AS 91523 · 2023 paper · diffraction glasses
Question 3(a) Achieved
Practises Diffraction & refraction
At a fair, children buy “Rainbow Glasses” made of diffraction gratings in a cardboard frame. Steve shines a laser pointer of wavelength 643 nm (6.43 × 10⁻⁷ m) through one of the gratings onto a wall. The pattern is a bright red spot at the centre, with slightly dimmer red spots either side. The wall is 1.43 m from the grating, and the distance from the central bright spot to the second dimmer spot is 1.75 m.
| λ | 643 nm (6.43 × 10⁻⁷ m) |
|---|---|
| grating to wall | 1.43 m |
| centre to 2nd spot | 1.75 m |
Describe diffraction.
Work it out
Try it on paper first. Stuck? Take one hint at a time.
-
1 What happens to a wave as it passes through a narrow gap?
It spreads out on the far side, instead of carrying on as a straight beam. That spreading is diffraction. -
2 When is the spreading greatest?
When the gap is about the same size as the wavelength, or smaller. Waves also spread round the edge of an obstacle.
What the marker wanted
- Achieved Describes diffraction as waves spreading out through a gap.
In our words, from NZQA’s 2023 marking schedule.
✕Common mistake
Describing interference instead: bright and dark spots. NZQA’s 2023 report says diffraction was poorly understood and often used to mean interference. Diffraction is the spreading; interference is what the spread-out waves do when they overlap.How did it go?
Got it moves it further out. Review brings it back tomorrow.