Waves and Optics
Waves and Optics tests wave properties, sound, light, reflection, refraction, lenses and mirrors, interference, diffraction, and polarization. Expect ray diagrams, the wave equation, Snell’s law, and image calculations.
Core rules
For any wave, v = fλ. Frequency is set by the source and does not change when a wave enters a new medium; speed and wavelength change. Reflection follows equal angles. Refraction follows Snell’s law, n1 sinθ1 = n2 sinθ2, with n = c/v. Total internal reflection occurs above the critical angle when light travels from higher to lower n. For thin lenses and mirrors, 1/f = 1/do + 1/di and magnification m = −di/do, with sign conventions. Constructive interference occurs when path difference is a whole number of wavelengths; destructive when it is a half-integer.
Worked example
An object is 15 cm from a converging lens with f = 10 cm. Use 1/f = 1/do + 1/di: 1/10 = 1/15 + 1/di, so 1/di = 1/10 − 1/15 = 3/30 − 2/30 = 1/30. Thus di = 30 cm. Magnification is m = −30/15 = −2. The image is real, inverted, and twice as large. The positive image distance and negative magnification are consistent with a real image formed beyond 2f.
Traps
Do not change frequency at a boundary. In lens problems, a virtual image has a negative image distance; mirror sign rules differ from lens rules, so state the convention. Sound requires a medium. Intensity is not the same as frequency. Diffraction is significant when wavelength is comparable to opening size.