiber optics is used in data transfers in buildings and short distances.

Plastic fiber optics makes pretty light displays.
This is also the basis of lighting systems that will illuminate an entire room with one bulb.
It is also used for bring sunlight into the interior of buildings.

Fiber Optics may also be used in surgeries.
One fiber to look what is inside and another fiber that can be used to pin point a laser pulse to a bad section of tissue.