Physics
Wave Reflection
Quick fact
A wave reflecting from a fixed (rigid) boundary inverts its phase—the reflected wave is flipped upside down—while reflection from a free (flexible) boundary preserves the original phase. This is easy to observe with a rope or slinky.
Why this is interesting
Have you ever wondered why you see your reflection in a calm lake, or why a shout echoes back from a distant cliff? These everyday experiences are both caused by the same simple but powerful phenomenon: wave reflection.
Read the full explanation
Understanding Wave Reflection
Imagine sending a single pulse along a rope tied to a wall. The pulse travels toward the wall, hits it, and then returns toward you—this is reflection. The same happens with water waves: create a ripple in a pond, and when it reaches a wall or edge, it bounces back. In general, a wave reflection occurs when a wave encounters a change in the medium it's traveling through. The original wave is called the incident wave, and the returning wave is the reflected wave. The boundary is the surface or transition between two materials (like air and water, or air and a wall). The law of reflection states that the angle at which the wave hits the boundary (angle of incidence) equals the angle at which it leaves (angle of reflection), measured from an imaginary perpendicular line called the normal. This law applies to all wave types: light off a mirror, sound off a canyon wall, and water waves off a barrier.
A deeper explanation
Why does reflection happen? When a wave reaches a boundary, the properties of the medium change, causing an impedance mismatch. Some of the wave's energy is transmitted into the new medium, but the rest is sent back into the original medium as a reflected wave. The exact proportion depends on the difference in wave speed or density between the two media. For example, a sound wave reflects strongly off a hard concrete wall because air and concrete have very different acoustic impedances. Reflection is crucial for many technologies: echoes help sea vessels navigate using sonar; mirrors form images by reflecting light; radar uses radio wave reflections to detect objects; and medical ultrasound relies on reflections from inside the body to create images. A deeper detail: when a wave reflects off a fixed boundary (like a rope attached to a rigid post), the reflected pulse is inverted (phase shift of 180 degrees). Off a free boundary (like a rope loosely tied to a ring on a pole), the reflection is upright. This phase behavior is important in wave superposition and the formation of standing waves.