Why grating maxima are sharper than double-slit fringes
For a given d, the positions of the maxima are the same for two slits and for a grating, but as the number of slits increases the maxima become much narrower and brighter, separated by broad dark regions. Just off the exact angle, the many beams get progressively further out of phase and cancel, so the intensity collapses almost immediately. This makes grating maxima far easier to locate precisely.
What this actually means
The intuition: with only two beams, a small angular error only makes them slightly out of step, so the brightness fades gradually. With a thousand beams, the same small error means the beam from slit 500 is antiphase with the beam from slit 1, and everything cancels in pairs.
So more slits means the same maxima positions, narrower peaks, higher peak intensity and much better contrast. Say all four in a describe-and-explain answer.
This is why the grating is the instrument of choice for measuring wavelength. Sharper maxima mean a smaller uncertainty in θ, and therefore a smaller uncertainty in λ.
One consequence people forget: on a grating there is no useful equally spaced fringe pattern to talk about. The maxima are widely and unevenly spaced, so fringe separation is not a meaningful quantity.
The single-slit diffraction envelope still applies, set by the width of each individual ruling, so higher orders remain dimmer even though they are just as sharp.
Claiming a grating moves the maxima to different angles from a double slit of the same d. It only sharpens them.
Prove it — watch it be true
- Open the sharper-than-double-slit comparison view with the same d for both.
- Confirm the maxima sit at identical angles in both patterns.
- Increase the number of slits and watch the peaks narrow and brighten while the dark gaps between them broaden.