If the potential difference applied to an X-ray tube is doubled while keeping the separation between the filament and the target as same, what will happen to the cut-off wavelength?

Which one of the following statements is true for the relation, $$F = {{G{m_1}{m_2}} \over {{r^2}}}$$ ? (All symbols have their usual meanings.)

Why is it difficult to measure the coefficient of expansion of a liquid than solid?

If the absolute refractive indices of glass and water are $${3 \over 2}$$ and $${4 \over 3}$$ respectively, what will be the ratio of velocity of light in glass and water?