Law of Reflection
A light ray incident upon a reflective surface will be reflected at an angle equal to the incident
angle. Both angles are typically measured with respect to the normal to the surface. This law of
reflection can be derived from Fermat's principle.
The law of reflection gives the familiar reflected image in a plane
mirror where the image distance behind the mirror is the same as
the object distance in front of the mirror.
Transmission of Light
Light transmission is the percentage of incident light that passes through a film. ExxonMobil
generally evaluates this property for OPPalyte films.
Refraction of Light
Refraction is the bending of a wave when it enters a medium where it's speed is different. The
refraction of light when it passes from a fast medium to a slow medium bends the light ray
toward the normal to the boundary between the two media. The amount of bending depends on
the indices of refraction of the two media and is described quantitatively by Snell's Law.
As the speed of light is reduced in the slower medium, the wavelength is shortened
proportionately. The frequency is unchanged; it is a characteristic of the source of the light and
unaffected by medium changes.
The index of refraction is defined as the speed of light in vacuum divided by the speed of light in
The indices of refraction of some common substances are given below with a more complete
description of the indices for optical glasses given elsewhere. The values given are approximate
and do not account for the small variation of index with light wavelength which is called
Scattering is a general physical process where some forms of radiation, such as light, sound, or
moving particles, are forced to deviate from a straight trajectory by one or more localized non-
uniformities in the medium through which they pass. In conventional use, this also includes
deviation of reflected radiation from the angle predicted by the law of reflection. Reflections that
undergo scattering are often called diffuse reflections and unscattered reflections are called
specular (mirror-like) reflections.
A Feynman diagram of scattering between two electrons by emission of a virtual photon.
Fluorescence is luminescence that occurs where the energy is supplied by electromagnetic
radiation, usually ultraviolet light. The energy source kicks an electronn of an atom from a lower
energy state into an "excited" higher energy state; then the electron releases the energy in the
form of light (luminescence) when it falls back to a lower energy state.
Interference (wave propagation)
Interference is the addition (superposition) of two or more waves that results in a new wave
pattern. Interference usually refers to the interaction of waves that are correlated or coherent with
each other, either because they come from the same source or because they have the same or
nearly the same frequency. Interference in physics corresponds to what in wireless
communications is called multi-path propagation and fading, while the term interference has a
different meaning in wireless communications.
The principle of superposition of waves states that the resultant displacement at a point is equal
to the vector sum of the displacements of different waves at that point. If a crest of a wave meets
a crest of another wave at the same point then the crests interfere constructively and the resultant
wave amplitude is increased. If a crest of a wave meets a trough of another wave then they
interfere destructively, and the overall amplitude is decreased.
This form of interference can occur whenever a wave can propagate from a source to a
destination by two or more paths of different length. Two or more sources can only be used to
produce interference when there is a fixed phase relation between them, but in this case the
interference generated is the same as with a single source; see Huygens' principle.
The simplest manifestation of polarization to visualize is that of a plane wave, which is a good
approximation of most light waves (a plane wave is a wave with infinitely long and wide
wavefronts). For plane waves Maxwell's equations, specifically Gauss's laws, impose the
transversality requirement that the electric and magnetic field be perpendicular to the direction of
propagation and to each other. Conventionally, when considering polarization, the electric field
vector is described and the magnetic field is ignored since it is perpendicular to the electric field
and proportional to it. The electric field vector of a plane wave may be arbitrarily divided into
two perpendicular components labeled x and y (with z indicating the direction of travel). For a
simple harmonic wave, where the amplitude of the electric vector varies in a sinusoidal manner
in time, the two components have exactly the same frequency. However, these components have
two other defining characteristics that can differ. First, the two components may not have the
same amplitude. Second, the two components may not have the same phase, that is they may not
reach their maxima and minima at the same time. Mathematically, the electric field of a plane
wave can be written as,
where Ax and Ay are the amplitudes of the x and y directions and φ is the relative phase between
the two components.
Amplitude is the magnitude of change in the oscillating variable, with each oscillation, within an
oscillating system. For instance, sound waves are oscillations in atmospheric pressure and their
amplitudes are proportional to the change in pressure during one oscillation. If the variable
undergoes regular oscillations, and a graph of the system is drawn with the oscillating variable as
the vertical axis and time as the horizontal axis, the amplitude is visually represented by the
vertical distance between the extrema of the curve.
Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as
temporal frequency. The period is the duration of one cycle in a repeating event, so the period is
the reciprocal of the frequency.
Complete spectrum of electromagnetic radiation with the visible portion highlighted
Electromagnetic radiation is classified according to the frequency (or wavelength) of the light
wave. This includes (in order of increasing frequency): natural electromagnetic waves, radio
waves, microwaves, terahertz radiation, infrared (IR) radiation, visible light, ultraviolet (UV)
radiation, X-rays and gamma rays. Of these, natural electromagnetic waves have the longest
wavelengths and gamma rays have the shortest. A small window of frequencies, called the
visible spectrum or light, is sensed by the eye of various organisms, with variations of the limits
of this narrow spectrum.
intensity is the luminous or radiant power per unit solid angle. This can cause confusion in
optics, where intensity can mean any of radiant intensity, luminous intensity or irradiance,
depending on the background of the person using the term. Radiance is also sometimes called
intensity, especially by astronomers and astrophysicists, and in heat transfer.
The phase of an oscillation or wave is the fraction of a complete cycle corresponding to an offset
in the displacement from a specified reference point at time t = 0. Phase is a frequency domain or
Fourier transform domain concept, and as such, can be readily understood in terms of simple
harmonic motion. The same concept applies to wave motion, viewed either at a point in space
over an interval of time or across an interval of space at a moment in time. Simple harmonic
motion is a displacement that varies cyclically, as depicted to the right.
It is described by the formula:
where A is the amplitude of oscillation, f is the frequency, t is the elapsed time, and θ is the
phase of the oscillation. The phase determines or is determined by the initial displacement at
time t = 0. A motion with frequency f has period