In a two-slit experiment, the slit separation is 6.60 × 10-5 m. The interference

Question

In a two-slit experiment, the slit separation is 6.60 × 10-5 m. The interference pattern is created on a screen that is 2.00 m away from the slits. If the 7th bright fringe on the screen is a linear distance of 10.0 cm away from the central fringe, what is the wavelength of the light?

I know the correct formulas to be using; what I'm not sure of is because it says the 7th bright fringe, does that mean it is the seventh order bright fringe? Or do you start counting from the central bright fringe which has an order of 0, in which case this would be the sixth order fringe?

Explanation / Answer

d = 6.6 * 10 ^-5 m

L = 2 m

Order of fringe, m = 7

distance of 7th bright fringe from cenrtal fringe, x = 10 cm = 0.1 m

So, tan = x / L = 0.05

= 2.86 degrees

We have a condition for bright fringe,

sin = m / d

= d sin / m = 0.05 * 10^-5 m

So the wave length of light, = 0.05 * 10^-5 m

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