AS 91523 · 2024 paper · interference

Question 2(a) Achieved

Jane's physics teacher shines a red laser through a diffraction grating placed at the centre of a semicircular screen. The pattern on the screen is a number of narrow, bright fringes.

200 lines per mm
λ	638 nm

The grating formula is nλ = d sinθ, where n is the order number and λ is the wavelength. What does the term nλ represent?

The 2024 paper, page 4 ↗ Marking schedule, page 3 ↗

See it move: Two sources, and the path difference between them

Work it out

Try it on paper first. Stuck? Take one hint at a time.

1 Picture the waves leaving two neighbouring slits towards a bright fringe at angle θ. Which travels further, and by how much?
The wave from the further slit travels an extra d sin θ: the slit spacing times sin θ.
2 For that fringe to be bright, how must the two waves arrive?
In phase. So the extra distance must be a whole number of wavelengths: d sin θ = nλ.
3 So what is nλ?
The extra distance itself: the path difference between waves from neighbouring slits, which for the nth bright fringe is n whole wavelengths.

Answer The path difference: how much further the light from one slit travels than the light from the next, which at the nth bright fringe is a whole number, n, of wavelengths.

What the marker wanted

Achieved The path difference.

In our words, from NZQA’s 2024 marking schedule.

How did it go?

Got it moves it further out. Review brings it back tomorrow.