AS Daily A Level Physics question
In a lab Young’s double-slit setup using a green laser and a distant screen, a thin transparent cover slip is slid directly in front of one slit only. Which statement must be true about the observed fringe pattern compared with before the cover slip was added?
Answer
The correct answer is B.
Correct: B — Every fringe shifts sideways as a set while the spacing between adjacent bright fringes stays the same. A The added glass introduces a constant extra optical path in one arm, which does not change fringe spacing, and the central bright is not fixed at the midpoint once a phase offset is introduced. B This is correct because the cover slip adds a fixed phase delay to one path, shifting the condition for constructive/destructive interference by the same amount for all orders; the spacing is set by wavelength in air and slit separation. C Any slight loss from reflection might reduce intensity, but the key effect is the phase change that repositions fringes, so positions do not stay the same. D A dark at the midpoint occurs only for a specific half-wavelength phase shift, not in general, and the spacing does not increase.