chemistry
posted by dave .
An electron in the n = 5 excited state of a hydrogen atom emits a photon of 1281 nm light. To what energy level does the electron move?
Any ideas on an equation for this? Is it some kind of backward version of Rydburg Eq?

conver nm to energy.
E=h*c/lambda
then, having E, find the n. 
h*c/lambda
would this be planck's constant times speed of light / wavelength? 
Yes. Another way to do it
(1/wavelength) = R(1/n^2  1/5^2)
where R is the Rydberg constant. Solve for n. 
How many photons of light are emitted when the electrons in 10 hydrogen atoms drop from energy level 5 to 3?