Light

In most everyday circumstances, the properties of light can be derived from the theory of classical electromagnetism, in which light is described as coupled electric and magnetic fields propagating through space as a traveling wave. However, this wave theory, developed in the mid-19th century, is not sufficient to explain the properties of light at very low intensities. At that level a quantum theory is needed to explain the interactions of light with atoms and molecules. In its simplest form, quantum theory describes light as consisting of discrete packets of energy, called photons. However, neither a classical wave model nor a classical particle model correctly describes light; light has a dual nature that is revealed only in quantum mechanics. This surprising wave-particle duality is shared by all of the primary constituents of nature (eg., electrons have both particle-like and wavelike aspects. Quantum electrodynamics (QED) has been regarded by physicists as complete. QED combines the ideas of classical electromagnetism, quantum mechanics, and the special theory of relativity.

While there is clear evidence that early civilizations used simple instruments such as plane, curved mirrors and convex lenses, ancient Greek philosophers thought about the nature of lights. Pythagoras (500 BCE) proposed that sight is caused by visual rays emanating from the eye and striking objects, whereas Empedocles (450 BCE) seems to have developed a model of vision in which light was emitted both by objects and the eye. Light is emitted by sources other than the eye and that vision is produced when light reflects off objects (Epicurus, 300 BCE). Euclid (300 BCE) presented a law of reflection and discussed the propagation of light rays in straight lines. With the decline of the Greco-Roman realm, scientific progress shifted to the Islamic world. Al-kindi extended the concept of rectilinearly propagating light rays and discussed the mechanism of vision. Ibn al Haytham (Optics , 1038) correctly attributed vision to the passive reception of light rays reflected from objects rather than an active emanation of light rays from the eye. He also drew detailed pictures of the optical components of the human eye. His work (Latinized as Alhazen) was translated into latine in the 13th century and was motivating influence on the Roger Becon . Becon studied the propagation of light through simple lenses and is credit as one of the first used lenses to correct vision. Newton discovered white light consists of mixture of colors ( in the 1660s) that we accept. Theoretical and experimental work in the mid to late 19th century convincingly established light as an electromagnetic wave, and the issue seeded to be resolved by 1900. With the arrival of quantum mechanics in the early decades of the 20th century, the controversy over the nature of light resurfaced. The scientific conflict between particle and wave models of light permeates the history of the subject.

Definition [1]. Light is the electromagnetic radiation that can be detected by human eye. Electromagnetic radiation occurs over an extremely wide range of wavelengths, from gamma rays with wavelengths less than about 1 x 10 exp – 11 metre to radio waves measured in metres. Within that broad spectrum the wavelengths visible to humans occupy a very narrow band, from about 700 nano-metres for red light down to about 400 nm for violet light. The spectral regions adjacent to the visible band are often referred to as light also, infrared at the one end and ultraviolet at the other. The speed of light in a vacuum is a fundamental physical constant, the currently accepted value of which is exactly 299, 792, 458 meters per second , or about 186, 282 miles per second.

Definition [2] . Through the sense of sight, light is a primary tool for perceiving the world and communicating within it. Light from the sun warms the earth, drives global weather patterns, and initiate the life-sustaining process of photosynthesis. On the grandest scale, light's interaction with the matter have helped shape the structure of the universe. Indeed, light provides a window on the universe, from cosmological to atomic scales. Almost all of the information about the rest of the universe reaches earth in the form of electromagnetic radiation. By interpreting that radiation, astronomers can glimpse the earliest epochs of the universe, measure the general expansion of the universe, and determine the chemical composition of stars and the interstellar medium. The analysis of the frequencies of light emitted and absorbed by atoms was a principal impetus for the development of quantum mechanics. Atomic and molecular spectroscopies continue to be primary tools for probing the structure of matter, providing ultrasensitive tests of atomic and molecular models of fundamental photochemical reactions.

Light transmits spatial and temporal information. This property forms the basis of the fields of optics and optical communications and a myriad of related technologies, both mature and emerging. Technological applications of light include lasers, holography, and fibre-optic telecommunications systems.

Light rays change direction when they reflect off a surface (reflection), move from one transparent medium into another (refraction), or travel through a medium whose composition is continuously changing.

Reflection: angel of incidence (θ1) = angel of reflection (θ2).

Refraction: (Snel's law) describes the relationship between the angle of incidence (θ1) and the angle of refraction(θ2). n1 sin θ1= n2 sin (θ2), where n1 and n2 are the index of refraction of the first and second media. The index of refraction for any medium is a dimensionless constant equal to the ratio of the speed of light in a vacuum to its speed in that medium.

