1. The problem statement, all variables and given/known data I want to convert wavenumbers into wavelength. The wavenumber is 400cm-1 and i used the expression v=vbar*c to get the frequency in Hz to be 1.2*10^13, I also remembered to convert c into cm/s so that frequency is in its SI units (seconds). I then simply thought that I could use the expression lambda=c/v to get the answer i require and then multiply that answer by 1*10^9 to get my answer in nm instead of ms. I know the answer should be 2500nm but i get 25000nm. Why do i have this factor of 10? where did it come from? 2. Relevant equations i began with vbar=400 cm-1 so then i said that v=vbar*c v=400*3*10^10 v=1.2*10^13 Hz I then said that lambda nm=(c/v)*1*10^9 lambda=(3*10^8/1.2*10^13)1*10^9 lambda = 25000 nm when it should be 2500nm or 2.5um 3. The attempt at a solution Please see above I know this forum is used mainly by graduates who set eachother calculus based challenges but any help for a simple folk like me would be much appreciated.