# Calculating wavelength

The energy levels for singly ionised helium can be constructed from the formula:
E = 54.4/n^2 eV
Use this to calculate the wavelength of the first line of the balmer series for singly ionized helium
E=54.4 for the first line. Whats the next step?

## Answers and Replies

Dick
Science Advisor
Homework Helper
Singly ionized helium is exactly like hydrogen, except with charge 2. So the ratios between lines are the same as the Balmer series for hydrogen.

can you tell me what equation i need to use?

Dick
Science Advisor
Homework Helper
You just gave it. Balmer lines are transitions between n>=3 and n=2. E=54.4eV isn't a line. It's the first energy level. Lines are the difference between energy levels.

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Im supposed to be calculating the wavelength, but none of the variables in that equation represent wavelength.

Dick
Science Advisor
Homework Helper
What's the relation between energy of a photon and it's wavelength?

is it e = hc/wavelength ?

Balmer lines are transitions between n>=3 and n=2

So what value do I use for n, 2?

Dick
Science Advisor
Homework Helper
Yes, for the first question. Though I don't know why you put a question mark after it. You don't use a value for n. You pick two values for n and find the energy difference. Pick the two values of n that make the difference largest. That's the first Balmer line.

You don't use a value for n

if n isn't assigned a value, how can i find E?

You don't use ONLY ONE value for n.

You need to calculate the energy levels for two values of n.

The difference between these energy levels is what you are interested in.