Thermal Sources : Our most important natural source of light is the sun. Nearly all the natural sources of light we receive comes from the sun; moonlight is sunlight reflected from the surface of the moon. Distant stars provide an extremely small amount of light. Example of other thermal sources are a body at a given temperature emits a characterstic spectrum of black-body radiation, incandescent lamp, burning candle, etc. When object is heated until it glows or becomes incandescent, it emits all visible wavelengths along with large quantity of infrared radiation. Hence, as producers of visible radiation (i.e. luminous energy), they have a low efficiency. Generally, the efficiency of such light sources improves as the operating temperature is increased.
Gas Discharge Sources : Example of gas discharge source are neon lamp, sodium lamps, etc. In this case, light is obtained by maintaining electric current in a gas at low pressure. Such a source emits only a few wavelength. The color and intensity of light of light depends upon the nature of gas or vapor only. It may be noted that in case of light emitted by a thermal source, the spectrum is continuous. However, when light is obtained from a gaseous discharge, the spectrum is discontinuous i.e. it consists of one or more colored lines. For examples, in the case of sodium lamp, the spectrum consists mainly of two yellow lines very close together with wavelengths of 5890Ĺ. These wavelengths are so close to each other that light from a sodium lamp is said to be monochromatic i.e. a light having only one wavelength.
Luminescent Sources : The familiar example of such a source is the fluorescent tube. A fluorescent tube consist of a thin-walled glass tube with fluorescent substance coated on the inside of the tube. An electric current is maintained in mercury vapors at low pressure. It emits visible radiation as well as ultraviolet radiations (invisible). The fluorescent material absorbs ultraviolet radiation and re-emits them at longer wavelengths of the visible spectrum.
As the temperature increases, the peak shhifts to shorter wavelengths, producing first a red glow, then a white glow, and finally a blue-white color as the peak moves out of the spectrum and into the iltraviolet. These colors can be seen when metal is heated to "red hot" or "white hot". Blue-thermal emissions is not often seen, except in stars.
Atoms emits and absorb light at characterstic energies. Atoms produce "emission lines" in the spectrum of each atom. Emission lines are spontaneous as in light-emitting diodes, gas discharge lamps, and flames (light from hot gas itself - sodium is a gas flame that emits characterstic yellow light).
Properties of Visible Light
Primary properties of visible light (44% of the sun's electromagnetic radiation that reaches the ground is the visible light range) are
.speed in vacuum is 299,792,458 meters per second.
Visible light, as with all types of electromagnetic radiation (EMR) is experimentally found to move at exactly the same speed in vacuum. Visible light is emitted and absorbed by tiny packets called photons, and exhibits properties of both waves and particles. The study of light is known as optices. The study of light and interaction of light and matter is called optics. The observation and study of optical phenomina such as rainbows and aurora boredis offers many clues to the nature of the light. The effective velocity of lights in various transparent bodies containing ordinary matter is less than vacuum. Speed of light in water is 3/4th of that in vacuum slowing down due to absorption and re-emission.
Speed of Visible Light
The speed of light in a vacuum is defined to be exactly 186,280 miles per second. The fixed value of the speed in SI units from the fact that the metre is now defined in terms of speed of the light. All forms of EMR are supposed to move at exactly the same speed in vacuum.
EMR spectrum and visible light
The behavior of EMR depends on its wavelength. Higher frequencies have shorter wavelengths, and lower frequencies have longer wavelengths.
Refraction is bending of light rays when passing through a surface between one trasparent material to another. When a beam of light crosses the boundary between vacuum and another medium, or between two different media, the wavelength of light changes, but the frequency remains constant. If the beam of light is not orthogonal to the boundary, the change in wavelength results in a change in direction of r=the beam. This change in direction is known as refraction.
Types of Radiation
Deceleration of a free charged particle such as electron can produce following types of radiation:
.Cherenkov Radiation (parts moving faster than light)
Certain chemicals can produce visible radiation by chemoluminescence
Living organisms can produce visible radiation by bioluminescence
Boats moving through water can produce a glowing flake
Certain substances produce light when they are illuminated by energetic radiation, a process known as fluoroscence
Some substances emit light slowly after excitation by more energetic radiation, this is known as phosphoroscence
Certain other mecj=hanisms can produce light:
Units and Measures
Light is measured with two main alternative sets of units: radiometry consists of measurement of light power at all wavelengths, while photometry measures light with wavelength weighted with respect to standarised model of human brightness perception.
Table 1. SI radiometry units
Table 2. SI photometry units
Important Terms: Radiance, Spectral, Luminous
Importantant Parameters: Wavelength, Frequency, Surface Area
Importantant Attributes: Energy, Flux, Intensity, Emittance, Exposure, Density, Efficacy, Efficiencies
Light exerts physical pressure on objects in its path - particle photons strike and transfer their momentum.
