The star-like appearance can be noticed only when a considerable amount of light reaches the glass (or any other transparent sheet). The phenomenon deals with two rays of light: the ordinary ray and the extraordinary ray. The ordinary ray obeys the law of light and gets diffracted. But the extraordinary ray does not. That’s why when we see light sources through a sheet of glass, the centre is bright.

As the ordinary ray falls on the outer surface of the glass, it gets diffracted and bends at an angle. It is further bent as it escapes from the inner side of the glass plate (towards our eye). This deviation in the path of the ray results in the characteristic star shaped appearance. Also, the diffracted light rays undergo interference and superposition. These phenomena take place only when the rays emerging are in phase with one another. The rays which do not have the same amplitude or wavelength do not superpose. Thus we get a glittering appearance resembling a star. 

            Tube lights are discharge lamps. To initiate a discharge, it requires a high voltage (about 1000 Volts) several times the mains voltage (about 220 V). To sustain a discharge it requires only about 100 V.

A choke is an inductance. When current through an inductance is abruptly interrupted it includes a high voltage. The interruption is done by the starter which works like a switch. The high voltage strikes an arc between the filaments at the ends of the tube light. Once an arc has struck, the choke takes half of the mains voltage and leaves the rest to maintain the arc.

The choke has a core made of thin laminated silicon steel sheets. When the sheets vibrate with the power frequency (50 Hz) or its harmonics it generates noise.

A starter is made of two electrodes one of them is a bimetallic strip. When a tube light is switched on, the voltage between the two electrodes produces a spark. 

This heats the bimetallic strip, forcing it to stretch and bridge the gap.  This in turn stops the sparking and cools the strip. The strip comes back to its initial position. This process interrupts the current in the choke.

When the discharge is established the starter will be in parallel (electrically) to the tube and gets only half the mains voltage which is insufficient to initiate a spark. The starter will now idle. If the tube fails to sustain the arc the starter will repeat the job. The job of the starter can be done manually by a switch. 

     While the speed of light in vacuum is the same for all wavelengths, the speed in a material substance is different for different wavelengths. As a result, the refractive index becomes a function of wavelength.

A glass Prism deviate a ray of light passing through it, at a particular angle which depends on the refractive index. This deviation increases with increasing refractive index so violet (380 nm) is deviated the most and red (600 nm) is deviated the least with other colours occupying intermediate positions. This light emerging from a prism is split into constituent colours.

            White light consists of seven colours – VIBGYOR. During foggy days air is highly humid and contains lots of tiny water droplets which can act like a prism. So, if white light is used in automobile headlights and street lamps, water droplets will split it into its component colours, thereby, creating a circular rainbow around each light source. This will affect clarity of vision and lead to accidents. In order to avoid this effect, a monochromatic (yellow) light source is used. Yellow is chosen because, being in the middle of the visible spectrum, it undergoes medium refraction and medium reflection while passing through a water droplet. Also, it is next only to white in brightly illuminating all objects in their original colour.

  However, mercury vapour lamps are used in the interiors because, white light is more pleasing to our eyes, and slight reduction of vision due to fog is not of great concern.

   The efficiency of sodium vapour lamps is good – between 40 and 50 lumens per watt. Hence they are used for street lighting.

Yes, we can save power with the electronic regulators but not with the old regulators based on rheostat. Speed of a fan, as we know, is controlled with the help of a regulator.

 The old regulators are based on rheostats which have 5 or 6 steps corresponding to different speeds. They consume a fixed power all the time. If we select the highest speed all the power is fed to the fan’s motor. If a lower speed is selected, electric power proportional to selected level is fed to the motor and the remaining power tapped from the mains is wasted as heat in the rheostat. That’s why when we run the fan at slow speeds for a long time the regulator becomes hot.

The new electronic regulator now available in the market, are based on a semiconductor device called triac. (Triac is a trademark of the General Electric Company.) It is an electronic switch designed for power control and phase control. According to the encyclopedia, using triac, power output can be continuously varied from about 5 per cent to 95 per cent of the maximum power without any loss. Depending on the speed selected, the electronic regulator draws, from the mains, only the required power. So it is prudent to use the new electronic regulator and save power.

 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.