What is relative density?

We compare the density of different substances with the density of water at 4°C.

Hence "the ratio of the density of a substance to the density of water at 4°C is called Relative density of the substance".

4°C should be mentioned because water has maximum density of 1g/cm^3 at 4°C only.

There is no unit of relative density as it is  the ratio of two similar quantities.

The value of density of any substances in g/cm^3 is equal to its relative density. For example if the density of a substance in g/cm^3 is 7.7gcm-3, then the relative density is 7.7.
It means that the substance is 7.7 times heavier than water.

LAW OF FLOATATION

"For a floating body, the upthrust experienced by it is equal to its weight."
Net force  on the floating  body is zero

What is Regelation?

The melting point of ice is 0°C at normal pressure but when you increase the pressure on ice it melts at the temperature below 0°C( i.e. melting point of ice decreases with pressure).
When the pressure applied is released then water formed from ice starts freezing. This phenomenon is called REGELATION.

Water seeks it's own level.( Hydrostatic paradox)

Irrespective of shape of vessels or containers, a connected liquid maintains same level in all containers.
This happens to maintain same pressure at a given depth. As the pressure depends on depth of a point from the liquid surface.

MIRAGE. HOW IS IT FORMED ?

MIRAGE is formed by the phenomenon of total INTERNAL reflection of light. Mirage is formed in extremely high or low temperature conditions.
For example: When a sand surface (desert) or road surface is heated, the air above this surface is also heated and rises up.
As a consequence, the density of air in the lower level is less than that of upper level.
  Now , the light from any object starting from the upper layers (denser medium) travels to lower layers ( rarer medium), total internal reflection takes place i.e, the light is reflected back towards the denser medium. When these light rays reflected back, it seems that the light rays are refected from some bright reflecting surfaces on the ground.  Hence the  image is seen below the object.

What is SMOG?

Literally,  smoke + fog= smog.
Smoke is the collection of unburnt fuel particles suspended in the air.
Fog is the suspended water particles in the air.
The combination of these suspended particles gives the smog.

A flute has a number of holes. Why?

The sound produced in a flute is due to the vibration of air column in the flute. The frequency of sound produced depends on length. The frequency is inversely proportional to the length of the air column. By closing or opening the holes , the length of the air column is changed and hence the frequency can be changed.

Plant leaves appear green. Why?

Sunlight contains seven colours, viz violet, indigo ,blue, green, yellow, orange and red. The leaves appear green because they absorb all the colours and reflect green as green colour is least used in photosynthesis. This green colour reaches our eyes. Hence leaves appear green.

Can unstable hydrogen isotope release alpha particles ?

Unstable hydrogen isotope contains
1 proton
2 neutrons

Now, alpha particles are the particles with
2 protons
2 neutrons

What do you think?
As there are insufficient particles in the hydrogen isotope, no alpha particles can be released.

Shallow water runs faster than deep river water. Why?

The equation of continuity is
area of cross-section of flow of water × velocity of water = constant.
Hence  the velocity of water is inversely proportional to the the area of cross-section of water flowing.
When water runs deep then the area of cross section of the flowing water is more than when its shallow so the velocity is less for deep water.

WHAT IS SUBLIMATION ?

Sublimation is a process of transformation of state of matter where a solid changes directly into its gaseous state.
The molecules in some solids have very weak force of attraction. So the molecules by gaining very little heat from the surrounding can change into gaseous particles.

Does specific resistance/resistvity change on stretching a wire?

Specific resistance or resistivity is the resistance of a wire of unit length and unit area of cross-section.

It depends on two factors.
1. Nature of the substance.
2.  Temperature of the substance

It is not effected by
1. Length of the wire.
2. Area of cross-section of the wire.

So., when you stretch a  wire its RESISTIVITY remains constant.

Can a solenoid be used to find the direction of earth's magnetic field ( magnetic North and magnetic south)?

A solenoid is a cylindrical/helical coil whose length is more than its diameter. When we pass current through the solenoid , a magnetic field is set up in the space around the solenoid similar to that of a bar magnet.
There is ,in fact, a strong and uniform field in the core of the solenoid.At the two ends of the solenoid, poles are developed as in a bar magnet.
So the current carrying solenoid if freely can show the direction of the magnetic field of a magnet.The north end of the magnet shows the geographic north and South end shows south .