2.  Light or visible light is electromagnetic radiation that is visible to
the human eye, and is responsible for the sense of sight.[1] Visible light
has wavelength in a range from about 380 nanometres to about 740 nm,
with a frequency range of about 405 THz to 790 THz. In physics, the
term light sometimes refers to electromagnetic radiation of any
wavelength, whether visible or not.[2][3]
 Primary properties of light are intensity, propagation
direction, frequency or wavelength spectrum, and polarisation, while
its speed in a vacuum, 299,792,458 meters per second (about 300,000
kilometre per second), is one of the fundamental constants of nature.
 Light, which is emitted and absorbed in tiny "packets" called photons,
exhibits properties of both waves and particles. This property is
referred to as the wave–particle duality. The study of light, known
as optics, is an important research area in modern physics.

3.  The speed of light in a vacuum is defined to be exactly
299,792,458 m/s (approximately 186,282 miles per
second). The fixed value of the speed of light in SI
units results from the fact that the metre is now defined
in terms of the speed of light.

4.  Certain other mechanisms can produce light:
 scintillation
 electroluminescence
 Sonoluminescence
 triboluminescence
 Cherenkov radiation
 There are many sources of light. The most common
light sources are thermal: a body at a
given temperature emits a characteristic spectrum
of black-body radiation. The most common examples
include Sun, artificial lights, incandescent light bulbs

5.  There are three main theories on light
 Particle theory
 Wave theory
 Quantum theory

6.  Newton's particle theory of light says that light is made
up of little particles. They obey the same laws of
physics as other particles.
 The elementary particle according to particle theory of
light is photon
 evidence of light as particle


7.  Light can exhibit both a wave theory, and a particle
theory at the same time. Much of the time, light
behaves like a wave. Light waves are also called
electromagnetic waves because they are made up of
both electric (E) and magnetic (H) fields.
Electromagnetic fields oscillate perpendicular to the
direction of wave travel, and perpendicular to each
other. Light waves are known as transverse waves as
they oscillate in the direction traverse to the direction
of wave travel.

8.  The Electromagnetic Wave
 Waves have two important characteristics - wavelength and
frequency.
 Wave length .this is the distance between peaks of a wave.
Wavelengths are measured in units of length - meters, When
dealing with light, wavelengths are in the order
of nanometres (1 x 10-9)
Frequency: This is the number of peaks that will travel past
a point in one second. Frequency is measured in cycles per
second. The term given to this is Hertz (Hz) named after the
19th century discoverer of radio waves - Heinrich Hertz. 1
Hz = 1 cycle per second

9.  A third anomaly that arose in the late 19th century
involved a contradiction between the wave theory of
light and measurements of the electromagnetic
spectrum emitted by thermal radiators, or so-
called black bodies. Physicists struggled with this
problem, which later became known as the ultraviolet
catastrophe, unsuccessfully for many years.
 In 1900, Max Planck developed a new theory of black-
body radiation that explained the observed spectrum

10.  . Planck's theory was based on the idea that black bodies emit light (and
other electromagnetic radiation) only as discrete bundles or packets
of energy. These packets were called quanta, and the particle of light
was given the name photon, to correspond with other particles being
described around this time, such as the electron and proton. A photon
has an energy, E, proportional to its frequency, f, by
 where h is Planck's constant, λ is the wavelength and c is the speed of
light. Likewise, the momentum p of a photon is also proportional to its
frequency and inversely proportional to its wavelength:
 As it originally stood, this theory did not explain the simultaneous
wave- and particle-like natures of light, though Planck would later
work on theories that did. In 1918, Planck received the Nobel Prize in
Physics for his part in the founding of quantum theory.

11.  Reflection is the change in direction of a wave front at
an interface between two different media so that the wave front
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 secular reflection the angle at
which the wave is incident on the surface equals the angle at
which it is reflected. Mirrors exhibit secular 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.

