Waves and Optics
Waves and Optics
Waves and Optics is Section C. It tests wave motion, sound, reflection and refraction, total internal reflection, and the action of mirrors and lenses.
Core rules and concepts
- Wave quantities. Amplitude is maximum displacement; wavelength λ is the distance between successive crests; frequency f is waves per second; period T = 1/f. Wave speed v = fλ.
- Types of wave. Transverse waves vibrate perpendicular to energy transfer; longitudinal waves vibrate parallel. Sound is longitudinal and needs a medium; light is transverse and can travel through a vacuum.
- Reflection and refraction. Angle of incidence equals angle of reflection, measured from the normal. Refraction occurs because speed changes; light bending towards the normal enters a denser medium. Snell's law: n = sin i / sin r, and n = c/v.
- Critical angle and TIR. Total internal reflection occurs only from a denser to a less dense medium, when i is greater than c. At the critical angle, sin c = 1/n.
- Lenses. A converging lens brings parallel rays to the principal focus; a diverging lens spreads them. Real images can be projected; virtual images cannot.
- Sound and hearing. Frequency determines pitch, amplitude determines loudness. Sound travels faster in solids than in air; the speed of sound in air is about 330–340 m/s.
Worked example
Light in air strikes glass of refractive index 1.50 at an angle of incidence of 30°. Find the angle of refraction. Reasoning: n = sin i / sin r, so sin r = sin i / n = sin 30° / 1.50 = 0.500 / 1.50 = 0.333. Thus r = sin⁻¹(0.333) ≈ 19.5°. The ray bends towards the normal because it enters the optically denser glass. The critical angle for this glass is given by sin c = 1/1.50, so c ≈ 41.8°.
Traps that cost marks
- Measuring angles from the mirror or surface rather than from the normal.
- Swapping i and r in Snell's law.
- Applying total internal reflection when light travels from less dense to denser.
- Confusing frequency and period, or using v = fλ with λ in cm and speed in m/s.
- Drawing a virtual image as if it could be caught on a screen.