# how can u control the speed of light? [closed]

Light traveling in a lake goes at 3 km/sec. Please tell me how to limit the speed in the medium (in this case the the lake).

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## closed as unclear what you're asking by Chris White, Manishearth♦Jul 23 '13 at 6:40

Please clarify your specific problem or add additional details to highlight exactly what you need. As it's currently written, it’s hard to tell exactly what you're asking. See the How to Ask page for help clarifying this question.If this question can be reworded to fit the rules in the help center, please edit the question.

Dear Anil, I'm having troubles reading your question. Can you make it, somewhat more readable? – Ali Jul 23 '13 at 6:07

Speed of light $c$ in medium is reduced by its refractive index $n$.
$$c=\frac{c_0}{n}$$ where $c_0 = 299\,792\,458\frac{m}{s}$ is speed of light in vacuum. You are asking for a material of refractive index $n=1,5$ which is e.g. the glass BK-7.