The solar radiation is

Energy required to remove an electron from aluminium surface is 4.2 eV . If light of wavelength $2000 \mathring{A}$ falls on the surface, the velocity of the fastest ejected electron the surface will be

If the binding energy of the electron in a hydrogen atom is 13.6 eV . Then, energy required to remove electron from first excited state of $\mathrm{Li}^{2+}$ is

A mixture consists of two radioactive materials $A_1$ and $A_2$ with half lives of 20 s and 10 s respectively. Initially, the mixture has 40 g of $A_1$ and 160 g of $A_2$. The amount of the two in the mixture will become equal after