Physics (AS & A Level) · May / June 2019 · Paper 13 · Question 28

Light of wavelength λ is emitted from two point sources R and S and falls onto a distant screen. [diagram showing sources R and S, a screen with point P, labelled (not to scale)] At point P on the screen, the light intensity is zero. What could explain the zero intensity at P?

1. ALight from the two sources is emitted 180° out of phase and the path difference to P is ½λ.
2. BLight from the two sources is emitted in phase and the path difference to P is λ.
3. CLight from the two sources is emitted 90° out of phase and the path difference to P is λ.
4. DLight from the two sources is emitted in phase and the path difference to P is ½λ.

As printed in the paper