Problem
Double slit. Coherent light passes through a double slit and forms an interference pattern on a screen. Which graph represents the intensity on the screen? (Epstein 2002)
Solution
The answer is graph B.
Why. The double-slit pattern is the product of two effects:
- the interference fringes of the two slits, oscillating rapidly (regular maxima and minima);
- the diffraction envelope of each single slit, which modulates the amplitude, giving a dominant central peak with progressively lower side lobes.
Graph B shows exactly this: oscillating fringes enclosed within a bell-shaped envelope, with a marked central maximum — the typical signature of double-slit interference.
Why not the others.
- A is a cosine of constant amplitude: this would be the interference of two ideal point sources (infinitely narrow slits), with no diffraction envelope. Not realistic.
- C is a step function: the distribution we would get with classical particles (two slit “shadows” added together), without interference. This is exactly what is observed if a detector is placed to determine which slit the light passes through.
Links
Topics: Quantum physics Concepts: Interference · Wave-particle duality Skills: Reading graphs