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**1. Homework Statement**

Blue light of wavelength 400 nm passes through two slits which are a distance

d = 1×10−4 m apart. This produces a double slit pattern on a screen L = 12 m away. (The

screen is parallel to the plane of the two slits.) If the central bright fringe is denoted the

“m=0 fringe”, what is the distance between the m = 15 and m = 14 bright fringe on the

screen? (All angles are suﬃciently small that tan θ ≈ sin θ ≈ θ.)

**2. Homework Equations**

dsin(θ) = mλ

y = L tan(θ)

**3. The Attempt at a Solution**

arcsin[(15*400e-9)/1e-4] = 3.44°

y= 12 * tan (3.44) = .72 m

arcsin[(14*400e-9)/1e-4] = 3.21°

y= 12 * tan (3.21) = .67 m

y2-y1 = .72-.67 = .05 m

That was my attempt. First to find out how far the 14th and 15th bright fringe were from the center and then to find the distance between them.

The answer given by the instructor is: .113 m or 11.3 cm.

But why? I still can't find anything wrong with my train of thought. Thanks!!