Assertion (A) The focal length of lens does not change when red light is replaced by blue light.

Reason (R) The focal length of lens does not depend on colour of light used.

Light of wavelength $$300 \mathrm{~nm}$$ in medium $$A$$ enters into medium $$B$$ through a plane surface. If the frequency of light is $$5 \times 10^{14} \mathrm{~Hz}$$ and the ratio of speed in medium $$A$$ to that in medium $$B$$ is $$4 / 5$$, the absolute refractive index of medium $$B$$ is