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LearnAQA7408First teaching 2015Paper 1 · Paper 1Waves

Paper 1 · Module

Waves

6 subtopics≈ 220 min in total

Start the module: Progressive waves and intensity Open graphing calculator

Progressive waves and intensity
A progressive wave transfers energy without transferring matter. Points one wavelength apart oscillate in phase; points half a wavelength apart are in antiphase. Phase difference (in degrees or radians) $=\dfrac{x}{\lambda}\times360^\circ$ . Intensity, the power per unit area, is proportional to amplitude squared, and falls with the inverse square of distance from a point source.
≈ 35 min
Stationary waves
When two identical progressive waves travel in opposite directions, they superpose to form a stationary wave. Nodes (zero amplitude) and antinodes (maximum amplitude) stay in fixed places, $\tfrac\lambda2$ apart. No energy is carried along. On a string fixed at both ends the fundamental has $\lambda=2L$ ; in a pipe closed at one end, $\lambda=4L$ .
≈ 40 min
Diffraction and interference
Waves spread out (diffract) through gaps about the size of their wavelength. Two coherent sources produce an interference pattern: bright fringes where the path difference is a whole number of wavelengths, dark fringes where it's a half-integer. For Young's double slits, fringe spacing $x=\dfrac{\lambda D}a$ .
≈ 40 min
Electromagnetic spectrum and polarisation
All electromagnetic waves are transverse and travel at $3.00\times10^8$ m s⁻¹ in free space. Wavelengths run from kilometres (radio) to less than $10^{-11}$ m (gamma); visible light is $400$ – $700$ nm. Only transverse waves can be polarised. A polariser halves unpolarised light; after that, Malus's law gives $I=I_0\cos^2\theta$ .
≈ 30 min
The diffraction grating
A diffraction grating has thousands of equally spaced slits. Maxima appear only at angles where light from every slit arrives in phase: $d\sin\theta=n\lambda$ , with $d$ the slit spacing. The maxima are much sharper and brighter than with two slits, so gratings measure wavelengths precisely and split light into spectra.
≈ 35 min
Refraction and optical fibres
Light slows down in a material; the refractive index is $n=\frac cv$ . At a boundary, $n_1\sin\theta_1=n_2\sin\theta_2$ (Snell's law). Going into a less dense medium, beyond the critical angle $\sin C=\frac{n_2}{n_1}$ all the light reflects: total internal reflection. Optical fibres trap light this way, using a core surrounded by cladding of lower refractive index.
≈ 40 min