What a fiber really is
An optical fiber is a hair-thin strand of ultra-pure glass that guides light. It has four layers you must know by heart: the core (9 µm single-mode; 50 or 62.5 µm multimode) where the light travels; the cladding, always 125 µm, whose lower refractive index traps the light inside the core; the coating, a 250 µm acrylate layer that protects the glass from moisture and micro-scratches; and often a 900 µm tight buffer for indoor handling.
Why the sizes matter
Every tool you will ever use — cleavers, splicers, connectors, adapters — is machined around the 125 µm cladding. Mixing core sizes is a classic rookie failure: launching light from a 62.5 µm core into a 50 µm core throws away power instantly, and single-mode to multimode mismatch is worse. Read the cable jacket print before you touch anything.
Strength and fragility
Glass fiber is stronger than steel of the same diameter in pure tension, yet it fails instantly from surface flaws when bent or scratched. That is why the coating is never removed except in the last centimeters before a splice or connector, and why the bare section is always protected again afterwards.
Field habits that follow from this lesson
Identify fiber type from the jacket (yellow = single-mode, orange/aqua = multimode indoors). Handle stripped fiber only by the coating. Dispose of every cleaved shard in a sharps container — bare glass slivers penetrate skin and are almost invisible.