State Huygens' principle. Using it, show with a diagram how a plane wavefront moves forward in a uniform medium.
How to lay out the answer. Follow the frame, then write it in your own words.
1 · State the principle. Every point on a wavefront is a source of secondary wavelets that spread with the wave speed v.
2 · Give the rule for the new wavefront. The forward envelope (common tangent) of the wavelets after time τ is the new wavefront.
3 · Draw the construction. [Plane wavefront F₁ with points A to D; equal semicircles of radius vτ on the forward side; common tangent F₂; rays at right angles]
4 · Explain the result. All wavelets have the same radius vτ, so the tangent is a plane parallel to F₁, a distance vτ ahead.
5 · Mention the backwave. Huygens assumed zero amplitude backwards, so no backward wavefront forms.
6 · Real life. Sunlight reaches the Earth as plane wavefronts moving forward in this way.
7 · Close in one line. So, a plane wavefront in a uniform medium stays plane and moves at speed v.
Scoring tip: Draw at least four wavelets of equal radius and write vτ on one of them.
