2
3
Reflection (physics)
The reflection of Mount Hood in Trillium Lake.
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
Double reflection: The sun is reflected in the water, which is reflected in the paddle.
4
Reflection of light is either specular (mirror-like) or diffuse (retaining the energy, but losing the image) depending on the nature of the interface. Furthermore, if the interface is between a
dielectric and a conductor, the phase of the reflected wave is retained, otherwise if the interface is between two dielectrics, the phase may be retained or inverted, depending on the indices of
refraction. [ citation needed]
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.
Diagram of specular reflection
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, θ i and the angle of reflection, θ r. The law of reflection states that θ i = θ r, or in other words, the angle of incidence equals the angle of
reflection.
An Indian triggerfish reflecting in the water surface through total internal 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 5
can be used to predict how much of the light reflected, and how much is refracted in a given
situation. 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 focussing 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 denser (with higher refractive index) 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 regular reflection
Specular reflection at a curved surface of sea foam, which is made out of plankton
Main article: Specular 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:
1. The incident ray, the reflected ray and the normal to the reflection surface at the point of
the incidence lie in the same plane.
2. The angle which the incident ray makes with the normal is equal to the angle which the
reflected ray makes to the same normal.
3. Light paths are reversible.
6
[] Other types of reflection
[] Diffuse reflection
Diffuse reflection
Main article: Diffuse reflection
When light strikes a rough or granular surface, it bounces off in all directions due to the
microscopic irregularities of the interface. Thus, an 'image' is not formed. This is called diffuse
reflection. The exact form of the reflection depends on the structure of the surface. 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
[] Retroreflection
Working principle of a corner reflector
Main article: Retroreflector
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-
7
reflection is created by the refractive properties of the curved droplet's surface and reflective
properties at the backside of the droplet.
Some animal' s retinas act as retroreflectors, as this effectively improves the animal's 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.
[] 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.
[] 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 atoms within a material is commonly used to determine its internal structures. [1]
[] Sound reflection
Sound deflection panel for midrange frequencies
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 (from 20 to about 17000 Hz),
8
and thus a very wide range of wavelengths (from about 20 mm to 17 m). 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 (where rough is relative to the
wavelength) 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 (such as explosions) 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.
[] Quantum interpretation
Light waves incident on a material induce small oscillations of polarisation in the individual
atoms, causing each atom to radiate a weak secondary wave (in all directions like a dipole
antenna). All of these waves add up to specular reflection (following Hero's equi-angular reflection law) and refraction. 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.
Refraction
An image of the Golden Gate Bridge is refracted and bent by many differing three dimensional pools of water
9
Refraction in a Perspex (acrylic) block.
