And after that he explained that we can still talk about entropy change in irreversible processes and we can calculate it by assuming that the initial and final state of the process whose entropy is to be calculated is achieved by reversible process and we can equate those two since entropy is a state function.Isn't he contradicting what he said before?

I was studying about entropy from khan academy, and in that he proved that in a carnot cycle entropy change is always zero when we complete a full cycle and hence entropy is a state function.But when we consider an irreversible cycle for example,if we decide to include friction energy lose when piston moves up and down,the process will become irreversible and in this case when we complete a full cycle ,from initial state and back to it,the entropy change is not zero and hence Sal concluded that entropy as a state function makes sense only for reversible processes.

Ohkay so we can calculate entropy change due to an irreversible process just by calculating the entropy change assuming the final state is achieved by a reversible process and since it is a state function,we can equate those values?

I meant the entropy change in both the processes.We say that entropy change is a state function and hence should not depend on path taken.

Yes i meant entropy change

Okay I got your point,but you mentioned that white light will act as an incoherent source but won't the two slits act as coherent sources as any abrupt change in original source will reflect in the two slits acting as source? I came across this question that if you replace the monochromatic light with visible light you ll get a fringe pattern of different wavelengths overlapping incoherently.Why would they overlap incoherently?

So there is no difference between any observations like the pattern of fringe etc?

To apply hugens construction we use the reflecting surface CD for the source of secondary wavelets.As the various points of CD are reached by wavefront AB,they become source of secondary wavelets.

I ll tell you what's mentioned in the book.For proving law of reflection we suppose that a parallel beam of light is incident upon a reflecting plane surface CD.The wavefronts of the incident wave will be planes perpendicular to the direction of incidence.After reflection,the light returns in the same medium.Consider a particular wavefront AB of the incident light wave at t=0.We shall construct the position of wavefront at time t.