Ray Optics and Optical InstrumentsNEET Physics · Class 12 · NCERT Chapter 9

You can expect 2 to 3 questions in NEET 2027. The chapter has high PYQ frequency. Common asks: image formation by spherical mirrors and thin lenses, lens-maker formula, total internal reflection, prism deviation and angular dispersion, and microscope or telescope magnification.

All distances are measured from the pole of the mirror or optical centre of the lens, along the principal axis. Distances measured in the direction of incident light (usually rightwards) are positive, and against it are negative. Heights above the principal axis are positive, below are negative. Focal length of a concave mirror or convex lens (used as converging) is negative or positive depending on convention. Apply consistently and you will not be confused.

1 over v plus 1 over u equals 1 over f. With sign convention: real object u is negative, real image v is negative for a concave mirror. Magnification m equals minus v over u. Positive m means erect image, negative m means inverted. |m| > 1 means magnified, |m| < 1 means diminished.

When light passes from medium 1 to medium 2, n_1 sin theta_1 equals n_2 sin theta_2. The angles are measured from the normal. Light bends towards the normal when going from rarer to denser (n increases) and away from the normal when going denser to rarer.

Only when light goes from a denser medium to a rarer medium. The critical angle is sin theta_c equals 1 over n where n is the refractive index of the denser medium relative to the rarer. For glass-air, theta_c ≈ 42°. Light at angles greater than theta_c is totally reflected. Used in optical fibres, prismatic binoculars and brilliance of diamonds.

1 over f equals (n - 1)(1 over R_1 minus 1 over R_2). Here n is the refractive index of the lens material, R_1 is the radius of the surface light hits first and R_2 is the second surface. Use Cartesian convention. For a biconvex lens with both R equal, 1 over f equals 2(n - 1) over R.

In contact: 1 over f equals 1 over f_1 plus 1 over f_2 (or P equals P_1 plus P_2 in diopters). For lenses separated by distance d: 1 over f equals 1 over f_1 plus 1 over f_2 minus d over (f_1 f_2).

At minimum deviation, the ray inside the prism travels parallel to the base, so r_1 = r_2 = A over 2 and i_1 = i_2 = (A + delta_m) over 2. Refractive index n equals sin((A + delta_m) / 2) divided by sin(A / 2). For small prism angles, delta = (n - 1) A.

Angular dispersion equals (n_v - n_r) A or delta_v - delta_r: the spread of colours in radians. Dispersive power equals (n_v - n_r) divided by (n_y - 1) where n_y is the refractive index for yellow (mean): a dimensionless property of the material. Dispersive power of crown glass is around 0.02; of flint glass around 0.03.

Compound microscope: M = (L / f_o)(D / f_e) for image at near point, where L is tube length, f_o is objective focal length, f_e is eyepiece focal length, D = 25 cm. Astronomical telescope (normal adjustment, image at infinity): M = f_o / f_e and length L = f_o + f_e. For maximum magnification, use a long-focal-length objective and a short-focal-length eyepiece.