Skip to content
GRE Physics overview

Worked example

Free to read

Double-Slit Bright Fringes

A public interference question showing how wavelength, slit separation, and screen distance set the position of a double-slit bright fringe.

Question

In a Young's double-slit experiment, light of wavelength 500 nm500\,\text{nm} passes through slits separated by 0.10 mm0.10\,\text{mm}, and the pattern is viewed on a screen 2.0 m2.0\,\text{m} away. How far from the central maximum is the second-order (m=2m = 2) bright fringe?

  1. a.1.0 cm1.0\,\text{cm}
  2. b.2.0 cm2.0\,\text{cm}
  3. c.2.5 cm2.5\,\text{cm}
  4. d.4.0 cm4.0\,\text{cm}

Correct answer

B. 2.0 cm2.0\,\text{cm}

Full reasoning

  1. 1Bright fringes satisfy dsin⁡θ=mλd\sin\theta = m\lambda; for small angles this gives screen positions ym=mλL/dy_m = m\lambda L/d.
  2. 2Insert the values: y2=2(500×10−9 m)(2.0 m)/(1.0×10−4 m)y_2 = 2(500\times 10^{-9}\,\text{m})(2.0\,\text{m})/(1.0\times 10^{-4}\,\text{m}).
  3. 3The numerator is 2×500×10−9×2.0=2.0×10−6 m22 \times 500\times 10^{-9} \times 2.0 = 2.0\times 10^{-6}\,\text{m}^2; dividing by 1.0×10−4 m1.0\times 10^{-4}\,\text{m} gives 2.0×10−2 m2.0\times 10^{-2}\,\text{m}.
  4. 4The second-order bright fringe sits 2.0 cm2.0\,\text{cm} from the central maximum, and y≪Ly \ll L confirms the small-angle approximation.

Why each choice is right or wrong

Choice A

This is the first-order position λL/d\lambda L/d — the fringe spacing — obtained by using m=1m = 1 instead of m=2m = 2.

Choice B

Correct. y2=2λL/d=2(500×10−9)(2.0)/(1.0×10−4)=2.0×10−2 my_2 = 2\lambda L/d = 2(500\times 10^{-9})(2.0)/(1.0\times 10^{-4}) = 2.0\times 10^{-2}\,\text{m}.

Choice C

This applies the half-integer dark-fringe condition (m+12)λ(m + \tfrac{1}{2})\lambda with m=2m = 2, which locates a minimum rather than a maximum.

Choice D

This is the separation between the two second-order maxima on opposite sides of center, double the distance from the central maximum.

Related formula

Double-slit bright-fringe positions

ym=mλLdy_m = \frac{m\lambda L}{d}

Assumptions: The small-angle approximation holds (L is much larger than d and y). The two slits act as coherent point sources.

Related topic and practice

Interference and DiffractionOptics and Waves Domain GuideCreate a free account to practice

This worked example is free to read. A free account unlocks practice questions for this exam.

Sources

Authored for this public explainer registry. It has no database question ID and is not copied from a protected or official exam bank.

  1. University Physics Volume 3, Section 3.1: Young's Double-Slit Interference — OpenStax. Accessed 2026-08-02. OpenStax textbook content is CC BY-NC-SA 4.0; attribute and avoid verbatim reuse beyond short cited references.
Share this worked solution

Sources and corrections

Sources last checked 2026-08-07

Every source cited on this page was checked on the date shown, and we update the page when a source changes. If something looks wrong, tell us and we'll recheck it.

Report an issue with this page