Diffraction grating viewed up close shows undiffracted point source

Steps:

• Monochromatic light from distant point source incidents normally on grating with vertical lines.
• Grating diffracts light into orders, but zeroth order transmits straight through undeviated.
• At position near grating, angular spread of diffracted orders has negligible linear separation.
• Observer sees only overlapping central beam as single point.

Why C is correct:

• Zeroth-order diffraction follows grating equation $d \sin \theta = m \lambda$; for $m=0$, $\theta=0$, so direct transmission appears as original point source per Huygens' principle.

Why the others are wrong:

• A: Point images require circular aperture diffraction, not linear grating lines producing slit-like patterns.
• B: Vertical spectral lines form only on distant screen where orders separate spatially.
• D: Grating alters light path via diffraction; "distant" describes source, not observed image.

Final answer: C