Ray Optics and Optical Instruments
What this domain tests
Ray Optics and Optical Instruments tests reflection, refraction, total internal reflection, lenses and mirrors, prisms, dispersion and the working of microscopes and telescopes.
Core rules
- Mirror: 1/f = 1/v + 1/u; lens: 1/f = 1/v − 1/u, using the sign convention with distances measured from the pole or optical centre.
- Magnification m = v/u for mirrors and for lenses with signs; power P = 1/f in dioptre when f is in metres.
- Refraction: n1 sin i = n2 sin r; critical angle sin C = 1/n for dense-to-rare travel.
- Lens maker: 1/f = (n−1)(1/R1 − 1/R2).
- Prism: deviation and dispersion; at minimum deviation n = sin((A+D_m)/2)/sin(A/2).
Worked example
A convex lens of focal length 20 cm has an object 30 cm in front. Using 1/v − 1/u = 1/f with u = −30 cm: 1/v = 1/20 − 1/30 = 1/60, so v = +60 cm. The image is real, inverted and magnified: m = v/u = 60/(−30) = −2.
Traps
- Sign errors; these are the most common loss of marks in lens and mirror numericals.
- Applying total internal reflection from rare to dense, or below the critical angle.
- Confusing power in dioptres with focal length in centimetres.
- For a telescope, using m = f_o/f_e without noting that the final image is inverted.