AS Daily A Level Physics question
A laser illuminates two identical narrow slits to form an interference pattern on a distant screen. At the centre of the screen (a bright fringe), the measured light intensity with both slits open is I0. One slit is then completely covered; nothing else is moved. What is the intensity at the same central point now?
Answer
The correct answer is B.
Correct: B — 0.25 I0. With both slits open and in phase at the centre, equal wave amplitudes add to double the single-slit amplitude, so the intensity there is four times that from one slit; blocking one slit leaves one contribution, so the intensity is I0/4. A Treats intensity as simply adding from two slits (doubling), ignoring that in coherent interference amplitudes add and intensity depends on amplitude squared. B Two equal in-phase amplitudes give 2A, so intensity ∝ (2A)^2 = 4A^2; hence one-slit intensity is a quarter of I0. C No change would require no coherent interference; here removing one slit removes one amplitude contribution. D It is not zero: a single slit still sends light to the centre, giving a bright single-slit central maximum rather than darkness.