It took me twenty minutes on Google before I realized I had lambda on MS-word.

anyways, here's my question:

ëƒ = c

where lambda is the wavelength, f is the frequency and c is the speed of light in a vacuum.

Now, in the book, it said:

If we solve the equation for f and differentiate with respect to ë we get

df/dë = -c/ë^2 ------------------------ (A)


If we now go to finite differences instead of differentials and only look at absolute values we get

Äf = cÄë/ë^2 -------------------------(B)


Thus given the width of a wavelength band, Äë we can compute the corresponding frequency band.

Ok, the part I'm lost at is how did they get (A) and how the **** did they get (B).

Someone please explain it!!