# Calculating wavelength

1. Dec 3, 2007

### Ry122

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?

2. Dec 3, 2007

### Dick

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.

3. Dec 3, 2007

### Ry122

can you tell me what equation i need to use?

4. Dec 3, 2007

### Dick

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.

Last edited: Dec 3, 2007
5. Dec 3, 2007

### Ry122

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

6. Dec 3, 2007

### Dick

What's the relation between energy of a photon and it's wavelength?

7. Dec 3, 2007

### Ry122

is it e = hc/wavelength ?

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

So what value do I use for n, 2?

8. Dec 3, 2007

### Dick

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.

9. Dec 4, 2007

### Ry122

You don't use a value for n

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

10. Dec 4, 2007

### tyco05

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.