Infrared Radiations

Infrared (IR) light is electromagnetic radiation with a wavelength longer than that of visible light, measured from the nominal edge of visible red light at 0.74 micrometres (µm), and extending conventionally to 300 µm. These wavelengths correspond to a frequency range of approximately 1 to 400 THz, and include most of the thermal radiation emitted by objects near room temperature. Microscopically, IR light is typically emitted or absorbed by molecules when they change the irrotational-vibrational movements.

Infrared light is used in industrial, scientific, and medical applications. Night-vision devices using infrared illumination allow people or animals to be observed without the observer being detected. In astronomy, imaging at infrared wavelengths allows observation of objects obscured by interstellar dust. Infrared imaging cameras are used to detect heat loss in insulated systems, observe changing blood flow in the skin, and overheating of electrical apparatus.

Much of the energy from the Sun arrives on Earth in the form of infrared radiation. Sunlight at zenith provides an irradiance of just over 1 kilowatt per square meter at sea level. Of this energy, 527 watts is infrared radiation, 445 watts is visible light, and 32 watts is ultraviolet radiation. The balance between absorbed and emitted infrared radiation has a critical effect on the Earth's climate.

Main Applications: 

1. Night Vision: Infrared is used in night vision equipment when there is insufficient visible light to see. Night vision devices operate through a process involving the conversion of ambient light photons into electrons which are then amplified by a chemical and electrical process and then converted back into visible light. Infrared light sources can be used to augment the available ambient light for conversion by night vision devices, increasing in-the-dark visibility without actually using a visible light source. The use of infrared light and night vision devices should not be confused with thermal imaging which creates images based on differences in surface temperature by detecting infrared radiation (heat) that emanates from objects and their surrounding environment.
2. Hyperspectral Imaging:     
   A hyperspectral image, a basis for chemical imaging, is a "picture" containing continuous spectrum through a wide spectral range. Hyperspectral imaging is gaining importance in the applied spectroscopy particularly in the fields of NIR, SWIR, MWIR, and LWIR spectral regions. Typical applications include biological, mineralogical, defence, and industrial   measurements.Thermal Infrared Hyperspectral Camera can be applied similarly to a Thermographic camera, with the fundamental difference that each pixel contains a full LWIR spectrum. Consequently, chemical identification of the object can be performed without a need for an external light source such as the Sun or the Moon. Such cameras are typically applied for geological measurements, outdoor surveillance and UAV applications 
   
3. Heating: Infrared radiation can be used as a deliberate heating source. For example it is used in infrared saunas to heat the occupants, and also to remove ice from the wings of aircraft (de-icing). FIR is also gaining popularity as a safe heat therapy method of natural health care & physiotherapy. Infrared can be used in cooking and heating food as it predominantly heats the opaque, absorbent objects, rather than the air around them. Infrared heating is also becoming more popular in industrial manufacturing processes, e.g. curing of coatings, forming of plastics, annealing, plastic welding, print drying. In these applications, infrared heaters replace convection ovens and contact heating. Efficiency is achieved by matching the wavelength of the infrared heater to the absorption characteristics of the material.

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Rainbow

When Sunlight falls on small water droplets suspended in air during or after a rain, it suffers refraction, Total Internal Reflection(TIR) and dispersion. If an observer has the Sun at the back and water droplets in front of himself, then the person may see two rainbows one inside the other. The inner one is called primary rainbow and the outer one is called the secondary rainbow.

A ray of light when passing from air to water suffers refraction, TIR and again refraction. Dispersion takes place at both refractions. It turns out that rays of a given colour are strongly returned by a droplet in a direction tat corresponds to maximum deviation in its path. For red light, the max deviation is 137.8°. So the light comes out of the droplet at an angle of 180°-137.8°= 42.2°. This angle for violet is 40.6°.

The order of colours is Violet, Indigo, Blue, Green, Yellow, Orange & Red inside out. This is the primary rainbow.

The secondary rainbow is formed by rays which suffer two TIRs before coming out of the drop. Therefore, the order of colours in reversed.

Multiple rainbows:

Although most people will not notice it because they are not actively looking for it, a dim secondary rainbow is often present outside the primary bow. Secondary rainbows are caused by a double reflection of sunlight inside the raindrops, and appear at an angle of 50–53°. As a result of the second reflection, the colours of a secondary rainbow are inverted compared to the primary bow, with blue on the outside and red on the inside. The secondary rainbow is fainter than the primary because more light escapes from two reflections compared to one and because the rainbow itself is spread over a greater area of the sky. The dark area of unlit sky lying between the primary and secondary bows is called Alexander's band, afterAlexander of Aphrodisias who first described it.