One interesting consequence of the law of refractions associated with light passing into a medium with a lower index of refraction. In this case, light rays are bent away from the normal of the interface between the media. The critical angel of incident (θ) is 90 ◦ . Then sin θ = n2/n1. For any incident angle greater than the critical angle, light rays are completely reflected inside the material. This phenomenon is called the total internal reflection. When light is directed down a narrow fibre of glass or plastic, the light repeatedly reflects off the fibre-air interface at a large incident angle (for a glass-air interface the critical angle is about 42◦ . Optical fibres with diameters from 10 to 50 micrometers can transmit light over long distances with little loss of intensity. Optical communications uses sequences of light pulses to transmit information through an optical fibre network. Medical instruments such as endoscopes rely on total internal reflection of light through an optical fibre bundle to image internal organs.

Theory 3. As long as the physical dimensions of the objects that light encounters and the apertures through which it passes are significantly greater than the wavelength of the electromagnetic wave, there is no need for the mathematical formalism of the wave model and you can see the objects.

Theory 4. Wave is a disturbance that propagate through space. The disturbance is a physical displacement of the medium. The time dependence of the displacement at any single point is often an oscillation about some equilibrium position. Unlike particles, which have positions and trajectories, waves are not localized in space, and their evolutions in time are not described by simple trajectories. Two broad classes are (1) wave pulse which is relatively localized disturbance and (2) periodic wave which can extend over great distances. A simple useful example of a periodic wave is a harmonic wave which has frequency f , period τ and maximum displacement A. f = 1/ τ .

The wave velocity v is equal to the distance between crests λ multiplied by the frequency : v = λ f. The speed of light is the same in all reference frames. For a harmonic wave traveling in the x-direction, the spatial and time dependence of the displacement ф (x, t) = A cos ( 2 π x / λ - 2 π f t).

Theory 5. The superposition describes the behavior of overlapping waves. When two or more waves overlap in space, the resultant disturbance is equal to the algebraic sum of the individual disturbances. This simple underlying behavior leads to a number of effects that are collectively called interference phenomena. The superposition principle determines the resulting intensity pattern on the illuminated screen.

Most light sources emit a continuous range of wavelengths, which result in many overlapping interference patterns, each with a different fringe spacing. Detectors of light, including the eye, cannot register the quickly shifting interference patterns, and only a time-averaged intensity is observed. Laser light is approximately monochromatic (consisting of a single wavelength) and is highly coherent; it is thus an ideal source for revealing interference effects. Technological applications of interference effects in light are coatings, consisting of multiple layers of thin films, are designed to transmit light only within a narrow range of wavelengths and thus act as wavelength filters. Multilayer coatings are also used to enhance the reflectivity of mirrors in astronomical telescopes and in the optical cavities of lasers. The light intensity is about 5000 lux. The transport of energy by light plays critical role in life. About 10 exp 22 joules of solar radiant energy reaches earth each day. In turn, the earth continuously reradiates electromagnetic energy. Together, these energy-transport processes determine earth's energy balance, setting its average temperature and driving its global weather patterns. The transformation of solar energy into chemical energy by photosynthesis in plants maintains life on earth. Also as we will see, the transformation of solar energy into electric is important .The fossil fuels, natural gas, petroleum and coal are ultimately stored organic forms of solar energy deposited on earth millions of years ago. The electromagnetic-wave model of light accounts naturally for the origin of energy transport. In an electromagnetic wave, energy is stored in the electric and magnetic fields; as the fields propagate at the speed of light, the energy content is transported.

Theory6.The electric and magnetic fields of the wave exert forces onthe bound electrons of the atom, causing them to oscillate at the frequency of the wave. Oscillating charges are sources of electromagnetic radiation; the oscillating electrons radiate waves at the same frequency as the incoming fields. The electrons initially absorb energy from the incoming wave as they are set in motion, and they redirect that energy in the form of scattered light of the same frequency.

Theory 7. Assume Fc is the set of color lights and C is the set of colors. The function F : Fc → C gives the color for each color light( LED) and the reverse function F-1 : C → F gives the color light for each color(digital camera).

Collary 6. When the light ray strikes a light transistor, current flows.

Concolusion:

White Light is an electromagnetic wave, stored energy in its electric and magnetic fields, consists of mix of all colors, has light intensity of 5000 lux and can be transformed in other energy forms, photosynthesis and solar energy (electric current). Electric and magnetic fields and optic are unified in one relation(Maxwell equations). Optical communications uses sequences of light pulses to transmit information through an optical fibre network. Medical instruments such as endoscopes rely on total internal reflection of light through an optical fibre bundle to image internal organs. Laser light is approximately monochromatic (consisting of a single wavelength) and is highly coherent.