(a) Amplitudes reaching a screen point are a₁ = 3 units, a₂ = 1 unit. Find I_max, I_min, and visibility. (b) What if a₂ = 3 units? (c) White light illuminates a double slit. Why are only the central few fringes coloured? Step 1: I_max = (3+1)² = 16, I_min = (3−1)² = 4. V = (16−4)/(16+4) = 12/20 = 0.6. Fringes visible but grey-ish — dark fringes never go black. Step 2: I_max = 36, I_min = 0, V = 1 — perfect contrast, dark fringes truly dark. Equal amplitudes maximise visibility. Step 3: Each wavelength makes its own fringe set with β = λD/d: red fringes wider, blue narrower. At n = 0 all colours coincide (white central fringe); at higher orders the sets shift apart and overlap — beyond 2–3 orders the colours mix back to whitish. Plus white light's tiny coherence length kills fringes at large path differences anyway.