How light forms images, interferes and spreads around obstacles
Ray optics treats light as straight rays. For spherical surfaces and thin lenses, the lens maker’s formula, 1/f = (n − 1)(1/R₁ − 1/R₂), links focal length to refractive index and curvature. Total internal reflection occurs when light travels from a denser to a rarer medium at an angle greater than the critical angle, where sin C = 1/n. Microscopes and telescopes combine lenses, and the magnifying power of an astronomical telescope in normal adjustment is f₀/fₑ.
Wave optics explains effects that rays cannot. Huygens’ principle treats every point on a wavefront as a source of secondary wavelets. In Young’s double-slit experiment, light from two coherent slits overlaps to give bright and dark fringes with fringe width β = λD/d. Diffraction is the bending of light around the edges of small slits and obstacles. Polarisation shows that light is a transverse wave; Malus’s law gives transmitted intensity as I = I₀ cos²θ.
Why it matters
Optical fibres carry internet data using total internal reflection, and endoscopes let doctors see inside the body. Interference is used in anti-reflection coatings on spectacles and camera lenses, polarising filters reduce glare in sunglasses and photography, and diffraction limits the detail any microscope or telescope can resolve. Optics links to the dual nature of light and modern physics.
Key terms
- Critical angle
- The angle of incidence in a denser medium beyond which light is totally internally reflected.
- Coherent sources
- Light sources that emit waves of the same frequency with a constant phase difference.
- Diffraction
- The bending and spreading of waves around obstacles or through narrow openings.
- Polarisation
- Restricting the vibrations of a transverse wave to a single direction.