Wave Optics notes

Wave optics explains optical phenomena that cannot be understood fully by treating light only as straight rays. It studies light as a wave and explains Huygens principle, interference, diffraction and polarisation. Huygens principle says every point on a wavefront acts as a source of secondary wavelets, helping derive reflection and refraction. Interference explains redistribution of intensity when coherent waves superpose, especially in Young’s Double Slit Experiment. Diffraction shows bending and spreading of light around obstacles or through narrow apertures. Polarisation proves that light is transverse because only transverse waves can be plane polarised. For NEET, the most important areas are fringe width, coherent sources, single-slit pattern, Brewster’s law and conceptual comparisons.

Huygens principle is a geometrical method to describe wave propagation. A wavefront is a surface on which all particles vibrate in the same phase. Every point on a wavefront acts as a source of secondary spherical wavelets, and the new wavefront at a later time is the forward envelope of these wavelets. Rays are perpendicular to wavefronts in homogeneous isotropic media. Plane wavefronts come from very distant sources, spherical wavefronts come from point sources, and cylindrical wavefronts come from line sources. Using wavefront construction, laws of reflection and refraction can be derived. In refraction, bending occurs because wave speed changes in the second medium.

Interference is the redistribution of light intensity when two or more coherent waves superpose. According to the principle of superposition, the resultant displacement at a point is the algebraic sum of individual displacements. In Young’s Double Slit Experiment, a single source illuminates two narrow slits, making them coherent secondary sources. Their waves overlap on a screen to produce alternate bright and dark fringes. Bright fringes occur when path difference is an integral multiple of wavelength, while dark fringes occur when path difference is an odd multiple of half wavelength. Fringe width depends directly on wavelength and screen distance, and inversely on slit separation. YDSE is one of the most important NEET numericals.

Diffraction is the bending and spreading of light when it passes through a narrow aperture or around an obstacle. It becomes prominent when the size of the aperture or obstacle is comparable to the wavelength of light. In single-slit diffraction, different parts of the same wavefront act as secondary sources and interfere with one another. The pattern consists of a broad and bright central maximum, followed by weaker secondary maxima on both sides separated by dark minima. The first minima satisfy a sin θ = ±λ. Unlike YDSE interference fringes, single-slit diffraction fringes are not equally bright, and the central maximum is twice as wide as the secondary maxima.

Polarisation is the phenomenon in which vibrations of light are restricted to a particular plane perpendicular to the direction of propagation. Ordinary light is unpolarised because its electric field vibrations occur randomly in all possible transverse directions. When only one vibration direction remains, the light is plane polarised. Since longitudinal waves cannot be polarised, polarisation proves that light is transverse in nature. Light can be polarised by selective absorption using Polaroids, by reflection, by scattering and by double refraction in some crystals. At Brewster’s angle, reflected light is completely plane polarised, and the reflected and refracted rays are perpendicular. Applications include Polaroid sunglasses, photography, LCDs and stress analysis.