Reflection is the change in direction of a wavefront at an interface between two different
media so that the wavefront returns into the medium from which it originated.
Common examples include the reflection of light, sound and water waves.
The law of reflection says that for specular reflection the angle at which the wave is
incident on the surface equals the angle at which it is reflected.
Mirrors exhibit specular reflection.
In acoustics, reflection causes echoes and is used in sonar.
In geology, it is important in the study of seismic waves.
Reflection is observed with surface waves in bodies of water.
Reflection is observed with many types of electromagnetic wave, besides visible light.
Reflection of VHF and higher frequencies is important for radio transmission and for radar.
Even hard X-rays and gamma rays can be reflected at shallow angles with special "grazing" mirrors.
Reflection of light
Reflection of light is either specular or diffuse 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 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 reflection actually 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.
In the diagram at left, 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.
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 above 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.
A conventional reflector would be useless as the X-rays would simply pass through the
intended reflector.
When light reflects off a material denser than the external medium, it undergoes a polarity
inversion.
In contrast, a less dense, lower refractive index material will reflect light in phase.
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 the classical electrodynamics, light is
considered as an electromagnetic wave, which is described by the Maxwell Equations.
Light waves incident on a material induce small oscillations of polarisation in the
individual atoms, causing each particle to radiate a small secondary wave.
All these waves add up to give specular reflection and refraction, according to the Huygens–Fresnel
principle.
In case of dielectric, the electric field of the light acts on the electrons in the
glass, the moving electrons generate a field and become a new radiator.
The refraction light in the glass is the combined of the forward radiation of the electrons
and the incident light and; the backward radiation is the one we see reflected from the surface
of transparent materials, this radiation comes from everywhere in the glass, but it turns
out that the total effect is equivalent to a reflection from the surface.
In metals, the electrons with no binding energy are called free electrons.
The density number of the free electrons is very large.
When these electrons oscillate with the incident light, the phase differences between the radiation
field of these electrons and the incident field are , so the forward radiation will
compensate the incident light at a skin depth, 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
When light strikes the surface of a material it bounces off in all directions due to multiple
reflections by the microscopic irregularities inside the 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 or radiance 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, 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.
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.
Complex conjugate reflection Light bounces exactly back in the direction
from which it came due to a nonlinear optical process.
In this type of reflection, 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.
Other types of reflection Neutron reflection
Materials that reflect neutrons, for example beryllium, are used in nuclear reactors and
nuclear weapons.
In the physical and biological sciences, the reflection of neutrons off of atoms within
a material is commonly used to determine the material's internal structure.
Sound reflection
When a longitudinal sound wave strikes a flat surface, sound is reflected in a coherent
manner provided that the dimension of the reflective surface is large compared to the
wavelength of the sound.
Note that audible sound has a very wide frequency range, and thus a very wide range of wavelengths.
As a result, the overall nature of the reflection varies according to the texture and structure
of the surface.
For example, porous materials will absorb some energy, and rough materials tend to reflect
in many directions—to scatter the energy, rather than to reflect it coherently.
This leads into the field of architectural acoustics, because the nature of these reflections
is critical to the auditory feel of a space.
In the theory of exterior noise mitigation, reflective surface size mildly detracts from
the concept of a noise barrier by reflecting some of the sound into the opposite direction.
Seismic reflection
Seismic waves produced by earthquakes or other sources may be reflected by layers within
the Earth.
Study of the deep reflections of waves generated by earthquakes has allowed seismologists to
determine the layered structure of the Earth.
Shallower reflections are used in reflection seismology to study the Earth's crust generally,
and in particular to prospect for petroleum and natural gas deposits.
See also
References
External links Acoustic reflection
Animations demonstrating optical reflection by QED
Simulation on Laws of Reflection of Sound By Amrita University