The straight-ray picture from the last chapter breaks down here — interference, diffraction, and polarisation only make sense once you treat light as a wave.
Ray optics (last chapter) treats light as straight lines and works well for mirrors and lenses, but it can't explain why light bends slightly around obstacles, or why two beams of light can cancel each other out. For that, you need to go back to treating light as a genuine wave.
This simple geometric idea, entirely independent of ray optics, turns out to correctly predict reflection, refraction, interference, and diffraction — all from one construction.
Applying the wavelet construction at a boundary between two media — where light travels at different speeds on either side — reproduces Snell's Law exactly, and gives a cleaner physical picture of why it holds:
Two sources are coherent if they maintain a constant phase relationship over time — same frequency, and a fixed (not randomly shifting) phase difference. Only coherent sources produce a stable, observable interference pattern; ordinary independent light sources (two separate bulbs) are incoherent, since their phases drift randomly relative to each other thousands of times a second, washing out any pattern before your eye can register it.
Young's classic setup gets around the coherence problem cleverly: split light from a single source through two closely-spaced slits, so both resulting beams are automatically coherent with each other — any phase relationship in the original source is shared identically by both.
Diffraction is the bending and spreading of waves around obstacles or through narrow openings — the same underlying wave behaviour as interference, but usually discussed for a single slit rather than two.
Because any aperture (a lens, a telescope mirror, even your pupil) diffracts light passing through it, there's a fundamental limit to how finely an optical instrument can distinguish two closely-spaced objects — no amount of magnification can beat this limit. Larger apertures diffract less, giving better resolving power, which is exactly why large telescopes see finer detail than small ones.
Ordinary light vibrates in all directions perpendicular to its direction of travel. Polarisation restricts that vibration to a single plane — and the fact that light can be polarised at all is itself strong evidence that light is a transverse wave (only transverse waves can be polarised; longitudinal waves like sound cannot).
Light reflecting off a surface like water or glass becomes partially polarised — and at one specific angle of incidence, the reflected ray becomes completely polarised.
In a Young's double slit experiment, the slit separation is 0.5 mm, the screen is 1 m away, and the light used has wavelength 500 nm. Find the fringe width.
Solution: β = λD/d = (500×10⁻⁹ × 1) / (0.5×10⁻³) = 1×10⁻³ m = 1 mm.
Unpolarised light of intensity I₀ passes through two polarisers whose transmission axes are set 60° apart. Find the intensity of the final transmitted light.
Solution: After the first polariser: I₁ = I₀/2 (any unpolarised light is halved passing through a single polariser).
After the second, using Malus's Law: I₂ = I₁ cos²(60°) = (I₀/2)(0.25) = I₀/8.