With the same double-slit setup as above, what is the fringe spacing Δy for the m = 2 bright fringe?

• A. 1.0 mm
• B. 4.0 mm
• C. 2.0 mm
• D. 8.0 mm

Answer: (B)

In a double-slit setup, bright fringes occur where the path difference equals mλ, which on the screen translates to y_m ≈ mλL/d. The spacing between adjacent bright fringes is constant and given by Δy = λL/d, independent of which bright fringe you’re looking at. So for the m = 2 bright fringe, the spacing between successive bright fringes is still Δy = λL/d, even though its position from the center is y_2 ≈ 2Δy. If the setup yields Δy = 4.0 mm, that is the fringe spacing. The m = 2 fringe would sit about 8.0 mm from the center, which matches why 8.0 mm might come up as a distance to that fringe, but the spacing itself remains 4.0 mm.