← all cardsTopic 11 · Wave Motion · Phase & phase difference
Oblique wavefronts → effective separation Δx
Two points along a boundary hit by wavefronts at angle θ: effective separation is Δx = d sin θ (perpendicular distance between wavefronts through the points), THEN Δφ = (Δx/λ)2π. Draw the right triangle — the setup mark is the hard mark (N95 sea-wall).
What this actually means
When wavefronts arrive at an angle to a line of points, the separation that matters is not the distance d between the points but its component along the travel direction: Δx = d sin θ.
Draw the right-angled triangle between a wavefront and the line of points; that setup is where the hard mark sits. Then feed Δx into (Δx/λ) × 2π exactly as usual.
The N95 sea-wall question is this card in exam clothing: two posts on a wall, waves arriving obliquely, one triangle to draw.
Prove it — watch it be true
- Switch to oblique incidence and place two points distance d apart
- Watch the Δx = d·sinθ triangle build itself between the wavefronts
- Vary θ: at 90° the full d counts; at 0° the points are hit together