Q4. Assertion: a ray passing through a parallel-sided glass slab surrounded by air on both sides comes out parallel to its original direction. Reason: the refractions at the two faces are equal and opposite. Are both true, and does the reason explain the assertion?
Both are true, and the reason explains the assertion: at the second face the angle of incidence is i2 = r1 (the faces are parallel), so sin r2 = n sin r1 = sin i1, so the ray leaves at the angle it entered and is only shifted sideways (NCERT page 229).
Q5. The speed of light in a medium is 2.0 × 108 m/s. Find its refractive index with respect to vacuum.
n = c/v = (3.0 × 108)/(2.0 × 108) = 1.5 (NCERT Eq. (10.4), page 259).
Q6. The refractive index of glass is 1.5 and of water 1.33, both with respect to air. Find the refractive index of glass with respect to water.
ngw = nga × naw = 1.5 × (1/1.33) ≈ 1.13 (NCERT Eq. (9.11), page 229).
Q7. A bird flies 3.0 m above a pond (n = 4/3), directly above a fish 1.2 m below the surface. How far away does the bird look to the fish?
Going from air into water the rays bend towards the normal, so near the normal the bird looks n times higher: 4/3 × 3.0 = 4.0 m above the surface. It looks 4.0 + 1.2 = 5.2 m away, farther than the real 4.2 m (reasoned from NCERT page 229).