WAEC Physics 2013 Theory — Question 13

Model answer

(a) Sound waves travel faster on a hot day, because the speed of sound (v) is proportional to the square root of the absolute temperature (T), i.e. v ∝ √T; so the higher the temperature, the greater the speed of sound. (b) Megaphones are shaped like funnels because at the smaller end of the funnel/cone, the sound energy is concentrated into a small mass of air, so as it comes out of the larger open end, the energy sets a larger mass of air into vibration, which makes the final sound louder. Also, since intensity of sound is energy per unit area, a greater intensity of sound is experienced at the larger open end before the sound spreads out. (c)(i) In the diagram, x = angle of incidence (between the incident ray and the normal), y = angle of reflection (between the reflected ray and the normal, on the same side), z = angle of refraction (between the refracted ray and the normal, on the opposite side within the glass). (ii) Refractive index of glass n(g) = 1.5, refractive index of air n(a) = 1 (ray moving from air to glass): n(g)/n(a) = sin(i)/sin(r). Angle of incidence i = (90−30)° = 60°. So 1.5 = sin60°/sin(z); sin(z) = sin60°/1.5 = 0.5774; z = sin⁻¹0.5774 = 35.3° ≈ 35° (to the nearest whole number).