Keshav Jain

 Only when lit by electrical lamps, table fans seem to rotate forwards and backwards. Wheels of cars running on the roads also give a similar illusion. These are a result of stroboscopic effect.

The illusion does not occur when the fan or wheel is lit by sunlight or candle light. This naturally leads us to a fundamental difference between these light sources.

Electrical lamps emit light according to the frequency (50 Hz) of the main supply (that is, the lamp is on for 10 milliseconds, goes off for the next 10 milliseconds, and the process repeats 50 times a second). But we don’t see the on-off processes because of our eye’s persistence of vision. (The eye has the ability to retain the impression of an image for short time even after the image has disappeared). But sunlight and candle light are continuous without periodically going off and on.

Hence when the light goes off, the eye involuntarily retains the image of the fan blades’ or wheel spokes’ position.

 Again when the light comes on, the fan’s blades would have moved to a new position and the eye records a new image. Depending on the speed the fan or wheel, the image retained by the eve gives us the illusion.

            Supposing we make a fan rotate at a speed of 50 rpms and place it under an electrical lamp, the fan would appear to be stationary. If the speed is different from 50 rpm, the fan seems to rotate slowly forwards or backwards depending on the speed.

Fan wings, also called blades, are curved for optimum air circulation which is determined by solidity ratio which is the ratio of the area of the blades to the area of the disc swept by them.

 If a flat plate is used as a blade, it will provide air circulation no doubt but the volumetric flow rate will be less compared to a blade which is suitably curved based on aerodynamic principles.

The cross-section of a blade is in the form of a circular arc and is called camber. It will vary from the root of the blade to its tip. One can see the blade twisted from the root to the tip.

The angle of attack (angle between the chord of the aerofoil and flow direction) will vary from the root to the tip. Engineers optimize these Para-meters, now-a-days using computer modeling, so that as the fan rotates there is enough air flow. The flow disc varies with rotational speed.

            The numbers of blades in fans vary between 2 and 4. Accordingly, their shapes differ some are slender and long while others are broad and short.

   A bird sitting on a live wire will get electrocuted only if electric current passes through its body. We can compare flow of electricity through a body to flow of water through a pipe or tube. Water will always flow from a higher level to a lower level. Similarly electric current will always flow or pass through from a higher potential or voltage level to a lower potential level. We can take two wires running on poles through a street. One wire which we can live will be at a potential of 230 volts which is called the phase wire and the other one which we call the neutral wire will be at a potential of zero volts. Immediately on sitting on the live wire the bird’s potential will also be raised to 230 volts and if by an accident it comes in contact with the neutral wire or touches it, a current will pass through its body from the live wire which is at a higher potential to the neutral wire which is at zero potential.

 Birds, animals, and human beings can withstand flow of a certain amount of electric current only through their bodies and excess flow of current will cause immediate death.

In the present case the flow of current throngs the bird’s body will be enormous and the bird will probably get burnt to death.

   If two crows, one sitting on the live wire and the other sitting on the neutral wire, happen to touch each other, both will meet the same fate as the bird mentioned above and will get electrocuted. But if both the sit on the live wire side by side and touch each other nothing will happen as both the crows will be at the same voltage level of 230 volts and there will be no flow of current through their bodies.

            Electric line testor is used for testing alternating current (AC). In an electric line, ‘phase’ line gives out AC which has both positive and negative components. Usually when current is allowed to pass through a bulb from the ‘phase’ to ‘neutral’, which is at lower potential, the bulb glows.

            In the case of a testor, when we touch its metal cap, a very small amount of current being tested passes through the neon bulb, a high resistance and through the body to the earth which is at zero potential.

            In other words the body helps to complete the circuit enabling the testor to glow. The high resistance inside the testor acts as a safety mechanism by restricting the amount of current passing through the body.

