# Wavelengths and Balmer series

## Homework Statement

In the Balmer series, how many spectral lines have the wavelength greater than 400 nm?

## Homework Equations

1/lambda= R(1/2^2-1/n^2)
lambda=wavelength
r=rhydberg constant 1.097x10^7
n= 3,4,5....
balmer series is visible light

## The Attempt at a Solution

ok so i know the balmer series starts with 400 nanometers because its visible light, but i dont understand how to see how many spectral lines have wavelength greater.

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Delphi51
Homework Helper
You could just look up the Balmer Series in a book or on the web and count them.
Or use the formula to calculate the wavelengths - put in n=3, calculate. Put in n=4, calculate. Pretty soon you will have all the wavelengths you need!

ok. but when i do the calculating up to what number do i go? because i thought it went from 2 on.

Delphi51
Homework Helper
Yes, it does go on forever - 2 to infinity. But after calculating 2 or 3 wavelengths, you will see that you don't have to do very many of them the answer your question.

o ok i got it. wavelength decreases when you increase n