# Double Slit Problem, Waves and Optics [closed]

Given d(slit separation)= $0.158\:\rm{mm}$, $\lambda _{red}= 665\:\rm{nm}$, $\lambda _{g/y}= 565nm$, L(distance from screen)= $2.24\:\rm{m}$

What is the distance between the third order red and green/yellow fringes?

I am using the equation $X _{3}= \frac{m \lambda L}{d}$ (just to find the distance to the third red fringe). My teacher told me this equation is approximate, but I think this is beyond simple approximation error... I keep getting the answer $2.8284 \times 10^{7}\:\rm{m}$. This seems unreasonable; what am I doing wrong?

$\:\rm{m} = \:\rm{mm}\times10^{-13}, \:\rm{m}= \:\rm{nm}\times10^{-9}$, right?

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## closed as too localized by dmckee♦, Sklivvz♦, David Z♦Apr 3 '11 at 21:13

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Put all your distances into the same units... – dmckee Apr 3 '11 at 19:04
I think I did, I put them all into meters! I added the prefix powers I used to the question – BKaylor Apr 3 '11 at 19:10
Hmm...mm is millimeters $= 10^{-3}$ m. What's ten orders of magnitude between friends? – dmckee Apr 3 '11 at 19:12
So the problem has been found... I guess that goes to show how well you can rely on the Physics Reference app on the android market. Thanks for your help. And friends? What?? – BKaylor Apr 3 '11 at 19:15
Since you already identified the problem yourself (kudos, by the way :P), we'll close this as "localized" because there's no need to collect answers anymore. – David Z Apr 3 '11 at 21:14