Waves, Sound, and Optics
Waves, Sound, and Optics
This domain tests wave properties, superposition, standing waves, sound, and geometric and physical optics. Exam items often require you to relate frequency, wavelength, and speed, or to apply Snell's law and lens equations with correct signs.
Core rules and concepts
- Wave speed v = fλ. When a wave crosses into a new medium, frequency stays the same and speed and wavelength change.
- Sound intensity level: β = 10 log(I/I0), with I0 = 10⁻¹² W/m².
- Standing waves on a string fixed at both ends have nodes at the ends; the fundamental has λ = 2L.
- Reflection: angle of incidence equals angle of reflection, measured from the normal. Refraction follows Snell's law: n1 sinθ1 = n2 sinθ2.
- Thin-lens equation: 1/f = 1/do + 1/di; magnification m = -di/do. Convex lenses converge (positive f for a real object), concave lenses diverge.
Worked example
Light in air (n ≈ 1.00) strikes water (n = 1.33) at 40° from the normal. sinθ2 = (1.00/1.33) sin40° = 0.483, so θ2 ≈ 29°. The ray bends toward the normal because it enters a denser medium.
Traps that cost marks
Measuring angles from the surface instead of the normal; assuming frequency changes on refraction; using the wrong sign for image distance in lens problems; and confusing nodes with antinodes. In interference, path difference determines bright or dark fringes.