Reflection Definition

When two different media come together at an interface, a wavefront might change direction so that it returns to the first medium, which is known as reflection. The reflection of light, sound, and water waves are typical examples. According to the law of reflection, the angle at which the wave incident on the surface equals the angle at which it is reflected for specular reflection (such as at a mirror).

Reflection produces echoes in acoustics and is used in sonar. It is crucial to the study of seismic waves in geology. Surface waves in bodies of water are used to observe reflection. In addition to visible light, reflection is seen with several different types of electromagnetic waves. VHF and higher frequency reflection is crucial for radio transmission and radar. With the use of special "grazing" mirrors, even hard X-rays and gamma rays can be reflected at shallow angle.

Reflection of light

Depending on the nature of the interaction, light reflection is either specular (mirror-like) or diffuse (retaining energy but losing image). While the relative phase of s and p (TE and TM) polarisations is fixed by the characteristics of the media and of their interface in specular reflection, the phase of the reflected waves varies on the choice of coordinate origin.

The most basic example of specular light reflection is a mirror, which is normally a glass sheet with a metallic coating where the main reflection takes place. Metals reflect more light because wave propagation beyond their surface depths is suppressed. At the surface of transparent media, like water or glass, reflection also takes place.

The angles of incidence and reflection can be determined by imagining a line passing through point O that is perpendicular to the mirror; this imaginary line is called the normal. The angle of incidence must match the angle of reflection, according to the rule of reflection, which is expressed as I = r.

In actuality, light reflection can happen whenever it passes through a medium with one refractive index and enters another media with a different refractive index. In the most basic scenario, a portion of the light is refracted and the remainder is reflected from the interface. The Fresnel equations can be derived by solving Maxwell's equations for a light ray striking a boundary and can then be used to forecast how much of the light will be refracted and how much will be reflected in a particular situation. This is comparable to how impedance mismatch results in signal reflection in an electric circuit. If the angle of incidence is greater than the critical angle, total internal reflection of light from a denser medium occurs.

When conventional methods of reflection are ineffective, total internal reflection is utilised to focus the waves. Building an overlapping "tunnel" for the waves allows for the construction of X-ray telescopes. The waves are reflected towards the focus point as they come into low angle contact with the tunnel's surface (or towards another interaction with the tunnel surface, eventually being directed to the detector at the focus). Because the X-rays would just flow through the intended reflector, a typical reflector would be ineffective.

Light experiences a 180° phase shift when it reflects off of a substance with a greater refractive index than the medium it is passing through. The reflected light is in phase with the incident light when it bounces off of a substance with a lower refractive index, in contrast. This is a key idea in the area of thin-film optics.

Images are created by spherical reflection. Mirror images are created when light reflects off of a flat surface. Because we compare the image we see to what we would see if we were rotated into the position of the image, the mirror image appears to be reversed from right to left. Curved mirrors have optical power; specular reflection at a curved surface creates an image that can be enlarged or diminished. Such mirrors might have parabolic or spherical surfaces.

Laws of reflection

The reflection of light that takes place is known as specular or regular reflection if the reflecting surface is extremely smooth. The following are the laws of reflection:

1. At the point of incidence, the normal to the reflecting surface, the incident ray, and the reflected ray all lie in the same plane.
2. The incident ray's angle with the normal is identical to the reflected ray's angle with the same normal.
3. The incident and reflected rays are on different sides of the normal.

The Fresnel equations can be used to determine all three of these laws.

Mechanism

Light is seen in classical electrodynamics as an electromagnetic wave that can be represented by Maxwell's equations. Each particle in a material emits a tiny secondary wave in all directions like a dipole antenna as a result of the small polarisation oscillations that light waves impacting on the material cause in the individual atoms (or oscillations of the electrons in metals). The Huygens-Fresnel principle states that the sum of all these waves results in specular reflection and refraction.

When light interacts with the electrons in a dielectric medium like glass, new fields are created and new radiators are created as a result of the moving electrons. The incident light and the electrons' forward radiation combine to form the refracted light inside the glass. The sum of all the electrons' backward radiation is the light that is reflected.

Free electrons in metals are those that have no binding energy. The forward radiation cancels the incident light and the backward radiation is only the reflected light when these electrons oscillate with the incident light because of the 180° phase mismatch between their radiation field and the incident field. Richard Feynman elaborates on the subject of light-matter interaction in terms of photons in his best-selling book, "QED: The Strange Theory of Light and Matter."

