What is the Correct Wavelength of Light in an Interference Experiment?

[stphillips]

Homework Statement

Monochromatic light from a point source illuminates two parallel, narrow slits. The centres of the slip openings are 0.8mm apart. An interference pattern forms on a screen placed parallel to the plane of the slits and 49 cm away. The distance between two adjacent dark interference fringes is 0.30 mm.
a) Calculate the wavelength of the light.

Thanks

Homework Equations

I used the formula X/L = lambda/d

The Attempt at a Solution

I got 490 nanometers, the book got 540. Whats the correct answer?

 

[Liquidxlax]

If you haven't done paraxial approximations you should be using

dsinθ = (n+1/2)λ where θ=x/l for destructive interference

you know d you know n and x is the distance from the central maximum to every successive light fringe

 

[stphillips]

I don't know theta in that eq'n though =/
I have 3 variables and i have an eq'n i just don't know why I am getting the wrong answer

 

[Theaumasch]

You have enough data to calculate theta

 

[stphillips]

Okay so what is the right answer then =/ I got an answer it's just different from the textbook and i don't know why

 

[Liquidxlax]

we cannot tell you how the answer, only to guide you to it. it says that the two dark fringes are .3mm apart. so this would be n=0 since these are the 0 order dark fringes.
What is the distance from one dark fringe to the central maximum?

To find theta, you have the adjacent and you can find the opposite. After that you have everything you need

 

[stphillips]

Why can't I use the formula X/L = lambda/d
I have 3 of those variables its just when i solve i don't get the right answer :S
I shouldn't have to solve for theta to solve this question

Also, this was homework due like a week ago, now I just want to know the right answer so I know if I'm doing something wrong or if the book is.