Reflection (physics)

Reflection happens when a wavefront hits the boundary between two different materials and bounces back into the original material. This can be seen with light, sound, and water waves. For specular reflection—like what happens at a mirror—the angle at which the wave arrives at the surface equals the angle at which it leaves, a rule known as the law of reflection. Beyond visible light, reflection occurs with many electromagnetic waves, including VHF and higher frequencies used in radio and radar, as well as hard X-rays and gamma rays, which can reflect off special grazing mirrors at shallow angles.

In acoustics, reflection creates echoes and is key to sonar. Geologists rely on it to study seismic waves. Water waves also reflect off surfaces. For light, reflection can be specular (mirror-like, preserving an image) or diffuse (scattering energy but losing the image), depending on the surface. A typical mirror is a glass sheet with a metallic coating where most reflection happens; metals reflect well because waves can't propagate far beyond their skin depth. Transparent materials like water or glass also reflect light, though less effectively.

When a light ray hits a mirror, the incident and reflected rays are measured relative to an imaginary line called the normal, which is perpendicular to the surface. The angle of incidence equals the angle of reflection. The reflected wave's vector has the same magnitude as the incident wave's, but its projection on the normal is reversed. Reflection occurs whenever light moves between materials with different refractive indices; some light reflects, and the rest refracts. The Fresnel equations, derived from Maxwell's equations, predict how much reflects versus refracts—similar to how impedance mismatch causes signal reflection in circuits. If light travels from a denser to a less dense medium and the angle of incidence exceeds the critical angle, total internal reflection happens. This principle is used to focus waves that ordinary mirrors can't handle, like in X-ray telescopes, which use a converging tunnel to reflect X-rays at shallow angles toward a detector.

When light reflects off a material with a higher refractive index than the one it's traveling through, it undergoes a 180° phase shift. If the material has a lower refractive index, the reflected light stays in phase with the incident light—an important idea in thin-film optics. Specular reflection from a flat surface creates a mirror image that appears reversed left-to-right because we compare it to what we'd see if rotated into the image's position. Curved surfaces, like spherical or parabolic mirrors, can magnify or demagnify images and have optical power.

The laws of reflection state that the incident ray, reflected ray, and normal all lie in the same plane; the angle of incidence equals the angle of reflection; and the incident and reflected rays are on opposite sides of the normal. These laws can be derived from the Fresnel equations. In classical electrodynamics, light is an electromagnetic wave described by Maxwell's equations. When light hits a material, it makes atoms or electrons oscillate, radiating secondary waves in all directions—like tiny dipole antennas. These waves combine to produce specular reflection and refraction, as explained by the Huygens–Fresnel principle. In dielectrics like glass, the electric field moves electrons, which then generate new fields; the refracted light is the sum of forward radiation from electrons and the incident light, while reflected light comes from their backward radiation. In metals, free electrons oscillate with the incident light, and the phase difference between their radiation and the incident field determines the reflection.