Light and Optics — Revision Notes

⚡ 30-Second Revision

Normal, index, image. Measure optical angles from the normal. Across a stationary boundary frequency remains fixed; speed and wavelength may change. Total internal reflection needs higher-to-lower index and incidence above the critical angle. A real image has actual ray convergence; a virtual image uses backward extensions. Lens power uses focal length in metres.

2-Minute Revision

Reflection compares two angles to the normal. Refraction connects index, speed and direction; at normal incidence a speed change need not produce bending. Total internal reflection is conditional, not a synonym for strong reflection.

For a thin lens with f = +20 cm and u = −30 cm, 1/v = 1/20 − 1/30 gives v = +60 cm; magnification is −2. The image is real, inverted and enlarged. A +0.20 m focal length has power +5 dioptres. Optical fibres use index contrast but still have attenuation.

Treat the ray model as an approximation with a stated domain.

5-Minute Revision

Draw a flat mirror and a ray 30° from its surface. Mark the normal and obtain 60° for incidence and reflection. Next, take a 600 nm vacuum wavelength entering index 1.5: calculate 400 nm while preserving frequency. Finally, solve f = +20 cm and u = −30 cm to obtain v = +60 cm and magnification −2. Explain why +12 cm mishandles the reciprocal terms and why positive v does not imply an upright image. End by naming both total-internal-reflection conditions and one real fibre limitation.

Vyyuha Quick Recall

NIM: Normal for angles; Index for propagation; Magnification for orientation. Keep image-distance and magnification signs separate.