Roman coronagraph: interference A-level challenge

A coronagraph is an instrument that reduces light from a bright star so a faint nearby planet can be detected. A student suggests placing only an opaque mask over the centre of the star's image. Explain why diffraction means this mask will not remove all the unwanted starlight from the surrounding image.

In a simplified monochromatic model, two components of the same stellar light overlap at one detector point. Their electric fields oscillate in the same direction. An adjustable mirror changes one component's optical path. State the amplitude and phase conditions for complete cancellation at this point, and explain why maintaining coherence matters for keeping the point dark.

A temperature change moves an optical component slightly. The amplitudes remain equal, but the phase difference at the previously dark detector point changes. Explain what happens to the starlight intensity at that point. Then evaluate this statement: 'Before the component moved, destructive interference destroyed the energy of the light.' Consider interference between the transmitted waves, not absorption by the mask.