Describe the wave model of light and apply the principle of superposition to constructive and destructive interference.

What this dot point is asking

You need to describe the wave model of light and apply the principle of superposition to predict where constructive and destructive interference occur.

The wave model of light

The wave model treats light as a transverse electromagnetic wave. Its key quantities are wavelength $\lambda$, frequency $f$ and speed $v$ (in a vacuum, $c = 3.00\times10^8\ \text{m s}^{-1}$), linked by:

Different colours correspond to different wavelengths; the visible range runs from about $400\ \text{nm}$ (violet) to $700\ \text{nm}$ (red). The wave model successfully explains reflection, refraction, diffraction and interference.

The principle of superposition

Superposition is what makes interference possible: the combined wave can be larger or smaller than either wave alone, depending on the phase relationship at that point.

Coherence and path difference

For a stable interference pattern the two sources must be coherent, meaning they have the same frequency and a constant phase relationship. The outcome at any point is then decided by the path difference, the difference in the distances travelled by the two waves to that point.

So you classify any point by dividing its path difference by the wavelength: a whole number gives a maximum, a half-integer gives a minimum.

Why interference is evidence for waves

Interference - the alternating reinforcement and cancellation of two overlapping disturbances - is a uniquely wave phenomenon. Particles travelling in straight lines cannot cancel one another out, yet two coherent light beams produce dark fringes where they overlap. The double-slit pattern (next dot point) is the classic demonstration and was historically decisive evidence for the wave nature of light.

How SACE assesses this

SACE Stage 2 questions on superposition give the path difference to a point (often from two antennas or two slits) and ask whether the interference is constructive or destructive. The reliable method is to compute the path difference, divide it by the wavelength, and classify the result: a whole number of wavelengths gives a maximum (constructive), an odd number of half-wavelengths gives a minimum (destructive). A frequent explanation part asks you to account for the dark fringes in a two-slit pattern, where the marking points are superposition of the two coherent waves, destructive interference (a crest meeting a trough), and the path-difference condition of an odd number of half-wavelengths. State your reasoning explicitly and quote the path-difference condition you are using.