How Reflection Works in Physics

Definition of Reflection in Physics

a woman staring ahead next to her reflection

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In physics, reflection is defined as the change in the direction of a wavefront at the interface between two different media, bouncing the wavefront back into the original medium. A common example of reflection is reflected light from a mirror or a still pool of water, but reflection affects other types of waves beside light. Water waves, sound waves, particle waves, and seismic waves may also be reflected.

The Law of Reflection

a diagram illustrating the law of reflection

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The law of reflection is usually explained in terms of a ray of light striking a mirror, but it applies to other types of waves as well. According to the law of reflection, an incident ray strikes a surface at a certain angle relative to the "normal" (line perpendicular to the mirror's surface).

The angle of reflection is the angle between the reflected ray and the normal and is equal in magnitude to the angle of incidence, but is on the opposite side of the normal. The angle of incidence and angle of reflection lie in the same plane. The law of reflection can be derived from the Fresnel equations.

The law of reflection is used in physics to identify the location of an image that is reflected in a mirror. One consequence of the law is that if you view a person (or other creature) through a mirror and can see his eyes, you know from the way reflection works that he can also view your eyes.

Types of Reflections

a woman standing in front of a mirror with infinite reflections

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The law of reflection works for specular surfaces, which means surfaces which are shiny or mirror-like. Specular reflection from a flat surface forms mirror mages, which appear to be reversed from left to right. Specular reflection from curved surfaces may be magnified or demagnified, depending on whether the surface is spherical or parabolic.

Diffuse Reflections

Waves can also strike non-shiny surfaces, which produce diffuse reflections. In diffuse reflection, light is scattered in multiple directions because of tiny irregularities in the surface of the medium. A clear image is not formed.

Infinite Reflections

If two mirrors are placed facing each other and parallel to each other, infinite images are formed along the straight line. If a square is formed with four mirrors face to face, the infinite images appear to be arranged within a plane. In reality, images aren't truly infinite because tiny imperfections in the mirror surface eventually propagate and extinguish the image.

Retroreflection

In retroreflection, light returns in the direction from whence it came. A simple way to make a retroreflector is to form a corner reflector, with three mirrors faced mutually perpendicular to each other. The second mirror produces an image that is the inverse of the first. The third mirror makes in an inverse of the image from the second mirror, returning it to its original configuration. The tapetum lucidum in some animal eyes acts as a retroreflector (e.g., in cats), improving their night vision.

Complex Conjugate Reflection or Phase Conjugation

Complex conjugate reflection occurs when light reflects back exactly in the direction from whence it came (as in retroreflection), but both the wavefront and the direction are reversed. This occurs in nonlinear optics. Conjugate reflectors may be used to remove aberrations by reflecting a beam and passing the reflection back through the aberrating optics.

Neutron, Sound, and Seismic Reflections

an anechoic chamber

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Reflections occur in several types of waves. Light reflection doesn't only happen within the visible spectrum but throughout the electromagnetic spectrum. VHF reflection is used for radio transmission. Gamma rays and x-rays may be reflected, too, although the nature of the "mirror" is different than for visible light.

The reflection of sound waves is a fundamental principle in acoustics. Reflection is somewhat different from sound. If a longitudinal sound wave strikes a flat surface, the reflected sound is coherent if the size of the reflecting surface is large compared to the wavelength of the sound.

The nature of the material matters as well as its dimensions. Porous materials may absorb sonic energy, while rough materials (with respect to wavelength) may scatter sound in multiple directions. The principles are used to make anechoic rooms, noise barriers, and concert halls. Sonar is also based on sound reflection.

Seismologists study seismic waves, which are waves that may be produced by explosions or earthquakes. Layers in the Earth reflect these waves, helping scientists understand the Earth's structure, pinpoint the source of the waves, and identify valuable resources.

Streams of particles may be reflected as waves. For example, neutron reflection off of atoms may be used to map internal structure. Neutron reflection also is used in nuclear weapons and reactors.