Which physical law underlies the first law of thermodynamics?

Conservation of energy

Explanation:

The first law of thermodynamics is given by :

Here,

Q is amount of heat added to the system

is change in internal energy

W is amount of work done by the system

It is clear that the energy can be neither created nor destroyed, it remains conserved. Hence, first law of thermodynamics shows that conservation of energy.      

the current is 0.6

voltage ÷ resistance = current

so 12 ÷ 20 = 0.6

speed of light in vacuum is 300,000 km/s

speed of light in other mediums is given by the expression v = c/n, where c is the speed of light in vacuum and n is the refractive index of medium.

for the first medium v = 200,000 km/s

  so, refractive index of medium = 300000/200000 = 1.5

    1.5 refractive index is usually for glass. so first medium is glass.

for the second medium v = 250,00 km/s

so, refractive index of medium = 300000/250000 = 1.2

    1.2 refractive index is usually for water. so second medium is water.

for the third medium v = 0 km/s

so, refractive index of medium = 300000/0 = ∞

    ∞ refractive index is usually for opaque mediums. so third medium is opaque medium.