Physical interpretation of wave function : Einstein proposed a historic relationship between energy and mass using postulates of STR. The variation of mass with velocity has modified the idea of energy. According to this relationship, energy and mass can be considered as different manifestation of same physical quantity i.e. mass and energy are equivalent to each other.

This relationship can be derived directly from the definition of kinetic energy (K.E.) of a moving body i.e. work done in bringing the body from rest to its present state of motion.

Importance: Entire nuclear energy process, fission, fusion and other nuclear rection are examples of mass energy relation. Using mass energy concept we can explain the mass defect and release of tremendous amount of energy in nuclear fission.

Inertial Frame of Reference: If in a given system a free object does not experience any acceleration or force, the system is called the inertial frame. Any reference frame that moves with the constant velocity relative to an inertial frame of reference is itself an inertial frame. Newton's laws are found to be valid in these frames of references. Examples, a passenger in a car, moving with uniform velocity experiences no force on him.