In a double-slit interference experiment, the slit separation is 0.20 mm and the screen is 2.0 m away. Light of wavelength 600 nm is used. What is the distance between the central bright fringe and the first-order bright fringe? (1 nm = 1 × 10⁻⁹ m)

Correct answer

B. 6.0 mm

Principle or equation

For small angles, the position of the m-th bright fringe is y_m = mλL/d.

Why this answer is correct

Convert: d = 0.20 × 10⁻³ m, λ = 600 × 10⁻⁹ m, L = 2.0 m. y₁ = (1 × 600 × 10⁻⁹ × 2.0) / (0.20 × 10⁻³) = 6.0 × 10⁻³ m = 6.0 mm.

Example

If d = 0.1 mm, L = 1 m, λ = 500 nm, then y₁ = (500e-9 × 1)/(0.1e-3) = 5 mm.