| Feature | Spherical Mirror | Thin Lens |
|---|---|---|
| Formula | 1/v + 1/u = 1/f | 1/v − 1/u = 1/f |
| Magnification | m = **−**v/u | m = v/u (no minus) |
| Converging type | Concave (f < 0) | Convex (f > 0) |
| Diverging type | Convex (f > 0) | Concave (f < 0) |
| u sign | Always negative | Always negative |
| Real image v sign | v < 0 (in front, same side) | v > 0 (other side) |
| Virtual image v sign | v > 0 (behind mirror) | v < 0 (same side as object) |
| Power | Not used (use 1/f) | P = 1/f (D) |
| Combination rule | Not additive | P = + (in contact) |
| Works on | Reflection | Refraction |
| Image erect? | Yes, only if virtual | Yes, only if virtual (for real object) |
Critical Differences to Memorize:
- Mirror formula has +, lens has −.
- Mirror magnification has − sign, lens has none.
- Real image from mirror → v < 0. Real image from lens → v > 0.
- Concave = converging for mirrors but concave = diverging for lenses.