Very dim tertiary (triple) and even quaternary (quadruple) rainbows have been photographed. These are caused by triple or quadruple reflections of sunlight inside the raindrops. Such rainbows appear on the same side of the sky as the sun, at about 40° from the sun for tertiary and 45° from the sun for quaternary rainbows. It is difficult to see these types of rainbows with the naked eye because of the sun's glare.

Higher-order rainbows were described by Felix Billet (1808–1882) who depicted angular positions up to the 19th-order rainbow, a pattern he called a "rose of rainbows". In the laboratory, it is possible to observe higher-order rainbows by using extremely bright and well collimated light produced by lasers. Up to the 200th-order rainbow was reported by Ng et al. in 1998 using a similar method but an argon ion laser beam.^[

Supernumerary rainbow:

A supernumerary rainbow—also known as a stacker rainbow—is an infrequent phenomenon, consisting of several faint rainbows on the inner side of the primary rainbow, and very rarely also outside the secondary rainbow. Supernumerary rainbows are slightly detached and have pastel colour bands that do not fit the usual pattern.

It is not possible to explain their existence using classical geometric optics. The alternating faint rainbows are caused by interference between rays of light following slightly different paths with slightly varying lengths within the raindrops. Some rays are in phase, reinforcing each other through constructive interference, creating a bright band; others are out of phase by up to half a wavelength, cancelling each other out through destructive interference, and creating a gap. Given the different angles of refraction for rays of different colours, the patterns of interference are slightly different for rays of different colours, so each bright band is differentiated in colour, creating a miniature rainbow. Supernumerary rainbows are clearest when raindrops are small and of similar size. The very existence of supernumerary rainbows was historically a first indication of the wave nature of light, and the first explanation was provided by Thomas Young in 1804.

Tertiary and quaternary rainbows

In addition to the primary and secondary rainbows seen in a direction opposite to the sun, it is also possible (but very rare) to see two faint rainbows in the direction of the sun. These are the tertiary and quaternary rainbows, formed by light that has reflected three and four times within the rain drops, respectively. Photographic evidence for the tertiary and quaternary rainbows was published, apparently for the first time, in 2011.

Reflected rainbow, reflection rainbow:

When a rainbow appears above a body of water, two complementary mirror bows may be seen below and above the horizon, originating from different light paths. Their names are slightly different. A reflected rainbow will appear as a mirror image in the water surface below the horizon, if the surface is quiet (see photo above). The sunlight is first deflected by the raindrops, and then reflected off the body of water, before reaching the observer. The reflected rainbow is frequently visible, at least partially, even in small puddles.

Where sunlight reflects off a body of water before reaching the raindrops, it may produce areflection rainbow (see photo at the right), if the water body is large, quiet over its entire surface, and close to the rain curtain. The reflection rainbow appears above the horizon. It intersects the normal rainbow at the horizon, and its arc reaches higher in the sky, with its centre as high above the horizon as the normal rainbow's centre is below it. Due to the combination of requirements, a reflection rainbow is rarely visible.

Six (or even eight) bows may be distinguished if the reflection of the reflection bow, and the secondary bow with its reflections happen to appear simultaneously.

Monochrome rainbow:

An unenhanced photo of a red (monochrome) rainbow.

Occasionally a shower may happen at sunrise or sunset, where the shorter wavelengths like blue and green have been scattered and essentially removed from the spectrum. Further scattering may occur due to the rain, and the result can be the rare and dramatic monochrome rainbow.

Rainbows under moonlight (Moonbows) are often perceived as white and may be thought of as monochrome. The full spectrum is present but our eyes are not normally sensitive enough to see the colours. So these are also classified (on the basis of how we see them) into seven coloured rainbow, three coloured rainbow and monochrome rainbow. Long exposure photographs will sometimes show the colour in this type of rainbow.

Circumhorizontal arc

The circumhorizontal arc is sometimes referred to by the misnomer "fire rainbow". As it originates in ice crystals, it is not a rainbow but a halo.

Rainbows on Titan

It has been suggested that rainbows might exist on Saturn's moon Titan, as it has a wet surface and humid clouds. The radius of a Titan rainbow would be about 49° instead of 42°, because the fluid in that cold environment is methane instead of water. A visitor might need infrared goggles to see the rainbow, as Titan's atmosphere is more transparent for those wavelengths.

