Overview

Reflection is the change in direction of a wave when it encounters a boundary between two different materials, causing part or all of the wave energy to travel back into the medium it came from. This behavior applies to many kinds of waves: mechanical and electromagnetic waves alike. Common, everyday examples include the bounce of visible light from a surface, echoes of sound, and the patterns seen when water waves strike a barrier.

Basic principles

The simplest statement of reflection is the law of reflection: the angle at which an incoming ray meets a surface equals the angle at which it departs, measured with respect to the surface normal. In geometric descriptions this applies to rays of light striking a plane mirror: the incident and reflected rays lie in the same plane and make equal angles with the perpendicular. For many wave problems, reflection follows from boundary conditions imposed by differing properties of the two media — for example, differing wave speed or acoustic impedance — which determine how much energy is reflected and how much is transmitted.

Types of reflection

  • Specular reflection: Mirror-like reflection where rays from a single direction are redirected into a single outgoing direction, producing clear images. Classical mirrors and polished metal surfaces show this behavior; see mirror applications.
  • Diffuse reflection: Occurs on rough surfaces that scatter incident waves in many directions, preventing the formation of sharp reflections and allowing objects to be visible from many angles.
  • Total internal reflection: A special case for waves that travel from a medium with higher wave speed to one with lower speed; beyond a critical angle all energy is reflected. This is widely used in optical fibers and some sensor designs.
  • Retroreflection: Structures that send light back toward its source, used on road signs and safety gear to improve visibility at night.

Physical determinants

The fraction of energy reflected depends on contrasts between media (for example, refractive index for light or acoustic impedance for sound), surface roughness relative to wavelength, and the wave's polarization. Phase changes on reflection can occur; for instance, light reflecting from a medium with a higher refractive index can undergo a half-wavelength phase shift. For guided waves or resonant systems, multiple reflections produce interference patterns and standing waves.

Applications and examples

Reflection is exploited in instruments and everyday devices. Optical systems such as reflecting telescopes rely on specular mirrors to form images, while remote sensing and navigation use reflected signals: radar and sonar detect objects by timing reflected pulses, and angle measurements are essential in surveying and optical metrology. Simple reflectors improve bicycle and vehicle visibility; designers use diffuse and specular surfaces to control lighting and minimize glare.

Historical and practical notes

Studies of reflection date back to antiquity with polished metal mirrors and early geometric treatments of light. The wave-based explanation and rigorous formulations of boundary conditions emerged later with the development of optics and acoustics. Today reflection remains a fundamental concept across physics and engineering, underlying imaging, communication, sensing, and architectural design.

Further reading and resources

  1. Introductory materials and demonstrations on wave behavior: media and boundaries.
  2. Practical guides to optical components and mirrors: mirror technology.
  3. Applications in remote sensing and measurement: waves, telescopes, and signal-based methods such as radar and sonar.