Category Science

Why do the Sun and Moon appear many times larger at the horizon?

The angular diameter of the sun and moon is about half a degree each. The celestial objects are seen with two eyes (binocular vision). When we observe the horizon, the terrestrial surface with all its objects, such as tree, houses, roads or ground give us a perspective view (geometrical phenomenon), i.e., farther the objects smaller they appear, as they subtend smaller and smaller angles at the retina.  Through this long distance perspective, which some time extends to several kilometres, our vision is able to realize a long perceptual distance. At the end of this perspective we locate the celestial objects and realize a particular size.

On the other hand, when we view these objects in the sky, the most important perspective is absent and consequently the two eyes are left with their own power to discern a distance. This binocular distance limit in the absence of the perspective has been found to be about 500 feet, which vary slightly from person to person. This can be checked by the absence of parallax shifts of objects beyond the distance when we view with alternate eyes. Therefore the sun and moon are located at short distance. Naturally a half degree object located at a distance of about 500 feet must appear much smaller  than  a similar half degree object at a considerably long, perceptual distance realized by the sense of vision with the help of perspective.

 In order to illustrate this phenomenon, we place a large ball at a distance from the eye and a much smaller ball closer to the eye in order that the smaller one just covers the angular dimension of the larger ball. Now both the balls subtend the same angle at the eye, but to our binocular vision the larger ball at that distance appears decidedly larger and it is physically larger. Through a telescope or a camera, the sun and moon appear of the same sizes either at the horizon or over the high sky. This view also applies to star constellations at the horizon and at the high sky. Monocular vision by one-eyed people has no illusion of this nature, because they have no perceptual depth. 

 

Why does a spinning top reverse its direction of rotation towards the end?

A spinning top process (or rotate) about its central axis due to the lateral forces given initially or due to lack of speed (as during the end of spinning). When the speed becomes too low (not zero) for it to hold it (its mass) upright, it falls down.

 As the body of the top touches the ground, due to friction it is unable to spin and so the remaining spin (force) forces it to roll on the ground, obviously in the opposite direction.

 Due to tapering shape of the top, it tends to rotate along a circle in the reverse                                                                                             direction, with the nail pointing towards the                                                                                    centre.

While light from a candle illuminates a room, the gas (LPG) flame does not. At the same time the gas stoves help cook faster. Can anyone explain?

Combustion (burning of fuel) can be classified into two: complete and incomplete.

 Combustion of the fuel will be complete when there is sufficient quantity of oxygen for the fuel to burn. These results in a blue flame and the heat generated will be maximum. Combustion is termed incomplete when the oxygen supply is insufficient. This results in a yellow flame. As a result, the heat produced is lesser than the maximum obtainable from the fuel.

In the case of a candle flame, as one can see, there are three distinct zones. The hottest zone surrounding the wick is blue in colour. A zone of unburnt volatile fuel forms the centre of the flame. The yellow luminous zone which is responsible for illuminating the room forms a major portion of the flame. This zone contains hot carbon particles which get heated by hotter zone below. They emit light due to incandescence. As they move out of the flame and escape into the air, they get oxidized and cooled. The process continues as more fuel is burnt.

In the case of LPG (liquefied petroleum gas, which is mostly propane and a little butane), the aim is to get the maximum heat energy from the fuel so that the cooking time is reduced and fuel conserved. Hence the incoming gas is mixed with sufficient quantities of air in the burner and then burnt to give a blue flame.

It is known that propane needs about 25 times its volume for complete combustion. Hence by designing the pinhole (which lets the gas into the burner) suitably, the flow of gas is controlled and ratio of gas and air is maintained. As there is no incandescent zone the flame does not illuminate the room. But, sometimes we see that dust and rust block the holes preventing free flow of air. This results in a yellow flame which illuminates the room just like the candle flame.

Also in the case of LPG stoves, as all the fuel that is injected is burnt fully, the heat generated is more and the cooking time is reduced.

                    

What is the difference between hydrogen and an atom bomb?

 

 

 

 

 

 

 

An atom bomb is a fission device. Here a heavy atom (such as uranium) is split into two or more lighter atoms. In such a nuclear fission process, one or two neutrons are also released from each atom. If these neutrons are captured by the surrounding heavy atom, further fission takes place leading to further neutron release. In atomic power reactors, this chain reaction is controlled so that the energy released during the reaction can be used beneficially.

But in the case of an atom bomb, the chain reaction is not controlled but left free. This leads to the release of large amounts of heat energy in a very short time resulting in a catastrophic explosion.

In a fusion bomb, such as the hydrogen bomb, two or more lighter atoms fuse together to form a heavier atom. Such fusion reactions can take place only at very high temperatures; say a few million degrees Celsius.

Only at such temperatures can the starting atoms overcome the repulsive forces acting between them.

 In this case also, when uncontrolled fusion takes place a large amount of energy is suddenly released resulting in an explosion far bigger than an atomic explosion.

 It may be noted that the temperature needed to initiate a fusion reaction is normally obtained by a fission reaction.

Why does boiling milk overflow but boiling water does not?

            The major component of milk is water (83-87%). Its other constituents are protein (3.5%), sugar (5%) and fat (4-7.4%). When heated, fat being lighter than water, floats as a creamy layer on the top and water vapour, in the form of steam bubbles, is trapped under it. Further heating results in the formation of more number of bubbles. These bubbles expand and lift the creamy layer causing it to overflow. In the case of water, the steam bubbles break as                                                                                          they reach the surface.

How do hearing aids work?

 Hearing aid is an electronic device used by those who are hard of hearing, to amplify sounds particularly speech. It’s essential components are a microphone, an electronic amplifier and a receiver, all operated by a battery. In simple terms, the hearing aid can be likened to a miniature public address system. Sounds are amplified by the amplifier and the output is fed to the receiver.

The gain and amplification of the device is determined by the amplifier. In fact, there are low gain, moderate gain and high gain hearing aids. We know that sound signals are wave patterns in which air molecules vibrate, and waves representing different sounds differ in their wave length (and frequency).

As the human ear can selectively respond to certain frequencies very well and not so well to others, manufacturers vary the frequency response of hearing aids with electronic filters, based on the audiometric configurations (low frequency, high frequency or flat loss of hearing) of the user.

Externally most of the aids are provided with a battery compartment, on- off switch, tone control (to adjust the frequency response) and a volume control (to adjust amplification).

In some aids the on-off switch will be provided with a telephone setting to enable the individual to use the telephone. This is made possible with a telecoil in the aid which selectively picks up telephone signals.