What is the principle of photo-chromatic glasses?

Photo-chromatic glasses become dark (activated) when kept under bright light especially when the wavelength of the incident light is in the range of 320-400 nm(nanometres).Darkened glasses get bleached when kept in the dark or under light of longer wavelenghts, 550-650 nm.Both these processes occur simultaneously during daylight and the glass's transparency depends on the predominant reaction taking place in it.
                     These processes are based on the possibility to split and recombine silver and chloride ions in silver chloride using light of different wavelenghts.It is the same principle which is used in photography also. However, in photography, the chlorine and silver ions are separated and not allowed to recombine. In photochromatic glasses, the chloride ions cannot diffuse far away from the silver ions as they are held tightly in a glass matrix. Hence when the 'activating energy' is withdrawn, they recombine to form silver halide. The speed(or rate) of the reaction depends on the doping level of the halide with a catalyst(cooper).
                   Choice of the halides, crystals' size (usually 40-100 Angstroms), concentration, glass type and sensitising dopants are all factors that decide the final behaviour of photochromatic glasses used in light-control, displays, memory applications and optical data processing.

In what way are spatial sound, wide sound and surround sound different from stereo sound?

Let's look at at the way a home music system work.The standard format for home music systems (or FM radio or even television) is the two-channel sound,which is otherwise known as Stereo sound.It is nothing but the two-channel recording,in which sound is played on speakers on either side of the receiver/listener.In the broader sense,stereo effect just recreates the experience of being present at the event.
          Surround sound refers to multi-channel systems.To be more specific, multi-channel systems designed by 'Dolby Laboratories'.It also refers to home theatre multi-channel audio systems, in which sound originates from multiple directions.
         Spatial sound is very much concerned with the concept of localisation.Humans and animals as well hear sound in three dimensions and the very fact is that the perception of spatial aspects of sound is essential to our survival.That is, Spatial waves or sound are well recognised by person since his ear-body-brain combination correctly decodes waves. Assume you are listening to a recorded sound where there is a lack of sensational ambiance, a separation of the instruments and a perception of context of the performance.
        A keen observation reveals that something is missing here. Yes. The missing stuff happens to the Spatial effect or detail. To quote an example, both Virtual Reality (VR) and High Definition Television(HDTV) require spatial sound reproduction apart from what ordinary stereo can offer.But there remains no proof that no commercially available system can successfully convey the natural spatial hearing experience or effect.
       Wide sound is more or less similar to Surround sound. To be more precise, it's a type of sound that is intermediate between stereo and surround effect. The frequency range lies between that of stereo and surround ones.

How does a compact fluorescent lamp consume less electricity than conventional fluorescent lamps and bulbs?

Fluorescent lamps are based on the phenomenon of gas discharge between two electrodes at the ends of the glass tube.Generally these tubes contain a little mercury in the low-vapour phase.When sufficiently large voltage is appiled between the electrodes,some atoms of the vapour get ionised.
The process of ionisation starts with stray electrons and ions that are generally present in the vapour.The electron-ion pair so formed gets accelerated towards electrodes of opposite electrical polarity,gaining kinetic energy.When they collide neutral mercury atoms,some of them are ionised and some are electronically exicted.Excited atoms release their energy in the form of electromagnetic radiation,part of which is in the visible and in the infrared regions of the spectrum.But it is rich in the visible ultraviolet region.
         A fluorescent light source has the inner surface of its glass tube painted with a material called phosphor.Zinc sulphide is the commonest example of a phosphor.But phosphor used in particle are complex mixtures of the sulphides and phosphates of barium,strontium and rare earth elements.These phosphors have the property of absorbing ultraviolet component of the radiation and re-emitting a major fractrion of the corresponding energy in the form of visible light.This enhances the lamp's efficiency of converting electrical energy into visible light.
         The ordinary fluorescent lamp works with a supply voltage of about 220 volt.Since the start of the discharge process demands a little higher voltage,it also employs a starter and ballast(a choke coil) that together produce the desired voltage.On the other hand, the compact tube works at about 400 volt(constant), which is produced by a transformer arrangement embedded in its base.
         Higher efficiency means low consumption of electrical energy.A filament lamp has the lowest efficiency, because is is based on the fact that a material body heated to a high temerature emits radition of all wavelenghts.This radition is richer in the infrared part of the spectrum and since there is no mechanism of converting this into visible light, it has poor efficiency.The three types of lamps may have a typical efficiency ratio of 8:6:3.