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How to Spot Venus and Jupiter

Venus (on the left) and Jupiter in the evening sky in 2008.
 This week their positions will be reversed.  

After the moon, they are the two brightest objects in the night sky, and for the next few evenings they will appear side-by-side in western skies in a dazzling heavenly spectacle.

Though Jupiter is seven times farther from Earth than Venus, the planets' orbits bring them into close approach on Tuesday evening, when they will appear only three degrees, or a few finger-widths, apart.

From Tuesday, the planets will gradually move apart, but remain within five degrees of one another until Saturday, after which their next heavenly meeting in fully dark skies will be on June 2015. Venus, the second rock from the sun, appears by far the brighter of the two, because it receives and reflects more intense sunlight than reaches Jupiter, the fifth planet out, beyond the orbit of Mars.

With binoculars or an amateur telescope, stargazers might glimpse three or four moons in orbit around Jupiter. Observations of these moons 400 years ago prompted Galileo to declare that not all heavenly bodies orbited the Earth.

All of the planets in the solar system orbit the sun in more or less the same plane, so they appear on a line in the sky called the ecliptic. Draw a line through Jupiter and Venus and it will eventually lead to the reddish dot of Mars, and later Saturn, in the eastern night sky.

18th March 2012

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Radio Waves

Radio waves are a type of electromagnetic radiation with wavelengths in the electromagnetic spectrum longer than infrared light. Radio waves have frequencies from 300 GHz to as low as 3 kHz, and corresponding wavelengths from 1 millimeter to 100 kilometers. Like all other electromagnetic waves, they travel at the speed of light. Naturally occurring radio waves are made by lightning, or by astronomical objects. Artificially generated radio waves are used for fixed and mobile radio communication, broadcasting, radar and other navigation systems, satellite communication, computer networks and innumerable other applications. Different frequencies of radio waves have different propagation characteristics in the Earth's atmosphere; long waves may cover a part of the Earth very consistently, shorter waves can reflect off the ionosphere and travel around the world, and much shorter wavelengths bend or reflect very little and travel on a line of sight.

Radio waves were first predicted by mathematical work done in 1867 by James Clerk Maxwell. Maxwell noticed wavelike properties of light and similarities in electrical and magnetic observations. He then proposed equations that described light waves and radio waves as waves of electromagnetism that travel in space. In 1887, Heinrich Hertz demonstrated the reality of Maxwell's electromagnetic waves by experimentally generating radio waves in his laboratory. Many inventions followed, making practical the use of radio waves to transfer information through space. 

Magnetic resonance imaging (MRI) uses radio frequency waves to generate images of the human body, is one of the main applications of radio waves in medicine. 

In order to receive radio signals, for instance from AM/FM radio stations, a radio antenna must be used. However, since the antenna will pick up thousands of radio signals at a time, a radio tuner is necessary to tune in to a particular frequency (or frequency range). This is typically done via a resonator (in its simplest form, a circuit with a capacitor and an inductor). The resonator is configured to resonate at a particular frequency (or frequency band), thus amplifying sine waves at that radio frequency, while ignoring other sine waves. Usually, either the inductor or the capacitor of the resonator is adjustable, allowing the user to change the frequency at which it resonates.

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Electromagnetic Radiations

The electromagnetic spectrum is the range of all possible frequencies of electromagnetic radiation. The "electromagnetic spectrum" of an object is the characteristic distribution of electromagnetic radiation emitted or absorbed by that particular object.

The electromagnetic spectrum extends from low frequencies used for modern radio communication to gamma radiation at the short-wavelength (high-frequency) end, thereby covering wavelengths from thousands of kilometers down to a fraction of the size of an atom. It is for this reason that the electromagnetic spectrum is highly studied for spectroscopic purposes to characterize matter. The limit for long wavelength is the size of the universe itself, while it is thought that the short wavelength limit is in the vicinity of thePlanck length,although in principle the spectrum is infinite and continuous.

Propagation: Electromagnetic Waves are interactions of electric and magnetic fields perpendicular to each other. This enables the propagation of the waves even in vacuum. The velocity of the EM wave is always a constant which is equal to 299,792,458 m/s or simply 3 x 10^8m/s or approx. 186,282 miles/s.

Thus for different waves, there are ranges of frequencies and wavelengths, the velocity of light being given by    
  c = fλ,
where c is velocity of light,
f = frequency of the wave &
λ = wavelength of the wave.


