In a double-slit experiment, under what condition is a bright fringe formed?

• A. Constructive interference when ΔL = m λ (m = 0,1,2,...)
• B. Destructive interference when ΔL = m λ
• C. Bright fringes occur when ΔL = (m + 1/2) λ
• D. Slits emit incoherent light

Answer: (A)

Bright fringes appear when the two waves from the slits arrive in phase. This happens when the extra distance one wave travels relative to the other is an integer multiple of the wavelength, so the path difference ΔL = m λ (with m = 0, 1, 2, …). When they align in phase, their peaks add up and you see bright spots, including the central maximum at m = 0 where ΔL = 0.

Destructive interference, which yields dark fringes, occurs when the path difference is a half-integer multiple of the wavelength: ΔL = (m + 1/2) λ. The option about incoherent light would not produce a stable fringe pattern at all, so it isn’t correct.