IB Physics HL C 3 6 Coherence Topic Practice

A beam of coherent monochromatic light from a distant galaxy is used in an optics experiment on Earth.

The beam is incident normally on a double slit. The distance between the slits is 0.300 mm . A screen is at a distance D from the slits. The diffraction angle $\theta$ is labelled.

Outline why the beam has to be coherent in order for the fringes to be visible.