Diffuse reflection

When light strikes the surface of a (non-metallic) material, it reflects multiple times from both the surface, if it is rough, and the microscopic irregularities inside the material (such as the boundaries between the grains of a polycrystalline material or the cell or fibre boundaries of an organic material). As a result, no "image" is created. Diffuse reflection is the name for this. The material's structure affects the reflection's precise shape. The Lambertian reflectance model, which is frequently used to describe diffuse reflection, assumes that light is reflected with equal brightness (in photometry) or radiance (in radiometry) in all directions. Our primary means of physical observation is by diffuse reflection from the surfaces of most things we observe, which sends light to our eyes.

Retroreflection

Retroreflection is shown by some surfaces. These surfaces' design allows light to return in the direction it originally came from.

The area surrounding an aircraft's shadow will appear brighter when flying over clouds that have received sunshine; a similar effect can also be noticed from dew on grass. The refractive qualities of the curved droplet's surface and the reflective qualities on the back of the droplet combine to produce this partial retro-reflection.

Some animals' retinas function as retroreflectors, which significantly enhances the animals' night vision. The effect is that the eyes work as a strong retroreflector, which is sometimes observed at night when roaming in the wild with a flashlight. This is because the lenses of their eyes affect the courses of the incoming and departing light in reciprocal fashion.

Three regular mirrors can be used to create a straightforward retroreflector by aligning them perpendicularly to one another (a corner reflector). The resultant image is the opposite of what a single mirror would produce. By adding a layer of tiny refractive spheres to a surface or by building tiny pyramid-shaped structures, a surface can be made somewhat retroreflective. In both situations, internal reflection causes the light to return to its original location.

This is done to make road signs and vehicle licence plates mostly reflect light back in the direction it originated from. Since the light would then be directed back into the headlights of an approaching car rather than into the driver's eyes, flawless retroreflection is not needed in this application.

Multiple reflections

One picture arises when light reflects off of a mirror. It appears as though there are an infinite number of pictures running in a straight line when two mirrors are put perfectly face to face. The circle is covered by the many pictures that can be viewed between two mirrors that are at an angle to one another. The imaginary point where the mirrors connect is where that circle's centre is situated. An endless number of pictures appear to be grouped in a plane when four mirrors are arranged in a square facing each other. Each pair of mirrors in the pyramid-shaped set of four mirrors is at an angle to the others, creating several images that cover the sphere. Images cover a portion of a torus if the pyramid's base is rectangular in shape.

Be aware that these are theoretical goals that call for perfectly aligned, perfectly flat, perfectly smooth reflectors that do not absorb any light. The impacts of any reflector surface flaws propagate and magnify, absorption gradually extinguishes the image, and any observation apparatus (biological or technological) will interfere, thus in fact, these conditions can only be approximated but not realised.

Complex conjugate reflection

Due to a nonlinear optical process, light bounces back exactly in the same direction it originally came from during this process (also known as phase conjugation). The wavefronts themselves are reversed in addition to the direction of the light. By reflecting a beam and then sending the reflection through the aberrating optics a second time, a conjugate reflector can be used to remove aberrations from a beam. A complicated conjugating mirror would seem black if you looked into it because only photons that left the pupil would enter the pupil.

Other type of reflection

Neutron reflection

Nuclear reactors and nuclear weapons both utilise materials like beryllium that reflect neutrons. Neutron reflection off of atoms within a substance is frequently utilised in the physical and biological sciences to ascertain the material's internal structure.

Sound reflection

When a longitudinal sound wave collides with a flat surface, sound is coherently reflected so long as the reflective surface's size is greater than the sound wave's wavelength. Keep in mind that the frequency range of audible sound (from 20 to around 17000 Hz) corresponds to a fairly broad range of wavelengths (from about 20 mm to 17 m). Because of this, the general nature of the reflection differs depending on the surface's roughness and structure. For instance, porous materials will absorb some energy, while rough materials (where rough is relative to the wavelength) have a tendency to reflect the energy in a variety of directions rather than coherently. The nature of these reflections is crucial to the auditory experience of a place, which introduces the topic of architectural acoustics. Reflective surface size mildly undermines the idea of a noise barrier in the notion of outside noise mitigation by reflecting part of the sound in the opposite direction. Acoustic space can be affected by sound reflection.

Seismic reflection

Layers within the Earth may reflect seismic waves generated by earthquakes or other sources (such as explosions). Seismologists have been able to identify the layered structure of the Earth by studying the deep reflections of earthquake waves. In reflection seismology, shallower reflections are used to analyse the Earth's crust in general and to search for natural gas and petroleum resources in particular.