12.  The bouncing back of light when it falls on an object is
called reflection of light.
 Reflection of light consist of incident ray ,reflected ray
and normal
 The ray that falls on a surface is called incident ray.
 The line perpendicular to the point of incidence is
called normal ray
 The ray that moves away after falling on a surface is
called reflected ray

13.  . 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.

14.  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 fibber 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 Lambert an 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.[1]

15.  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. [2] The centre 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. [3]

16.  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 wave fronts 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 abating optics a
second time

18.  A mirror whose polished, reflecting surface is a part of
a hollow sphere of glass or plastic is called a spherical
mirror.
 Depending upon the nature of the reflecting surface of
a mirror, the spherical mirror is classified as:
 Concave mirror
 Convex mirror

19.  A concave mirror, or converging mirror, has a reflecting
surface that bulges inward (away from the incident light).
Concave mirrors reflect light inward to one focal point.
They are used to focus light. Unlike convex mirrors,
concave mirrors show different image types depending on
the distance between the object and the mirror.
 These mirrors are called "converging" because they tend to
collect light that falls on them, refocusing parallel
incoming rays toward a focus. This is because the light is
reflected at different angles, since the normal to the surface
differs with each spot on the mirror.

20.  A convex mirror, fish eye mirror or diverging mirror, is
a curved mirror in which the reflective surface bulges
toward the light source. Convex mirrors reflect light
outwards, therefore they are not used to focus light. Such
mirrors always form a virtual image, since the focus (F) and
the centre of curvature (2F) are both imaginary points
"inside" the mirror, which cannot be reached. As a result,
images formed by these mirrors cannot be projected on a
screen, since the image is inside the mirror.
 A collimated (parallel) beam of light diverges (spreads out)
after reflection from a convex mirror, since the normal to
the surface differs with each spot on the mirror.

21.  A lens is an optical device with perfect or approximate axial
symmetry which transmits and refracts light, converging or
diverging the beam.[citation needed] A simple lens consists of a
single optical element. A compound lens is an array of
simple lenses (elements) with a common axis; the use of
multiple elements allows more optical aberrations to be
corrected than is possible with a single element. Lenses are
typically made of glass or transparent plastic. Elements
which refract electromagnetic radiation outside the visual
spectrum are also called lenses: for instance,
a microwave lens can be made from paraffin wax.
 The variant spelling lens is sometimes seen. While it is
listed as an alternative spelling in some dictionaries, most
mainstream dictionaries do not list it as acceptable

22.  Most lenses are spherical lenses: their two surfaces are parts of the surfaces of
spheres, with the lens axis ideally perpendicular to both surfaces. Each surface
can be convex (bulging outwards from the lens), concave (depressed into the
lens), or planar (flat). The line joining the centres of the spheres making up the
lens surfaces is called the axis of the lens. Typically the lens axis passes
through the physical centre of the lens, because of the way they are
manufactured. Lenses may be cut or ground after manufacturing to give them a
different shape or size. The lens axis may then not pass through the physical
centre of the lens.
 Tonic or sphere-cylindrical lenses have surfaces with two different radii of
curvature in two orthogonal planes. They have a different focal power in
different meridians. This is a form of deliberate astigmatism.
 More complex are aspheric lenses. These are lenses where one or both surfaces
have a shape that is neither spherical nor cylindrical. Such lenses can produce
images with much less aberration than standard simple lenses

23.  Lenses are classified by the curvature of the two optical surfaces.
A lens is biconvex (or double convex, or just convex) if both
surfaces are convex. If both surfaces have the same radius of
curvature, the lens is convex. A lens with two concave surfaces
is biconcave (or just concave). If one of the surfaces is flat, the
lens is Plano-convex or plane-concave depending on the
curvature of the other surface. A lens with one convex and one
concave side is convex-concave or meniscus. It is this type of
lens that is most commonly used in corrective lenses.
 If the lens is biconvex or plane-convex, a collimated beam of
light travelling parallel to the lens axis and passing through the
lens will be converged (or focused) to a spot on the axis, at a
certain distance behind the lens (known as the focal length). In
this case, the lens is called a positive or converging lens.

