Unit 6: Geometric and Physical Optics

Executive Summary

Light reflects, refracts, diffracts, and interferes — geometric optics handles mirrors and lenses; physical optics handles waves.

Reflection and refraction

Law of reflection: angle in = angle out. Snell's law: $n_1\sin\theta_1 = n_2\sin\theta_2$. Light bends toward the normal entering a denser medium. Total internal reflection occurs past the critical angle — the mechanism of fiber optics.

Mirrors and lenses

The thin-lens equation $\frac{1}{f} = \frac{1}{d_o} + \frac{1}{d_i}$ and magnification $m = -\frac{d_i}{d_o}$ work for both (with sign conventions). Converging lenses/mirrors focus; diverging ones spread. Ray diagrams locate real (inverted) vs virtual (upright) images.

Wave optics

Young's double slit: bright fringes at $d\sin\theta = m\lambda$. Single-slit diffraction spreads light ($a\sin\theta = m\lambda$ minima). Thin films interfere by path difference — soap bubbles and oil slicks. Higher frequency means shorter wavelength and greater resolution.

Exam traps

Real images are inverted; virtual are upright. Concave mirrors are like convex lenses (converging) — don't confuse the sign conventions. Refraction bends toward the normal in the SLOWER medium. Interference needs coherent light. The image distance sign tells you the side — set it up carefully.