Ray Optics and Optical Instruments
What this domain tests
Ray Optics and Optical Instruments tests reflection, refraction, total internal reflection, lenses, mirrors, magnification and simple optical instruments. Numerical questions rely on correct sign convention.
Core rules
- Mirror formula: 1/f = 1/v + 1/u; lens formula: 1/f = 1/v - 1/u. Use Cartesian sign convention: distances measured against incident light are negative.
- Magnification m = h_i/h_o. For mirrors m = -v/u; for lenses m = v/u.
- Refraction: n1 sin i = n2 sin r. Critical angle: sin C = 1/n for rarer medium air.
- Lens power P = 1/f (metre), unit dioptre. For thin lenses in contact, P = P1 + P2.
- A simple microscope uses a convex lens as magnifier; a compound microscope uses objective and eyepiece; a telescope uses objective of large focal length and eyepiece of short focal length.
Worked example
An object is 30 cm from a concave mirror of focal length 10 cm. With the object in front, u = -30 cm and f = -10 cm. 1/v = 1/f - 1/u = -1/10 + 1/30 = -2/30 = -1/15. So v = -15 cm. The image is real, 15 cm in front of the mirror. Magnification m = -v/u = -(-15)/(-30) = -0.5: inverted and half the size.
Traps
Do not mix mirror and lens sign rules. A negative image distance for a mirror means a real image in front; for a lens it means a virtual image on the same side as the object. Do not use n = 1 for the denser medium. Power in dioptres requires focal length in metres.