You look at a distant lamp through a narrow gap between two fingers held close to your eye. The light spreads into coloured bands. (a) Is this Fresnel or Fraunhofer diffraction, and why? (b) In a laboratory Fraunhofer setup, a lens of focal length 50 cm focuses light of wavelength 600 nm through a 0.1 mm slit. How wide is the central maximum on the focal plane? Solution: (a) This is Fresnel diffraction: the source (lamp) and screen (your retina) are at finite distances and no lenses make the wavefronts flat — you see near-field fringes from curved wavefronts. (b) For a single slit, half-angular width θ = λ/a = 600×10⁻⁹/1.0×10⁻⁴ = 6×10⁻³ rad. Linear half-width on the focal plane = fθ = 0.5 × 6×10⁻³ = 3 mm, so the full central maximum is 6 mm wide.