Reason (R): In case of reflection of light from a denser medium the phase changes by \(\pi\).
Solution:
The focal length of a spherical mirror ((f = R/2)) depends only on its geometric curvature, not on the wavelength of light or the medium's refractive index. Thus, (A) is true.
When light reflects from the surface of an optically denser medium (e.g., from air to glass), there is a phase change of \(\pi\) (or \(180^\circ)\). Thus, (R) is true.
However, the phase change on reflection does not explain why the focal length of a mirror is independent of wavelength. This independence is due to the nature of reflection itself, where the angle of incidence equals the angle of reflection regardless of wavelength.
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