Reflection of light is either
specular (mirror-like) or
diffuse (retaining the
energy, but losing the image) depending on the nature of the interface. In specular reflection the
phase of the reflected waves depends on the choice of the origin of coordinates, but the relative phase between
s and p (TE and TM) polarizations is fixed by the properties of the media and of the interface between them. A mirror provides the most common model for specular light reflection, and typically consists of a glass sheet with a metallic coating where the significant reflection occurs. Reflection is enhanced in metals by suppression of wave propagation beyond their
skin depths. Reflection also occurs at the surface of
transparent media, such as water or
glass, although the reflection is generally less effective compared with mirrors. In the diagram, a
light ray PO strikes a vertical mirror at point
O, and the reflected ray is
OQ. By projecting an imaginary line through point
O perpendicular to the mirror, known as the
normal, we can measure the
angle of incidence,
θi and the
angle of reflection,
θr. The
law of reflection states that
θi =
θr, or in other words, the angle of incidence equals the angle of reflection. The
wave vector of the reflected wave is such that its
vector projection on the mirror normal is the negation of that of the incident wave vector while the magnitude (
wavenumber) is the same. In fact, reflection of light may occur whenever light travels from a medium of a given
refractive index into a medium with a different refractive index. In the most general case, a certain fraction of the light is reflected from the interface, and the remainder is
refracted. Solving
Maxwell's equations for a light ray striking a boundary allows the derivation of the
Fresnel equations, which can be used to predict how much of the light is reflected, and how much is refracted in a given situation. This is analogous to the way
impedance mismatch in an electric circuit causes reflection of signals.
Total internal reflection of light from a denser medium occurs if the angle of incidence is greater than the
critical angle. Total internal reflection is used as a means of focusing waves that cannot effectively be reflected by common means.
X-ray telescopes are constructed by creating a converging "tunnel" for the waves. As the waves interact at low angle with the surface of this tunnel they are reflected toward the focus point (or toward another interaction with the tunnel surface, eventually being directed to the detector at the focus). A conventional reflector would be useless as the X-rays would simply pass through the intended reflector. When light reflects off a material with higher refractive index than the medium in which is traveling, it
undergoes a 180° phase shift. In contrast, when light reflects off a material with lower refractive index the reflected light is
in phase with the incident light. This is an important principle in the field of
thin-film optics. Specular reflection forms
images. Reflection from a flat surface forms a
mirror image, which appears to be reversed from left to right because we compare the image we see to what we would see if we were rotated into the position of the image. Specular reflection at a curved surface forms an image which may be
magnified or demagnified;
curved mirrors have
optical power. Such mirrors may have surfaces that are
spherical or
parabolic.
Laws of reflection If the reflecting surface is very smooth, the reflection of light that occurs is called specular or regular reflection. The laws of reflection are as follows: • The incident ray, the reflected ray and the normal to the reflection surface at the point of the incidence lie in the same
plane. • The angle which the incident ray makes with the normal is equal to the angle which the reflected ray makes to the same normal. • The reflected ray and the incident ray are on the opposite sides of the normal. These three laws can all be derived from the
Fresnel equations.
Mechanism In
classical electrodynamics, light is considered as an electromagnetic wave, which is described by
Maxwell's equations. Light waves incident on a material induce small oscillations of
polarisation in the individual atoms (or oscillation of electrons, in metals), causing each particle to radiate a small secondary wave in all directions, like a
dipole antenna. All these waves add up to give specular reflection and refraction, according to the
Huygens–Fresnel principle. In the case of dielectrics such as glass, the electric field of the light acts on the electrons in the material, and the moving electrons generate fields and become new radiators. The refracted light in the glass is the combination of the forward radiation of the electrons and the incident light. The reflected light is the combination of the backward radiation of all of the electrons. In metals, electrons with no binding energy are called free electrons. When these electrons oscillate with the incident light, the phase difference between their radiation field and the incident field is π radians (180°), so the forward radiation cancels the incident light, and backward radiation is just the reflected light.
Light–matter interaction in terms of photons is a topic of
quantum electrodynamics, and is described in detail by
Richard Feynman in his popular book
QED: The Strange Theory of Light and Matter.
Diffuse reflection by a solid surface When light strikes the surface of a (non-metallic) material it bounces off in all directions due to multiple reflections by the microscopic irregularities
inside the material (e.g. the
grain boundaries of a
polycrystalline material, or the
cell or
fiber boundaries of an organic material) and by its surface, if it is rough. Thus, an 'image' is not formed. This is called
diffuse reflection. The exact form of the reflection depends on the structure of the material. One common model for diffuse reflection is
Lambertian reflectance, in which the light is reflected with equal
luminance (in photometry) or
radiance (in radiometry) in all directions, as defined by
Lambert's cosine law. The light sent to our eyes by most of the objects we see is due to diffuse reflection from their surface, so that this is our primary mechanism of physical observation.
Retroreflection Some surfaces exhibit
retroreflection. The structure of these surfaces is such that light is returned in the direction from which it came. When flying over clouds illuminated by sunlight the region seen around the aircraft's shadow will appear brighter, and a similar effect may be seen from dew on grass. This partial retro-reflection is created by the refractive properties of the curved droplet's surface and reflective properties at the backside of the droplet. Some animals'
retinas act as retroreflectors (see
tapetum lucidum for more detail), as this effectively improves the animals' night vision. Since the lenses of their eyes modify reciprocally the paths of the incoming and outgoing light the effect is that the eyes act as a strong retroreflector, sometimes seen at night when walking in wildlands with a flashlight. A simple retroreflector can be made by placing three ordinary mirrors mutually perpendicular to one another (a
corner reflector). The image produced is the inverse of one produced by a single mirror. A surface can be made partially retroreflective by depositing a layer of tiny refractive spheres on it or by creating small pyramid like structures. In both cases internal reflection causes the light to be reflected back to where it originated. This is used to make traffic signs and automobile license plates reflect light mostly back in the direction from which it came. In this application perfect retroreflection is not desired, since the light would then be directed back into the headlights of an oncoming car rather than to the driver's eyes.
Multiple reflections When light reflects off a
mirror, one image appears. Two mirrors placed exactly face to face give the appearance of an infinite number of images along a straight line. The multiple images seen between two mirrors that sit at an angle to each other lie over a circle. The center of that circle is located at the imaginary intersection of the mirrors. A square of four mirrors placed face to face give the appearance of an infinite number of images arranged in a plane. The multiple images seen between four mirrors assembling a pyramid, in which each pair of mirrors sits an angle to each other, lie over a sphere. If the base of the pyramid is rectangle shaped, the images spread over a section of a
torus. Note that these are theoretical ideals, requiring perfect alignment of perfectly smooth, perfectly flat perfect reflectors that absorb none of the light. In practice, these situations can only be approached but not achieved because the effects of any surface imperfections in the reflectors propagate and magnify, absorption gradually extinguishes the image, and any observing equipment (biological or technological) will interfere.
Complex conjugate reflection In this process (which is also known as phase conjugation), light bounces exactly back in the direction from which it came due to a nonlinear optical process. Not only the direction of the light is reversed, but the actual wavefronts are reversed as well. A
conjugate reflector can be used to remove
aberrations from a
beam by reflecting it and then passing the reflection through the aberrating optics a second time. If one were to look into a complex conjugating mirror, it would be black because only the photons which left the pupil would reach the pupil. ==Other types of reflection==