24.  If the lens is biconcave or plane-concave, a collimated beam of light
passing through the lens is diverged (spread); the lens is thus called
a negative or diverging lens. The beam after passing through the lens
appears to be emanating from a particular point on the axis in front of
the lens; the distance from this point to the lens is also known as the
focal length, although it is negative with respect to the focal length of a
converging lens. Convex-concave (meniscus) lenses can be either
positive or negative, depending on the relative curvatures of the two
surfaces. A negative meniscus lens has a steeper concave surface and
will be thinner at the centre than at the periphery. Conversely, a positive
meniscus lens has a steeper convex surface and will be thicker at the
centre than at the periphery. An ideal thin lens with two surfaces of
equal curvature would have zero optical power, meaning that it would
neither converge nor diverge light. All real lenses have a nonzero
thickness, however, which affects the optical power. To obtain exactly
zero optical power, a meniscus lens must have slightly unequal
curvatures to account for the effect of the lens' thickness

25.  Lenses are used as prosthetics for the correction of visual impairments such
as myopia, hyperopic, presbyopia, and astigmatism. (See corrective
lens, contact lens, eyeglasses.) Most lenses used for other purposes have
strict axial symmetry; eyeglass lenses are only approximately symmetric. They
are usually shaped to fit in a roughly oval, not circular, frame; the optical
centres are placed over the eyeballs; their curvature may not be axially
symmetric to correct for astigmatism. Sunglasses' lenses are designed to
attenuate light; sunglass lenses that also correct visual impairments can be
custom made.
 Other uses are in imaging systems such
as monocular, binoculars, telescopes, microscopes, cameras and projectors.
Some of these instruments produce a virtual image when applied to the human
eye; others produce a real image which can be captured on photographic
film or an optical sensor, or can be viewed on a screen. In these devices lenses
are sometimes paired up with curved mirrors to make a catadioptric
system where the lenses spherical aberration corrects the opposite aberration in
the mirror (such as Schmidt and meniscus correctors).

26.  Convex lenses produce an image of an object at infinity at their
focus; if the sun is imaged, much of the visible and infrared light
incident on the lens is concentrated into the small image. A large
lens will create enough intensity to burn a flammable object at
the focal point. Since ignition can be achieved even with a poorly
made lens, lenses have been used as burning-glasses for at least
2400 years.[16] A modern application is the use of relatively large
lenses to concentrate solar energy on relatively
small photovoltaic cells, harvesting more energy without the
need to use larger, more expensive, cells.
 Radio astronomy and radar systems often use dielectric lenses,
commonly called a lens antenna to refract electromagnetic
radiation into a collector antenna.
 Lenses can become scratched and abraded. Abrasion
resistant coatings are available to help control this.[17]

27. 
Refraction is the change in direction of a wave due to a change
in its speed. This is most commonly observed when a wave
passes from one medium to another at any angle other than 90°
or 0°. Refraction of light is the most commonly observed
phenomenon, but any type of wave can refract when it interacts
with a medium, for example when sound waves pass from one
medium into another or when water waves move into water of a
different depth. Refraction is described by Snell's law, which
states that the angle of incidence θ1 is related to the angle of
refraction θ2 by
 where v1 and v2 are the wave velocities in the respective media,
and n1 and n2 the refractive indices. In general, the incident wave
is partially refracted and partially reflected; the details of this
behaviour are described by the Fresnel equations.

28.  In optics, refraction occurs when waves travel from a
medium with a given refractive index to a medium with
another at an angle. At the boundary between the media, the
wave's phase velocity is altered, usually causing a change in
direction. Its wavelength increases or decreases but
its frequency remains constant. For example, a light ray will
refract as it enters and leaves glass, assuming there is a
change in refractive index. A ray traveling along the normal
(perpendicular to the boundary) will change speed, but not
direction. Refraction still occurs in this case. Understanding
of this concept led to the invention offenses and
the refracting telescope.

