Light - Young's Expirement - The equation

[b^3]

I need a bit of help understanding an example from the Heinemann book. See the attached diagram.

Here is the question.
Light of an unknown wavelength emitted by a laser is directed through a pair of thin slits separated by 50 micrometers (um). The slits are 2.0 m from a screen pn which five bright fringes are seen over a width of 12.5cm. The wavelenght of the laser light can be determined.

They show us to use the equation .
Now I want to make sure, W is the width of a single slit and is measured in meters right? If this is correct then how do we know that dividing the width of five bright fringes by 5 gives us the width of the gap?

If I have this wrong then what is W or have I just completely got no idea what is going on here?

EDIT: Sorry typed wrong thing for W.

[Lasercookie]

W is the fringe spacing. The fringe spacing is the distance between each 'fringe' in the interference pattern produced.
L is the length from the screen.
d is the distance between slits.

The given distance for fringe spacing was the distance for 5 fringes (12.5cm), so 5W = 12.5cm, W = 2.5cm (convert to metres to get into SI units).

I think that example was the first time Heinemann introduced this relationship in this form, they didn't do an overly good job of explaining it. This equation is really important - makes your life easier when trying to figure out how changing one value (e.g. wavelength) will affect the interference pattern.

[b^3]

Thanks laseredd, we actually did this last year in 1/2 physics (we weren't meant to though) but I couldn't remember or find my notes. Thanks again, acutally makes moer sense to be that anyway.

[Lasercookie]

Thanks laseredd, we actually did this last year in 1/2 physics (we weren't meant to though) but I couldn't remember or find my notes. Thanks again, acutally makes moer sense to be that anyway.

Yeah, we ending up covering it last year as well. We had this substitute teacher come in and he's the really passionate, crazy science teacher type

It's definitely something I'll write down on my cheat sheet, I'll probably forget what letter stands for what.

[b^3]

Yeh I basically spent all of today trying to learn the whole light chapter out of Heinemann, only released an hour ago that I have a methods SAC tomorrow. So I'm hoping all the theory is going to stay stuck in my head or it will have all been for nothing.