Reflection is the change in direction of energy from one point to another so that its motion is turned back toward the point of its origination. Common examples include the reflection of light, sound and water waves. The law of reflection says that for smooth surfaces, the angle at which light is incident on the surface equals the angle at which it is reflected.
Reflections may occur in a number of wave and particle phenomena; these include acoustic (as in common echos and underwater sonar), seismic waves in geologic structures, surface waves in bodies of water, and various electromagnetic waves, most usefully from VHF and higher radar frequencies, progressing upward through centimeter to millimeter-wavelength radar and the various light frequencies and (with special "grazing" mirrors, to X-ray frequencies and beyond to gamma rays.
Reflection of light
Reflection of light may be specular (that is, mirror-like) or diffuse (that is, not retaining the image, only the energy) depending on the nature of the interface. Furthermore, if the interface is between dielectric-conductor or dielectric-dielectric media, the phase of the reflected wave may or may not be inverted, respectively.
A mirror provides the most common model for specular light reflection, and typically consists of a glass sheet in front of a metallic coating where the reflection actually occurs. Reflection is enhanced in metals by suppression of wave propagation beyond their skin depths. It is also possible for reflection to occur from the surface of transparent media, such as water or glass.
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 reflected, 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 a 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 180° phase reversal. 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 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
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.
Other types of 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 cosine law.
Some surfaces exhibit retroreflection. The structure of these surfaces is such that light is returned in the direction from which it came. 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.
Materials that reflect neutrons, for example beryllium, are used in nuclear reactors. In the physical and biological sciences, the reflection of neutrons off atoms within a material is commonly used to determine its internal structures. 
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), 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 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.
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 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.
is the self-observation and reporting of conscious inner thoughts, desires and sensations. It is a conscious mental and usually purposive process relying on thinking, reasoning, and examining one's own thoughts, feelings, and, in more spiritual cases, one's soul. It can also be called contemplation of one's self, and is contrasted with extrospection, the observation of things external to one's self. Introspection may be used synonymously with self-reflection and used in a similar way.
Behaviorists claimed that introspection was unreliable and that the subject matter of scientific psychology should be strictly operationalized in an objective and measurable way. This then led psychology to focus on measurable behavior rather than consciousness or sensation. Cognitive psychology accepts the use of the scientific method, but rejects introspection as a valid method of investigation for this reason. Herbert Simon and Allen Newell identified the 'thinking-aloud' protocol, in which investigators view a subject engaged in introspection, and who speaks his thoughts aloud, thus allowing study of his introspection.
On the other hand, introspection can be considered a valid tool for the development of scientific hypotheses and theoretical models, in particular in cognitive sciences and engineering. In practice, functional (goal-oriented) computational modeling and computer simulation design of meta-reasoning and metacognition are closely connected with the introspective experiences of researchers and engineers.
Introspection was used by German physiologist Wilhelm Wundt in the experimental psychology laboratory he had founded in Leipzig in 1879. Wundt believed that by using introspection in his experiments he would gather information into how the subjects' minds were working, thus he wanted to examine the mind into its basic elements. Wundt did not invent this way of looking into an individual's mind through their experiences; rather, it can date to Socrates. Wundt's distinctive contribution was to take this method into the experimental arena and thus into the newly formed field of psychology.
In psychology, phenomenology is used to refer to subjective experiences or their study. The experiencing subject can be considered to be the person or self, for purposes of convenience. However, it is important to note that in phenomenological philosophy (and particularly in the work of Heidegger and Merleau-Ponty) 'experience' is a considerably more complex concept than it is usually taken to be in everday use. Instead, experience (or Being, or existence itself) is an 'in-relation-to' phenomena, and it is defined by qualities of directedness, embodiment and worldliness which are evoked by the term 'Being-in-the-World'.
Nevertheless, one abiding feature of 'experiences' is that, in principle, they are not directly observable by any external observer. The quality or nature of a given experience is often referred to by the term qualia, whose archetypical exemplar is "redness". For example, we might ask, "Is my experience of redness the same as yours?" While it is difficult to answer such a question in any concrete way, the concept of intersubjectivity is often used as a mechanism for understanding how it is that humans are able to empathise with one another's experiences, and indeed to engage in meaningful communication about them. The phenomenological formulation of Being-in-the-World, where person and world are mutually constitutive, is central here.
The concepts of phenomenological philosophy have influenced at least two main fields of contemporary psychology: the qualitative psychology of Giorgi, Smith , Kvale, and others; and the experimental approaches associated with Varela, Gallagher, Thompson, and others .
Difficulties in considering subjective phenomena
The philosophical psychology prevalent before the end of the nineteenth century relied heavily on introspection. The speculations concerning the mind based on those observations were criticized by the pioneering advocates of a more scientific approach to psychology, such as William James and the behaviorists Edward Thorndike, Clark Hull, John B. Watson, and B. F. Skinner. However, introspection is not intrinsically problematic, as Varela's attempts to train experimental participants in the structured 'introspection' of phenomenological reduction have demonstrated .
Philosophers have long confronted the problem of "qualia". Few philosophers believe that it is possible to be sure that one person's experience of the "redness" of an object is the same as another person's, even if both persons had effectively identical genetic and experiential histories. In principle, the same difficulty arises in feelings (the subjective experience of emotion), in the experience of effort, and especially in the "meaning" of concepts. As a result, many qualitative psychologists have claimed phenomenology inquiry to be essentially a matter of 'meaning-making' and thus a question to be addressed by interpretative approaches.
Psychotherapy and the phenomenology of emotion
Carl Rogers' person-centered psychotherapy theory is based directly on the "phenomenal field" personality theory of Combs and Snygg (1949). That theory in turn was grounded in phenomenological thinking. . Rogers attempts to put a therapist in closer contact with a person by listening to the person's report of their recent subjective experiences, especially emotions of which the person is not fully aware. For example, in relationships the problem at hand is often not based around what actually happened, but instead is based around the perceptions and feelings of each individual in the relationship. The phenomenal field focuses on "how one feels right now".
Introspections (also referred to as internal dialogue, interior monologue, self-talk) is the fiction-writing mode used to convey a character's thoughts. As explained by Renni Browne and Dave King, "One of the great gifts of literature is that it allows for the expression of unexpressed thoughts…". A character's thoughts can greatly enhance a story: deepening characterization, increasing tension, and widening the scope of a story. Thought plays a critical role in both scene and sequel. Among authors and writing coaches, there appears to be little consensus regarding the importance of introspection Fiction writing: The importance of your characters thoughts - Thoughts on Writing - Helium - by Mike Klaassen and how it is best presented.Fiction writing: How to write your characters thoughts - Writing Tips - Helium - by Mike Klaassen
- Frank C. Keil, Robert Andrew Wilson (eds) The MIT Encyclopedia of the Cognitive Sciences p.xx
- Fiction writing: The importance of your characters thoughts - Thoughts on Writing - Helium - by Mike Klaassen
- Fiction writing: How to write your characters thoughts - Writing Tips - Helium - by Mike Klaassen
- Schultz, D. P. & Schultz, S. E. (2004). A history of modern psychology (8th ed.). Belmont, CA: Wadsworth/Thomson Learning.
- Bickham, Jack M. (1993). Scene & Structure. Writer's Digest Books. ISBN 0-89879-551-6.
- Browne & King (2004). Self-Editing for Fiction Writers: How to Edit Yourself into Print. New York: Harper Resource. ISBN 0-06-054569-0
- Kress, Nancy (August 2003), Writer's Digest