Observing the travel of light
The falling edge of a single bit on a some inches long 115200 kbps UART connection between a 4D Systems OLED and a bluetooth module.

some more inches → longer signal propagation time

Bandwidth test with NEC 78K0 PWM
RIGOL RP2200 probe @ 10× with ground lead

Ground spring

Resistive Z0 probe @ 100× built from old 10Base2 T adaptor, terminator, 3 m RG-58 cable and 4.7 KΩ metal film resistor

@ 50×, 1.5 m RG-58 cable and 1 KΩ carbon composition resistor

The character A from a FT232RQ at 269531 bps. Cursor: 1 start + 8 data = 9 bits → 3.71 µs/bit → 269461 bps
Measurements with a SFH 213 photodiode
Samsung 244 T BLU PWM

Chromoflex 3

Sven Killig 2010