Q4. Assertion: a real wave runs back towards the source from every wavefront. Reason: Huygens’ secondary wavelets spread out in all directions. Are both true, and does the reason explain the assertion?
The assertion is false and the reason is true. The wavelets do have a backward envelope (D1D2 in NCERT Figure 10.2), but no backwave exists. Huygens removed it by assuming the wavelets are zero backwards; NCERT calls this ad hoc and says a more rigorous wave theory explains why there is no backwave (NCERT page 258).
Q5. Two points lie on the same wavefront of a light wave. What is the phase difference between them?
Zero: a wavefront is a surface of constant phase, so all its points vibrate in phase (NCERT page 257).
Q6. A plane wavefront moves through glass of refractive index 1.5. How far apart are the old and new wavefronts after 2.0 ns?
v = c/n = 2.0 × 108 m/s, so the new plane wavefront is vτ = 2.0 × 108 × 2.0 × 10−9 = 0.40 m ahead, parallel to the old one (NCERT page 258).
Q7. For a point source, how do the intensity and the amplitude change when the distance from the source doubles?
The same power spreads over a sphere four times larger, so the intensity falls to a quarter. Intensity goes as the square of the amplitude (NCERT page 263), so the amplitude halves (reasoned; beyond the NCERT text).