Sunlight takes about 8 minutes 20 seconds to reach Earth (distance 1.50×10¹¹ m). (a) Verify this using c = 3.00×10⁸ m/s. (b) Find the speed and wavelength of yellow light (vacuum wavelength 580 nm) in glass of refractive index 1.50. Step 1: time = distance/speed = 1.50×10¹¹ / 3.00×10⁸ = 500 s = 8 min 20 s ✓. Step 2: Speed in glass v = c/n = 3.00×10⁸/1.50 = 2.00×10⁸ m/s — light slows by a third. Step 3: Frequency is unchanged: f = c/λ = 3.00×10⁸/580×10⁻⁹ ≈ 5.17×10¹⁴ Hz. Wavelength in glass λ' = v/f = 2.00×10⁸/5.17×10¹⁴ ≈ 387 nm (= 580/1.5 ✓). The colour stays yellow because colour is set by frequency, not wavelength — the wave oscillates just as fast, only squeezed shorter in space.