Question Corner

Audible sound

QUESTION:How does audible sound pass through hard material such as a wall and is audible on the other side ?

A.G.Durai, Chennai

ANSWER:Sound is nothing but a form of energy. It travels in the form of waves. When these waves strike the wall, the molecules or the atoms vibrate due to the sound energy. The vibration passes from one atom onto its adjacent atom, and thus the travelling vibration travels through the whole wall and when the wall ends the vibration in turn takes place in the air molecules. Thus the sound travels through the wall and further into the air.Due to this,the sound created on one side of the wall can be heard on the other side.This phenomena proves that sound needs medium to travel. In vacuum the sound cannot travel because there are no atoms or molecules present to vibrate.

In solids the atoms are packed together more tightly than in liquids and gases. The vibrations need much less gap between the atoms in order to travel faster. Sound travels more intensely in solids. That is why audible sound passes through hard materials and is audible on the other side.

A.Sriharsha,Chennai

Shadow bigger than the object

QUESTION: Why is the shadow bigger than the object?

G.Srinath, Chennai

ANSWER: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 radially in all directions. As light travels only in straight line paths if obstructed by any object it creates a shadow.

If we consider the light source to be at infinite distance, 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 increases ie the size of the shadow also increases.

K.Rakesh, Erode, T.N.

White oil

QUESTION:What is white oil ?

E.R.Subramanyam,Amalapuram

ANSWER :White oil is a non-fuel petroleum product obtained from crude petroleum after heavy distillation.It is manufactured for pharmaceutical purposes like creams, ointments etc.It is also used as a common adultrant for adultrating edible oil. Usually hydrocarbon when partially burnt produces some amount of carcinogenic agents. Since white oil is a long chain hydrocarbon compound, when used as an adultrant in edible oil it produces carcinogenic agents when cooked.

S.Palaniappan, Pudukottai, T.N.

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