A2 Daily A Level Physics question
In a lab double-slit setup using a 600 nm red laser, the initial screen is 1.5 m from the slits (slit separation unchanged throughout). A very thin glass film of thickness 3.0 µm and refractive index 1.50 is then placed over only slit S1, and at the same time the screen is moved to 1.0 m from the slits. Which statement must be true about the new interference pattern compared with the original?
Answer
The correct answer is A.
Correct: A — The central bright shifts by 2.5 fringes towards the covered slit; the fringe spacing becomes 2/3 of its original value. The added optical path in S1 is (n−1)t = 0.5×3.0 µm = 1.5 µm = 1500 nm, which is 1500/600 = 2.5 wavelengths, so the pattern shifts by 2.5 fringes towards the covered slit, and moving the screen from 1.5 m to 1.0 m scales the spacing in proportion to distance, giving a factor 2/3. B is wrong because the shift is towards, not away from, the slit with the added delay. C is wrong because 1.7 fringes confuses spatial displacement (which changes with screen distance) with fringe count (which stays at 2.5); the spacing does not remain unchanged when the screen distance is reduced. D is wrong because the direction is wrong and the factor 3/2 inverts the correct proportional change (it should decrease by 2/3, not increase).