The material used for solar cell should have band gap

The ratio of the distance of $n^{\text {th }}$ bright band and $\mathrm{m}^{\text {th }}$ dark band from the central bright band in an interference pattern is

An ideal gas expands adiabatically, $(\gamma=1.5)$. To reduce the r.m.s. velocity of the molecules 4 times, the gas has to be expanded

Out of the following statements which is NOT the characteristics of electric lines of force?