29.  In optics the refractive index or index of refraction of a substance or medium is a
measure of the speed of light in that medium. It is expressed as a ratio of the speed of
light in vacuum relative to that in the considered medium.[1] [2] [3] This can be written
mathematically as:
 n = speed of light in a vacuum / speed of light in medium. For example, the refractive
index of water is 1.33, meaning that light travels 1.33 times faster in vacuum than it does
in water. (See typical values of materials here).
 As light moves from a medium, such as air, water, or glass, into another it may change
its propagation direction in proportion to the change in refractive index.
This refraction is governed by Snell's law, and is illustrated in the figure to the right.
Refractive index of materials varies with the wavelength of light. This is
called dispersion and results in a slightly different refractive index for each colour.[4]
 The wavelength λ of light in a material is determined by the refractive index according
to λ = λ0 / n, where λ0 is the wavelength of the light in vacuum. Brewster's angle, the
critical angle for total internal reflection, and the reflectivity of a surface is also affected
by the refractive index. These material parameters can be calculated using the Fresnel
equations.[4]
 The concept of refractive index can be used with wave phenomena other than light,
e.g. sound. In this case the speed of sound is used instead of that of light and a reference
medium other than vacuum must be chosen.[5]

30.  The refractive index, n, of a medium is defined as the ratio of the
speed, c, of a wave phenomenon such as light or sound in a reference
medium to the phase speed, up, of the wave in the medium in question:
 It is most commonly used in the context of light with vacuum as a
reference medium, although historically other reference media
(e.g. air at a standardized pressure and temperature) have been
common. It is usually given the symbol n. In the case of light, it equals
 where is the material's relative permittivity, and or is its
relative permeability. For most naturally occurring materials, or is very
close to 1 at optical frequencies,[6] therefore n is approximately .
Contrary to a widespread misconception, the real part of a
complex n may be less than one, depending upon the material and
wavelength (see dispersion (optics)). This has practical technical
applications, such as effective mirrors for X-rays based on total
external reflection.

31.  The phase speed is defined as the rate at which the crests of the waveform propagate;
that is, the rate at which the phase of the waveform is moving. The group speed is the
rate at which the envelope of the waveform is propagating; that is, the rate of variation of
the amplitude of the waveform. Provided the waveform is not distorted significantly
during propagation, it is the group speed that represents the rate at which information
(and energy) may be transmitted by the wave (for example, the speed at which a pulse of
light travels down an optical fibber). For the analytic properties constraining the unequal
phase and group speeds in dispersive media, refer to dispersion (optics).
 Another common definition of the refractive index comes from the refraction of a light
ray entering a medium. The refractive index is the ratio of the sins of the angles of
incidence θ1 and refraction θ2 as light passes into the medium[7] or mathematically
 The angles are measured to the normal of the surface. This definition is base on Snell's
law and is equivalent to the definition above if the light enters from the reference
medium (normally vacuum).
 A complex refractive index is often used to take absorption into account. This is further
discussed in the Dispersion and absorption section below.
 A closely related quantity is refractivity, which in atmospheric applications is
denoted N and defined as N = 106(n - 1). The 106 factor is used because for
air, n deviates from unity at most a few parts per thousand.

32.  The refractive index of a material is the most important property of
any optical system that uses refraction. It is used to calculate the focusing
power of lenses, and the dispersive power of prisms. It can also be used as a
useful tool to differentiate between different types of gemstone, due to the
unique chatoyance each individual stone displays.
 Since refractive index is a fundamental physical property of a substance, it is
often used to identify a particular substance, confirm its purity, or measure its
concentration. Refractive index is used to measure solids (glasses and
gemstones), liquids, and gases. Most commonly it is used to measure the
concentration of a solute in an aqueous solution. A refract meter is the
instrument used to measure refractive index. For a solution of sugar, the
refractive index can be used to determine the sugar content (see Bricks).
 In GPS, the index of refraction is utilized in ray-tracing to account for the radio
propagation delay due to the Earth's electrically neutral atmosphere. It is also
used in Satellite link design for the Computation of radio wave attenuation in
the atmosphere