Basic and Clinical Optics
This section covers basic and clinical optics: vergence, thin lenses, prisms, magnification, aberrations, and instrument optics. The exam tests sign conventions and clinical translation rather than pure mathematics.
Core rules and concepts
- Vergence is V=n/r; in air V=1/r. Object vergence is negative and image vergence positive after a converging surface.
- Thin-lens power is P=1/f in metres. At a lens, V_out=V_in+P.
- A thin prism deviates light by δ=(n−1)A; the Prentice rule is Δ=cF, with c in centimetres and F in dioptres.
- Spectacle magnification, telescope magnification, and lateral magnification are distinct; an astronomical telescope has M=−F_objective/F_eyepiece.
- Spherical aberration, coma, astigmatism, and chromatic aberration reduce image quality; small pupils and aspheric surfaces modify these effects.
Worked example
An object is 50 cm in front of a +5.00 D thin lens. Object vergence is 1/(−0.50)=−2.00 D. Adding the lens gives +3.00 D, so the image is real at 1/3.00=0.33 m, or 33 cm, behind the lens. The positive image vergence confirms a real, inverted image. If the object were at the focal point, outgoing vergence would be zero and the image would be at infinity.
Traps that cost marks
Mixing centimetres and metres is the commonest error. Light is deviated toward the prism base. Do not apply the Prentice rule to large decentrations. In telescope questions, identify objective and eyepiece before substituting.