Q4. Assertion: the central fringe in Young’s experiment is bright. Reason: at the centre of the screen the path difference is zero. Are both true, and does the reason explain the assertion?
Both are true, and the reason explains the assertion: zero path difference means the waves from S1 and S2 arrive in phase, which is the n = 0 case of NCERT Eq. (10.13) (page 266).
Q5. Light of wavelength 600 nm passes through slits 0.30 mm apart. Find the angular fringe width.
θ = λ/d = (600 × 10−9)/(0.30 × 10−3) = 2.0 × 10−3 rad, about 0.11° (reasoned from NCERT Eq. (10.13)).
Q6. With β = 0.50 mm, find the distance between the third bright fringe on one side of the centre and the third dark fringe on the other side.
The third bright fringe is at 3β and the third dark fringe at (2 + 1/2)β on the other side, so the distance is 3β + 2.5β = 5.5β = 2.75 mm (NCERT Eqs. (10.13)–(10.14), page 266).
Q7. Why does a Young’s experiment with white light show a white centre but only a few coloured fringes?
Every colour has zero path difference at the centre, so all are bright there and add to white. Away from the centre each colour’s bright fringes are at different places, because β = λD/d grows with λ, so the colours separate and soon overlap into an even white (reasoned; beyond the NCERT text).