Physics (AS & A Level) · May / June 2026 · Paper 13 · Question 27

Light from a laser is incident normally on a double slit. A wave of light emerges from each slit. Bright and dark interference fringes are observed on a screen that is placed parallel to the double slit. Which statement explains the formation of a dark fringe?

1. AA dark fringe is formed where none of the light reaches the screen because it was blocked at the double slit.
2. BA dark fringe is formed where the waves of light from the two slits do not overlap and so cannot superpose.
3. CA dark fringe is formed where a trough from one wave of light meets a trough from the other wave of light.
4. DA dark fringe is formed where the two waves of light meet and have a phase difference of 180°.

As printed in the paper