a signal reflection

Shout into a canyon and part of your voice bounces back as an echo. A signal travelling down a transmission line does the same thing: when it reaches the far end, if the load does not look exactly like the line, part of the wave turns around and travels back toward the source. That returning wave is a reflection, and it is the central villain of high-frequency design. On a poorly matched line you do not get one clean signal but the original plus its echoes, which add and subtract along the line.

How much bounces back is captured by the reflection coefficient, written with the Greek letter gamma. Its size is gamma = (ZL - Z0) / (ZL + Z0), where ZL is the load impedance and Z0 the line's characteristic impedance. Three cases tell the whole story. If ZL equals Z0, the top is zero, gamma is zero, and nothing reflects, the wave is fully absorbed. If the far end is an open circuit (ZL infinite), gamma is +1 and the whole wave bounces back in phase. If it is a dead short (ZL = 0), gamma is -1 and the whole wave bounces back inverted. A 50 ohm line feeding a 75 ohm load gives gamma = (75-50)/(75+50) = 25/125 = 0.2, so 20 percent of the wave's voltage reflects and 4 percent of its power is sent back.

Reflections matter because they waste power, distort signals, and can damage the source. On a digital line they cause ringing and false edges; on an RF power line a big reflection can overheat the transmitter's output stage. Reflections are also what build a standing wave on the line, measured as VSWR. The cure is impedance matching, making the load look like Z0 so gamma goes to zero. Honest caveat: a real load is rarely perfectly matched across a band, so some reflection almost always remains, and the engineer's job is to keep it acceptably small, not zero.