# Wave Optics > Wave Optics for NEET-UG Physics — Huygens' principle, Young's double slit, fringe width, diffraction, resolving power and polarisation from NCERT. - Canonical URL: https://prepelephant.com/topics/neet-ug/physics/wave-optics - Exam / course: NEET-UG · Subject: Physics - Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team - First published: 2026-10-02 - Last updated: 2026-10-02 - How to cite: "Wave Optics", PrepElephant, https://prepelephant.com/topics/neet-ug/physics/wave-optics ## Direct answer Young's double-slit experiment — one source, two slits, a steady alternation of bright and dark fringes — is the display piece of wave optics: coherent sources interfere, and the fringe width β = λD/d is the single most examined result of the chapter. Huygens' construction explains reflection and refraction as wave behaviour, single-slit diffraction and resolving power carry the wave account forward, and polarisation — possible only for transverse waves — closes it with Brewster's and Malus's laws. ## What you must remember - Wavefront: locus of points vibrating in phase; Huygens treats every point of it as a secondary source, and the laws of reflection and refraction follow from the construction. - Coherent sources keep a constant phase difference; two independent lamps never qualify, which is why Young split one beam through two slits. - YDSE: bright fringes where path difference = nλ; dark where it equals (n + ½)λ; fringe width β = λD/d — wider for longer wavelength, bigger D, smaller d. - Intensity with equal sources: I = 4I0 cos^2(φ/2), ranging from 4I0 at bright fringes to zero at dark ones — the maximum is four times a single slit's contribution. - Single-slit diffraction: central maximum of angular width 2λ/a, flanked by weak secondary maxima; the pattern broadens as the slit narrows. - Interference versus diffraction: interference fringes are equally spaced with equal-intensity maxima; diffraction's central maximum is twice as wide and intensity falls rapidly in the side lobes. - Resolving power: telescope minimum angle 1.22λ/d (bigger objective, better); microscope limit about 1.22λ/(2μ sinθ) — and the electron microscope wins because electron de Broglie wavelengths are far shorter than light's. - Polarisation: transverse waves only; Brewster's law n = tan i_B (reflected and refracted rays then perpendicular); Malus's law I = I0 cos^2θ between polariser axes; polaroid sunglasses cut glare. ## Working Young's experiment with numbers Light of 600 nm falls on slits 1 mm apart with the screen 1 m away. Fringe width β = λD/d = (600 × 10^-9 × 1)/10^-3 = 0.6 mm — a comfortably visible stripe. Now interrogate the arrangement: at 0.3 mm from the centre, the path difference is dy/D = 10^-3 × 3 × 10^-4/1 = 3 × 10^-7 m = 300 nm, exactly λ/2, so that position is dark. Next, drown the apparatus in water (n = 1.33): the wavelength shortens to λ/n ≈ 450 nm, the fringes crowd to β/n ≈ 0.45 mm, and a point that was dark becomes bright if the new path difference is an integral multiple of the new λ — the medium changes the geometry not at all and the answer completely. Finally, cover one slit and the fringes vanish, replaced by the broad single-slit diffraction envelope: interference needs both, and the exam loves asking what survives when one slit is shut. ## Where students slip The fringes are a competition of path difference, not of distance from centre: a point 0.3 mm left and 0.3 mm right of centre are mirror twins, and shifting the source sideways shifts the whole pattern rather than blurring it. Second, the d and D roles: β = λD/d puts slit separation below and screen distance above, and the inverted-memory version d/λD is the planted wrong option. Third, why the slits must be close and the screen far: only then are the angles small and the fringe spacing uniform — the formula's secret assumption. Fourth, Brewster questions carry a hidden gift: at the polarising angle, reflected plus refracted rays make 90°, and i_B + r = 90° with Snell's law delivers tan i_B = n; the option asking for the reflected ray's polarisation (fully polarised) versus the refracted ray's (partial) is the intended distinction. Fifth, Malus's law uses the angle between the polariser's axis and the analyser's axis, and cos^2 (not cos) is what survives — intensity, not amplitude. ## Frequently asked questions ### What are coherent sources, and why does Young's experiment need them? Sources of the same frequency maintaining a constant phase difference, so the pattern stays fixed on the screen; independent sources drift randomly and wash the fringes out. ### What is the expression for fringe width in Young's experiment? β = λD/d — wavelength times screen distance over slit separation; it sets the spacing of both bright and dark fringes alike. ### What happens to the fringe pattern when the apparatus is immersed in water? The wavelength falls to λ/n, so the fringe width shrinks by the factor n ≈ 1.33; the geometry is unchanged, only the wavelength in the medium has. ### State Brewster's law and its consequence. tan i_B = n for light incident from air; at this polarising angle the reflected light is completely polarised, and the reflected and refracted rays are perpendicular to each other. ### How do interference fringes differ from diffraction fringes? Interference gives equally spaced maxima of equal intensity from two wavefronts; diffraction from a single aperture gives a broad central maximum, twice the width of the rest, with rapidly weakening side maxima.