In case of spherical mirrors (both concave and convex), there will be no change in the focal length of the mirrors when immersed in water. This is because the focal length of mirrors does not depend on the external medium in which it is held.
In case of lenses, refraction takes place. When a lens is immersed in water, the value of refractive index decreases. Hence its focal length increases.

Superposition of waves is the process in which two or more different waves travelling through a media simultaneously overlap one another without losing their individual nature and shape. 

Principle of superposition: When two or more waves of the same nature travel past a point simultaneously, the resultant displacement at the point is vector sum of the instantaneous displacements due to individual wave. 

If  are the displacement vectors due to individual waves at a point at a certain instant, then the resultant displacements due to individual wave is given by, 

                    

Some important cases of superposition of waves are:

(i) Stationary waves,
(ii) Beats,
(iii) Interference

Transmitter : A collection of electronic components and circuits designed to convert the information into a signal suitable for transmission over a given communication medium.
 

Receiver : Another collection of electronic components and circuits that accept the transmitted message from the channel and convert it back into a form understandable by humans.
 

Noise : Any random, unwanted energy that enters the communications system via the communications medium and interferes with the transmitted signal.
 

Channel : It provides means of transporting signals from a transmitter to a receiver.

Zero potential energy means the point at which the perfectly rigid body has zero internal energy.

Perfectly rigid bodies usually gain or loose internal energy (internet energy is a term kept vague here) as they enter electromagnetic(if charged) and gravitational(if of mass) fields. This is due to the inherent properties of said fields and their effects on the given quantity of respective material in the body.

So a perfectly rigid body is said to have zero potential energy if the internal energy inside the body is exactly zero.

But wait, electromagnetic and gravitational fields are pretty much universal, i.e they can reach anywhere even if their strength diminishes. The equation has an 1/r² part. So bring this to zero, we need to make rr extremely high, likely infinity.

So the only point in the universe where this is possible is at some infinite distance from said source of field. But since the universe isnt of infinite size, we don’t have a point where the body can have zero potential energy.

Principle:-

Transformers work on the principle of Mutual induction.

Mutual Induction :- Suppose there are 2 inductors if some current flows through coil(1) ,there will be change in the current as a result there will be change in the magnetic flux, as a result there will be  change in the magnetic flux in the coil (2) and because of which emf is induced in the coil(2).

Construction:-

A Transformer consists of  :-

1. Primary coil:-
   1. Primary coil has ‘n’ number of turns of wire over a piece of soft iron core.
   2. It is the input end.
2. Secondary coil :-
   1. Secondary coil has ‘n’ number of turns of any wire(like copper etc.) .
   2. It is the output end as we receive output from this end.
3. Soft iron core :-
   1. The hysteresis curve for iron is extremely thin because of which it covers minimum possible area.
   2. As the  area  of  the  hysteresis  loop  of  iron is very less  therefore the energy lost by the transformer will be very less.
   3. Permanent magnet is not suitable to use in transformers because the energy lost will be huge.

   Working :-

   1. An input voltage(AC source)  is applied across the primary coil. As  a result alternating current is produced in the primary coil.
   2. The alternating current will give rise to alternating flux is produced in the coils.
   3. Because of change in the magnetic flux emf will be induced .
   4. There will be 2 Emfs produced in the circuit. 1. Self –induction 2. Mutual induction.
   5. There will be self - induced emf in the primary coil , because of change in the magnetic flux in the primary coil there will be corresponding change in the magnetic flux  associated with the secondary coil which will give rise to induced emf in the secondary coil.
   6. Mutual induction takes place in the secondary coil.
   7. Induced emf in the primary coil = e_p = -N_p (df/dt)  
      • Where (df/dt) = rate of change magnetic flux and N_p = number of turns in the primary coil.
   8. Mutual induction in the secondary coil e_s = - N_s (df/dt)
   9. • Where N_s = number of turns in the secondary coil.
   10. Assuming  resistance =0 in both primary and secondary coils.
   11. Therefore e_p = V_p (Voltage across primary coil)  
   12. V_p = -N_p (df/dt)  (equation(1))  and
   13. e_s = V_s (Voltage across secondary coil) = - N_s (df/dt)  (equation(2))
   14. Dividing equation(1) with (2):-
   15. • • (V_p/V_s) = (N_p/ N_s)
         • =>  V_s = (N_s/ N_p) V_p
   16. Power at the input end is same as the power at the output end.
   17. Therefore P_intput = P_output
   18. => I_pV_p =  I_sV_s