Ray Optics and Optical Instruments — Class 12 Physics

Treating light as rays, reflection and refraction explain image formation by mirrors and lenses. Spherical mirrors obey the mirror equation with magnification . Refraction at a plane surface follows Snell's law ; when light passes to a rarer medium beyond the critical angle, total internal reflection occurs, the basis of optical fibres and brilliant gems. Refraction at spherical surfaces leads to the lens maker's formula and the thin-lens equation ; lens powers add for combinations. A prism deviates light, the minimum deviation giving the refractive index, and dispersion separates white light into colours. The eye focuses by changing lens curvature, and defects such as myopia and hypermetropia are corrected with lenses. Optical instruments—the simple and compound microscope and the astronomical telescope—extend vision, their magnifying powers determined by focal lengths and the near point, allowing us to observe the very small and the very distant.