Wave OpticsNEET Physics · Class 12 · NCERT Chapter 10

You can expect 1 to 2 questions in NEET 2027, often with the bigger ask coming from Young's double slit experiment (fringe width, intensity at a point), single-slit diffraction width, and polarisation laws (Brewster's, Malus').

A wavefront is a surface where all points are at the same phase of oscillation. A point source produces a spherical wavefront. Far from a source, the wavefront becomes a plane. The direction perpendicular to the wavefront is the direction of propagation (the ray).

Every point on a wavefront acts as a source of secondary spherical wavelets. The new wavefront at a later time is the envelope of these secondary wavelets. Used to derive reflection (angle equal), refraction (Snell's law follows from speed difference) and propagation in general.

Two sources are coherent if they have the same frequency and a constant phase difference. Coherence is required to see a stable interference pattern. Sunlight from two different filaments is incoherent: the phase between them changes randomly. A laser is highly coherent.

beta = lambda D over d, where lambda is wavelength, D is the slit-to-screen distance and d is the separation between the slits. Fringe width is the distance between consecutive bright (or dark) fringes. Same for both. The pattern is periodic with bright at x = n lambda D / d.

The slab introduces an extra optical path of (n - 1) t, where n is the refractive index of the glass and t is its thickness. The whole fringe pattern shifts towards the side with the slab by (n - 1) t D over d, equivalent to ((n - 1) t / lambda) fringes.

2 lambda D over a, where a is the slit width and D is the slit-to-screen distance. First minima on either side at a sin theta = lambda. The central max is twice as wide as the secondary maxima.

Interference is between coherent waves from two (or more) discrete sources (e.g. two slits). Fringes are equally bright and equally spaced. Diffraction comes from many wavelets across one aperture. Central max is brightest, secondary maxima decrease in brightness, central max is twice the width of the others.

tan theta_B = n. At this angle of incidence (from the rarer side), the reflected light is completely plane-polarised perpendicular to the plane of incidence. The reflected and refracted rays are exactly 90 degrees apart. Polaroid sunglasses reduce glare from horizontal surfaces (water, road) by blocking this polarised reflection.

When polarised light of intensity I_0 passes through a polariser whose axis makes angle theta with the wave's polarisation direction, the transmitted intensity is I = I_0 cos^2 theta. Crossed polarisers (theta = 90°) block all light. Unpolarised light through an ideal polariser has intensity halved (I_0 / 2).