Correct option is C
Correct Answer: (c) Focal length does not vary in any media
The focal length of a concave mirror remains constant regardless of the surrounding medium (vacuum, air, or water).
Explanation
Focal length of a spherical mirror is determined solely by its radius of curvature via the formula f = R/2, where R is independent of the medium's refractive index.
Unlike lenses (which depend on relative refractive indices via the lensmaker's formula), mirrors operate purely by reflection. Light rays reflect according to the law of reflection (angle of incidence = angle of reflection), which holds in any medium.
Information Booster
Object position change: Image characteristics (position, size, nature) change via mirror formula 1/v + 1/u = 1/f, but f remains fixed.
Medium change: No effect on f; any apparent image shift when submerged results from refraction of incident rays before they reach the mirror surface, not a change in the mirror's focal length.
Mirror deformation: Only physical change to curvature alters f; temperature expansion or mechanical stress could theoretically affect R, but this is not medium-related.
Additional Information
Options
| Option | Scenario | Focal Length | Reason |
|---|---|---|---|
| A | Vacuum | Unchanged | f = R/2 purely geometric |
| B | Air | Unchanged | Standard reference condition |
| C | None | Unchanged | Correct: Independent of medium |
| D | Water | Unchanged | Reflection unaffected by refractive index = 4/3 |
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