Total internal reflection
Light bends when it crosses between materials of different refractive index. When it tries to leave a dense material (the core, n≈1.468) into a less dense one (the cladding, n≈1.463) at a shallow angle, it cannot exit — it reflects back completely. This total internal reflection repeats millions of times per kilometer and is the entire physics of fiber optics.
Numerical aperture and launch
Only rays entering within the fiber's acceptance cone are captured; its measure is the numerical aperture (NA). Multimode fiber has a large core and NA, so cheap LEDs and VCSELs couple easily. Single-mode's tiny 9 µm core demands precise laser alignment — which is exactly why single-mode connectors and splices need more care.
The decibel — your daily language
Power changes in fiber are always expressed in decibels: dB = 10·log₁₀(P_out/P_in). Memorize the anchors: 3 dB = half the power, 10 dB = one tenth, 20 dB = one hundredth. Absolute power uses dBm, referenced to 1 milliwatt: 0 dBm = 1 mW, −10 dBm = 0.1 mW, +3 dBm ≈ 2 mW. A receiver reading of −28 dBm on a link launched at −3 dBm means the link consumed 25 dB.
Why technicians must own this math
Every acceptance test, every loss budget, every troubleshooting decision compares measured dB against expected dB. If the arithmetic is not automatic, the meter readings are just numbers.