30-05-2013, 02:47 PM
Electro Magnetic spectrum
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Regions of the spectrum
The types of electromagnetic radiation are broadly classified into the following classes:[3]
1. Gamma radiation
2. X-ray radiation
3. Ultraviolet radiation
4. Visible radiation
5. Infrared radiation
6. Terahertz radiation
7. Microwave radiation
8. Radio waves
Infrared radiation
The infrared part of the electromagnetic spectrum covers the range from roughly 300 GHz (1 mm) to 400 THz (750 nm). It can be divided into three parts:[3]
Far-infrared, from 300 GHz (1 mm) to 30 THz (10 μm). The lower part of this range may also be called microwaves. This radiation is typically absorbed by so-called rotational modes in gas-phase molecules, by molecular motions in liquids, and by phonons in solids. The water in Earth's atmosphere absorbs so strongly in this range that it renders the atmosphere in effect opaque. However, there are certain wavelength ranges ("windows") within the opaque range that allow partial transmission, and can be used for astronomy. The wavelength range from approximately 200 μm up to a few mm is often referred to as "sub-millimeter" in astronomy, reserving far infrared for wavelengths below 200 μm.
Mid-infrared, from 30 to 120 THz (10 to 2.5 μm). Hot objects (black-body radiators) can radiate strongly in this range, and human skin at normal body temperature radiates strongly at the lower end of this region. This radiation is absorbed by molecular vibrations, where the different atoms in a molecule vibrate around their equilibrium positions. This range is sometimes called the fingerprint region, since the mid-infrared absorption spectrum of a compound is very specific for that compound.
Near-infrared, from 120 to 400 THz (2,500 to 750 nm). Physical processes that are relevant for this range are similar to those for visible light. The highest frequences in this region can be detected directly by some types of photographic film, and by many types of solid state image sensors for infrared photography and videography.
Visible radiation (light)
Electromagnetic radiation with a wavelength between 380 nm and 760 nm (400-790 terahertz) is detected by the human eye and perceived as visible light. Other wavelengths, especially near infrared (longer than 760 nm) and ultraviolet (shorter than 380 nm) are also sometimes referred to as light, especially when the visibility to humans is not relevant. White light is a combination of lights of different wavelengths in the visible spectrum. Passing white light through a prism splits it up in to the several colors of light observed in the visible spectrum between 400 nm and 780 nm.
MAGNETISM
Some of the natural magnets which are the examples of Permanent magnets include lodestone or magnetite. A magnet always have two poles i.e. north and south poles.
A magnet with single pole never exists in nature.
A moving charge, or a current, creates a magnetic field around it.
1. The magnetic field exerts a force on a moving charge or a current in the field.
1. Electric charges attract or repel one another with a force inversely proportional to the square of the distance between them: unlike charges attract, like ones repel.
2. Magnetic poles (or states of polarization at individual points) attract or repel one another in a similar way and always come in pairs: every north pole is yoked to a south pole.
3. An electric current in a wire creates a circular magnetic field around the wire, its direction (clockwise or counter-clockwise) depending on that of the current.
4. A current is induced in a loop of wire when it is moved towards or away from a magnetic field, or a magnet is moved towards or away from it, the direction of current depending on that of the movement.
Scientists understood why forces acted the way they did when objects touched. The idea that confused them was forces that acted at a distance without touching. Think of examples such as gravitational force, electric force, and magnetic force. To help them explain what was happening, they used the idea of "field". They imagined that there was an area around the object, and anything that entered would feel a force. We say, for example, that the Moon has a gravitational field around it, and if you get close to the Moon, it will pull you down to its surface.