Question

In perturbation theory, what defines "small?"

Question 1 options:

a)	The absolute size of the perturbation is small.

b)	The size of the perturbation is small as compared to the main component.

c)	The perturbation must be smaller than the Heisenberg uncertainty limit.

d)	The perturbation should be smaller than 1 nanometer.

e)	The number of degenerate states

Answer 1

In case of perturbation theory we basically add small correction term to the main component to get the accurate result, because sometimes there may be some instability in the system which can be easily solvable by adding small perturbation term.for example, by adding a perturbative electric potential to the quantum mechanical model of the hydrogen atom cause tiny shifts in the spectral lines  of hydrogen caused by the presence of an electric field (the Stark effect) can be calculated. So correct answer is b.