Two light sources against a wall

[menco]

Homework Statement

Two light sources half a millimetre apart with a wavelength of 550nm shine on a wall exactly 50cm away. What is the distance between the centre point on the wall and the 10th bright spot out?

Homework Equations

x=nLv/d

The Attempt at a Solution

x=10x0.5x(5.5x10^-7)/0.0005
= 0.0055m = 5.5mm

 

[Delphi51]

I agree with that answer. The assumption that the angle is sufficiently small so that nλ = d*sin(θ) equals dx/L appears to be reasonable. Working it out with the exact nλ = d*sin(θ) yields x = 5.5003 mm.