[WK95]

1/λ= R(1/n_1 - 1/n_2)

---vs---

1/λ= (RH/hc)(1/n_1 - 1/n_2)

What is the difference in usage? In my problem sets, I keep getting questions usin these equations and sometimes one is used while other times, another is used.

[Vidya]

you will get the same answer by using any of the equation
In one case the constant is R which is equal to RH/hc
Basic equation is E= RH(1/n^)2-  1/n^2_2)
and E = hc/λ If you plug in the values
then 1/λ= RH/hc(1/n^2_1-n^2_2)
so basically they are the same equation and R = RH/hc
RH is the Rydberg constant with units of J
and R is the Rydberg constant with units of m^-1