Light of wavelength 650 nm passes through a slit of width 0.25 mm and falls on a screen 2.0 m away. (a) Find the angular position of the first minimum. (b) Find the width of the central maximum on the screen. Solution: (a) First minimum: a sin θ = λ → sin θ = λ/a = 650×10⁻⁹ / 2.5×10⁻⁴ = 2.6×10⁻³. For small angles, θ ≈ 2.6×10⁻³ rad ≈ 0.149°. (b) The central maximum stretches from the first minimum on one side to the first on the other: full angular width = 2θ = 5.2×10⁻³ rad. Linear width = D × 2θ = 2.0 × 5.2×10⁻³ = 0.0104 m ≈ 1.04 cm. So the bright central band is about a centimetre wide.