Masking changes sharpness, not centre separation

A grating resolves a nearby wavelength pair in a fixed nonzero order. A mask then covers half the illuminated lines while leaving their spacing, incident direction and wavelengths unchanged.

What is the ratio of the angular separation of the line centres after masking to before masking?

Organize the givens & assumptions

Illuminated count changes from N to N/2
Spacing and order unchanged

Ideal far-field diffraction, normal incidence on a regular transmission grating, equal coherent illumination over the stated width, and vacuum wavelengths in air. Resolution uses the stated ideal Rayleigh criterion; aberrations and detector limitations are neglected.

A $1/2$
B $0$
C $1$
D $2$

Try it on paper first. Free sample problems in every collection include the complete reasoning: find one in this collection →