Why does a mushroom shaped cloud form after a nuclear bomb explosion?

When a nuclear weapon explodes,there is a rapid release of a large amount of energy within a small volume.This results in significant increase in temperature and pressure.The temperature may be a few tens of million degrees and pressure a few million times the atmospheric pressure.At this temperature,all the material present in the weapon will be converted into hot compressed gases.
Within a fraction of a millionth of a second of the explosion, the weapon's residues emit large amounts of energy mainly in the form of X-rays.The surrounding atmosphere absorbs this energy.This results in the formation of a blazzing, highly luminous, spherical mass of air and gaseous weapon residue called the fireball.
             Within an extremely short time after the explosion,the fireball from a high yield nuclear weapon will be about 130 meters across increasing to about 1700 meters in ten seconds.The fireball expands rapidly engulfing the surround air.
             The ball of hot air is less dense than the surrounding air.It rises swiftly like a hot air balloon.This rising column pulls up debris of the weapon,dust and moisture along with it forming cloud.As it moves up,it cools gradually and reaches about 10km where the atmosphere is extremely stable.The ball of air mass moving up does not have enough energy to penetrate this stable layer.It flattens out.As the relatively warmer layers at the bottom push up,the top layer spread laterally and equally in all directions and the cooler denser layer descend at he edges,giving a distint mushroom shape.
    

Why are all the planets in the universe spherical in shape?

Dip a finger into a bowl of water and then take it out.Now watch the drop of water as it trickles down the finger and drops down. You will notice that the drop of water acquires a spherical shape.Place a drop of mercury on a smooth surface.It attains a spherical shape,this phenomena is due to the cohesive force (force of attraction between the molecules of same element) which brings the molecule closer.
               But then why does it take a spherical shape, why not a cubic one? This is because, out of all multidimensional geometric figures, sphere has the least surface area. Thus all particles try their best to attain a spherical shape in order to reduce their surface area. It is for the same reason that when you pour any liquid in space they attain a spherical shape. But this is not the case with solids since their molecules are compactly packed and cannot be displaced. When a new star is born the gas clouds around it condenses   to form planets. The dense gas passes through three states, the gaseous state , the molten state and finally the rigid solid state. When a planet is in the first two states it attains a spherical structure due to the cohesive force as mentioned in the illustration. Later the molten planet solidifies into a rigid planet like Earth.

What is RDX?

RDX, widely used as an explosive and as a base charge for detonators,is the British abbreviation for Research Department Explosive or Royal Demolition Explosive . Chemists call it by various names such as cyclo-tri-methylene tri-nitramine, 2,4,6 tri-nitro N-methyl aniline, tetryl, hexa-hydro-1,2,3-trinitro-5-triazine, hexagon, T4 and cyclonite.
             RDX is odourless and so it can not be detected by conventional devices.Unlike other explosive, it carries fuel and oxidiser in the same molecule.On detonation it creates a powerful blast that sends out shock waves.It is used as energy boosting explosive in double base solid propellant where smokelessness is an advantage.
             RDX is prepared by reacting hexamethylene tetramine or its dinitrate salt with nitric acid.The famous Bachman process uses hexamine, nitric acid,ammonium nitrate and acetic anhydride.RDX can be easily moulded into any shape,for example, like a long thread or a sheet, and that is why it comes in handy for terrorists.

Is it necessary to change the water in an aquarium periodically?

Like all other creatures fishes too need oxygen for breathing.But unlike other creatures fishes respire by their pair of gills.Their gills separate the oxygen from water and release out carbon dioxide.In lake or pond there is not much considerable change in the Oxygen level because: in a huge amount of water only negligible ratio of oxygen is used.Direct and indirect rainwater keeps the oxygen ratio stable.
          Water plants like Hydrilla and Vallisneria use the carbon  dioxide and deliver oxygen during their photosynthetic activity.But in the aquarium there is less amount of water and there are no natural plants to maintain the oxygen level.Another reason for changing the water is to keep the water clean from fish waste in aquarium.