Geometrical and Physical Optics
This domain tests refraction at surfaces, lens and mirror imaging, prisms, and physical optics including diffraction and polarization. Calculations and sign conventions are common.
Core rules
- Snell law: n1 sin(theta1) = n2 sin(theta2). Light bends toward the normal when entering a denser medium; total internal reflection occurs beyond the critical angle.
- Thin lens equation: 1/f = 1/do + 1/di, using real objects and real images as positive in the conventional form. Magnification m = -di/do.
- Diopters are reciprocal meters: F = 1/f. A +5.00 D lens has a focal length of 0.20 m.
- Prisms bend light toward the base and displace the image toward the apex. Prism diopter = 100 tan(deviation angle).
- Aberrations include spherical, coma, astigmatism, field curvature, and chromatic; diffraction limits resolution, and polarization describes the orientation of the electric field.
Worked example
A +5.00 D thin lens has a focal length of 0.20 m. An object is 0.50 m away. Reasoning: 1/di = 1/f - 1/do = 5.00 - 2.00 = 3.00 D, so di = 0.333 m. Magnification is -di/do = -0.333/0.50 = -0.67. The image is real, inverted, and reduced.
Traps that cost marks
- Confusing the prism base and apex: light bends toward the base, while the image appears displaced toward the apex.
- Using centimeters directly in the diopter formula without converting to meters.
- Reversing sign conventions for real and virtual images.