Sound and Light
Sound
SEC Physics tests sound as a longitudinal wave that needs a material medium. Speed is greatest in solids and least in gases; sound cannot travel through a vacuum. Frequency determines pitch; amplitude determines loudness. Echoes are reflected sound; ultrasound has a frequency above human hearing, about 20 kHz.
Light and rays
Light travels in straight lines in a uniform medium. Reflection gives i = r, with angles measured to the normal. Refraction at a boundary follows Snell's law, n₁ sin θ₁ = n₂ sin θ₂; for air to glass, n = sin i/sin r. Frequency is unchanged; speed and wavelength change. Total internal reflection occurs above the critical angle C, where sin C = 1/n for glass to air. Converging and diverging lenses form real or virtual images; dispersion separates white light because refractive index varies with colour.
Worked example
A ray passes from air into glass of refractive index 1.50 at an angle of incidence of 30°. Find the angle of refraction.
sin r = sin i/n = sin 30°/1.50 = 0.500/1.50 = 0.333.
r = sin⁻¹(0.333) ≈ 19.5°.
Reasoning: the ray bends towards the normal because glass is optically denser; the angle is measured from the normal.
Traps
Using the angle to the surface instead of the normal; writing n = sin r/sin i; claiming frequency changes on refraction; using sin C = n instead of 1/n; forgetting that sound cannot cross a vacuum.