Some Important Theories About Light
Light travels in straight lines
Beam from the eye
Light rays are taken to be stream of high velocity of atoms
Momentary flashes of light or energy
Light is a mechanical property of luminous body
Emitted in all directions from a source light travelled only in straight lines
Light was emitted in all directions as a series of waves in a medium call luminiferousether
The Light consists of waves
Different colors were caused bu different wavelengths of loight
Color vision in terms of three-colored recoptors in the eye
Behavior of light waves: reflection, refraction, diffraction, interference
The behavior between light and lighwaves is same as sound and soundwaves
Propagation of Light
The light is composed of tiny particles aw well as that it is made up of waves
Light is a form of electromagnetic radiation: light and similar forms of radiance are made up of moving electric and magnetic fields
Light radiates from its source in all directions until absorbed or diverted by some substance. The lines drawn from the light source to any point on one of these waves indicate the direction in which the waves are moving. These lines, called the radii of the spheres, are formed by the waves and are called light rays.
Although single rays of light do not exist, light "rays" as used in illustrations are convinient method to show the direction in which light is traveling at any point.
A large volume of light is called a beam; a narrow beam is called pencil; and the smallest portion of a pencil is called a light ray. A ray of light can be illustrated as a straight line. The straight line drawn from a light source will represent an infinite number of rays radiating in all directions from the source.
Frequencies and Color
The wavelength of a light deterines the color of the light. The light with a 700 millimicrons is red, and the light with a wavelength of 500 millimicrons is blue-green. The color of light depends upon frequency and not the wavelength.
Object absorbs all color except whose frequency corresponds to that particular color.
Those waves are reflected from the surface strike your eye and cause you to see the particular color.
The color of an object therefore depends on the frquency of the electro magnetic wave reflected.
Self-Luminous Bodies: Sun, flame, electric light filament
Non-Luminous Objects: become luminous in presence of other self-luminous bodies - they diffuse or reflect.
Black Objects: donot diffuse or deflect light. They are only visible only when outlined against a background of light from a luminous or diffusing body.
Property of Light
When light waves travel in straight lines through - transparent, translucent and opaque bodies.
In a medium
In a medium, light usually does not propagate at a speed equal to c; further, different types of light wave will travel at different speeds. The speed at which the individual crests and troughs of a plane wave (a wave filling the whole space, with only one frequency) propagate is called the phase velocity vp. An actual physical signal with a finite extent (a pulse of light) travels at a different speed. The largest part of the pulse travels at the group velocity vg, and its earliest part travels at the front velocity vf.
The phase velocity is important in determining how a light wave travels through a material or from one material to another. It is often represented in terms of a refractive index. The refractive index of a material is defined as the ratio of c to the phase velocity vp in the material: larger indices of refraction indicate lower speeds. The refractive index of a material may depend on the light's frequency, intensity, polarization, or direction of propagation; in many cases, though, it can be treated as a material-dependent constant. The refractive index of air is approximately 1.0003.] Denser media, such as water, glass,] and diamond,] have refractive indexes of around 1.3, 1.5 and 2.4, respectively, for visible light. In exotic materials like Bose-Einstein condensates near absolute zero, the effective speed of light may be only a few meters per second. However, this represents absorption and re-radiation delay between atoms, as does all slower-than-c speeds in material substances. As an extreme example of this, light "slowing" in matter, two independent teams of physicists claimed to bring light to a "complete standstill" by passing it through a Bose-Einstein Condensate of the element rubidium, one team at Harvard University and the Rowland Institute for Science in Cambridge, Mass., and the other at the Harvard-Smithsonian Center for Astrophysics, also in Cambridge. However, the popular description of light being "stopped" in these experiments refers only to light being stored in the excited states of atoms, then re-emitted at an arbitrarily later time, as stimulated by a second laser pulse. During the time it had "stopped," it had ceased to be light. This type of behaviour is generally microscopically true of all transparent media which "slow" the speed of light.]
In transparent materials, the refractive index generally is greater than 1, meaning that the phase velocity is less than c. In other materials, it is possible for the refractive index to become smaller than 1 for some frequencies; in some exotic materials it is even possible for the index of refraction to become negative. The requirement that causality is not violated implies that the real and imaginary parts of the dielectric constant of any material, corresponding respectively to the index of refraction and to the attenuation coefficient, are linked by the Kramers–Kronig relations. In practical terms, this means that in a material with refractive index less than 1, the absorption of the wave is so quick that no signal can be sent faster than c.
A pulse with different group and phase velocities (which occurs if the phase velocity is not the same for all the frequencies of the pulse) smears out over time, a process known as dispersion. Certain materials have an exceptionally low (or even zero) group velocity for light waves, a phenomenon called slow light, which has been confirmed in various experiments. The opposite, group velocities exceeding c, has also been shown in experiment. It should even be possible for the group velocity to become infinite or negative, with pulses travelling instantaneously or backwards in time.
None of these options, however, allow information to be transmitted faster
than c. It is impossible to transmit information with a light pulse any
faster than the speed of the earliest part of the pulse (the front velocity). It can
be shown that this is (under certain assumptions) always equal to c.
It is possible for a particle to travel through a medium faster than the phase velocity of light in that medium (but still slower than c). When a charged particle does that in a dielectric material, the electromagnetic equivalent of a shock wave, known as Cherenkov radiation, is emitted.