On the basis of frequencies and wavelengths, the EM radiations is divided into different types of radiations.

Types of Radiations: 

1. Gamma radiation
2. X-ray radiation
3. Ultraviolet radiation
4. Visible radiation
5. Infrared radiation
6. Microwave radiation
7. Radio waves

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Newton's Laws of motion

Newton's laws of motion are three physical laws that form the basis for classical mechanics. They describe the relationship between the forces acting on a body and its motion due to those forces. They have been expressed in several different ways over nearly three centuries, and can be summarized as follows:

1. First law: Every body persists in its state of being at rest or of moving uniformly straight forward, except insofar as it is compelled to change its state by force impressed
2. Second law:  The second law states that the net force on a particle is equal to the time rate of change of its linear momentum p in an inertial reference frame
3. Third law:  To every action there is always an equal and opposite reaction: or the forces of two bodies on each other are always equal and are directed in opposite directions.

The three laws of motion were first compiled by Sir Isaac Newton in his work Philosophiæ Naturalis Principia Mathematica, first published in 1687. Newton used them to explain and investigate the motion of many physical objects and systems. For example, in the third volume of the text, Newton showed that these laws of motion, combined with his law of universal gravitation, explained Kepler's laws of planetary motion.

Newton's laws are applied to bodies (objects) which are considered or idealized as a particle, in the sense that the extent of the body is neglected in the evaluation of its motion, i.e., the object is small compared to the distances involved in the analysis, or the deformation and rotation of the body is of no importance in the analysis. Therefore, a planet can be idealized as a particle for analysis of its orbital motion around a star.

In their original form, Newton's laws of motion are not adequate to characterize the motion of rigid bodies and deformable bodies. Leonard Euler in 1750 introduced a generalization of Newton's laws of motion for rigid bodies called the Euler's laws of motion, later applied as well for deformable bodies assumed as a continuum. If a body is represented as an assemblage of discrete particles, each governed by Newton’s laws of motion, then Euler’s laws can be derived from Newton’s laws. Euler’s laws can, however, be taken as axioms describing the laws of motion for extended bodies, independently of any particle structure.

Newton's Laws hold only with respect to a certain set of frames of reference called Newtonian or inertial reference frames. Some authors interpret the first law as defining what an inertial reference frame is; from this point of view, the second law only holds when the observation is made from an inertial reference frame, and therefore the first law cannot be proved as a special case of the second. Other authors do treat the first law as a corollary of the second. The explicit concept of an inertial frame of reference was not developed until long after Newton's death.

In the given interpretation mass, acceleration, momentum, and (most importantly) force are assumed to be externally defined quantities. This is the most common, but not the only interpretation: one can consider the laws to be a definition of these quantities.

Newtonian mechanics has been superseded by special relativity, but it is still useful as an approximation when the speeds involved are much slower than the speed of light

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Cohesive and Adhesive Forces

Cohesion: Cohesion or cohesive attraction or cohesive force is the action or property of like molecules sticking together, being mutually attractive. This is an intrinsic property of a substance that is caused by the shape and structure of its molecules which makes the distribution of orbiting electrons irregular when molecules get close to one another, creating electrical attraction that can maintain a macroscopic structure such as a water drop.

Cohesion

Cohesion, along with adhesion (attraction between unlike molecules), helps explain phenomena such as meniscus, surface tension and capillary action.Water, for example, is strongly cohesive as each molecule may make four hydrogen bonds to other water molecules in a tetrahedral configuration. This results in a relatively strong Coulomb force between molecules. Van der Waals gases such as methane, however, have weak cohesion due only toVan der Waals forces that operate by induced polarity in non-polar molecules.

Mercury in a glass flask is a good example of the effects of the ratio between cohesive and adhesive forces. Because of its high cohesion and low adhesion to the glass, mercury does not spread out to cover the top of the flask, and if enough is placed in the flask to cover the bottom, it exhibits a stronglyconvex meniscus, where the meniscus of water is concave. Mercury will not wet the glass, unlike water and many other liquids, and if the glass is tipped, it will 'roll' around inside.

Adhesion: It is the tendency of dissimilar particles and/or surfaces to cling to one another. The forces that cause adhesion and cohesion can be divided into several types. The intermolecular forces responsible for the function of various kinds of stickers and sticky tape fall into the categories of chemical adhesion, dispersive adhesion, and diffusive adhesion. In addition to the cumulative magnitudes of these intermolecular forces, there are certain emergent mechanical effects that will also be discussed at the end of the article.

Adhesion

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