Radio was a technological revolution that changed forever the way we lived our lives. Its invention gave us a whole new medium of getting both information and entertainment. News broadcasts, music programmes, variety shows, serials… radio offer us a dazzling variety of ways with which to keep abreast of what is happening in the world, and also to enjoy ourselves. Some radio stations like the BBC and Voice of America transmit around the globe. There are also smaller radio stations that broadcast to a local area.  Radio also changed the way we communicate. Two way radios help people like ambulance drivers to communicate with hospitals. Policemen use radios to keep in touch, and pilots use them while in the air and to fly safely in and out of airports. Children in Australia are sometimes taught by two way radios, because they live so far from any school. When man landed on the moon, the words that he spoke could be heard on our radios almost immediately… that is the magic of radio!

The shadow is bigger than the object only when the object (o) is nearer to the source of light. Let us consider a point source (P) in which the light emanates from almost a single point and goes out radically in all directions. As light travels only in straight line paths if obstructed by an object it creates a shadow. If we consider the light source to be at infinite distant, the light rays reaching the object will be parallel to one another and cause a shadow with the same size as that of the object.

If the light source is nearer to the object, only the light rays emitted at an angle greater than the angle made by the line joining the point source and a point on the corner of the object would go unobstructed by the object. So the rays travelling within this angle gets obstructed and a bigger shadow is cast. As the distance between the object and the source decreases, the angle of the rays to go unobstructed also increase i.e. size of the shadow also increases. 

            This can be explained using a simple diagram. Filament bulbs are point sources, that is, the light emanates from almost a single point and goes out radically in all the directions. As light travels only along straight line paths, if obstructed by any object, it creates a shadow. This is true of any source of light whether it is a filament bulb or a fluorescent bulb.

   In the case of filament bulb, there is no light ray falling on the shadowed area and so the shadow is sharp. Also there is no chance for the light to diffuse into the shadowed region (even if does, it is very small) in the case of a tube light, the light source is long.

            So even if the light from one end of the bulb causes a shadow, there is a possibility for the shadowed area to be lit by a light rays coming from the other end or part of the bulb. Hence the shadow is blurred.

The value 6500 K marked on fluorescent lamps represents a parameter called correlated colour temperature. It means that the spectral (light colour) distribution from that lamp is similar to that of a black body at that temperature. Any black body when heated gives different copious at different temperatures—at 2000 K it emits red light, at 4000 K it is yellowish white. At 2700 K it gives warm light and at 6500 K it gives the impression of cool day light, according to illuminination engineers.

  Based on colour appearance fluorescent tubes come under three classifications, viz, daylight white (above 5000 K), neutral white (4000 K) and warm white (below 3300 k).

  The colour of light (and colour rendering index) depends on the fluorescent coating inside the tube. Three types of coatings are generally given – tri-phosphor, standard phosphor and multi-phosphor.

 Standard phosphor is used in ordinary tube lights. Tri-phosphor coated lamps, emit yellow light similar to sunlight. Standard tube lights render colour similar to daylight.

The graying of the inner surfaces of incandescent bulbs is the result of gradual evaporation of tungsten from the filament while the light is on. This evaporation eventually makes the filament so thin it burns out.

Various methods have been developed to reduce greying. Filaments of the first incandescent lamps burnt in a vacuum, but it was soon found that introducing inert gas to the bulb reduced the rate of greying.

A mixture of nitrogen and argon is used today. In addition, “getters” reactive metal such as tantalum and titanium can be placed near the filament to attract the tungsten so that it is not deposited on the glass.

Alternatively, a small amount of abrasive tungsten powder can be placed in the bulb. Shaking it occasionally will remove the grey coating from the surface of the glass.

Graying can almost eliminated by introducing a small amount of the halogens iodine and bromine. As tungsten evaporates from the filament, it reacts with the halogens which then redeposit the tungsten on the filament. This keeps the bulb wall clean.

To prevent the tungsten halides from condensing on the bulb and breaking the cycle, the temperature of the bulb wall must be at least 500 – C. this is too hot for glass bulbs, which normally operate at about 150 – C, so fused quartz (silicon dioxide) must be used instead

  Compared with ordinary incandescent lamps, quartz-halogen lamps have longer lives and maintain their light output over time. For example, a quartz-halogen lamp with a 200-hour life will have dimmed by less than 5 percent by the time it burns out. When an incandescent lamp with 1000-hour life burns out, it will have dimmed by more than 15 percent.