The amplitude of the electric field associated with a light beam of intensity $\frac{15}{\pi} \mathrm{Wm}^{-2}$ is

When photons incident on a photosensitive material of work function 1.5 eV , the maximum velocity of the emitted photoelectrons is $8 \times 10^5 \mathrm{~ms}^{-1}$. The stopping potential of the photoelectrons is

(Mass of the electron $=9 \times 10^{-31} \mathrm{~kg}$ and charge of the electron $=1.6 \times 10^{-19} \mathrm{C}$ )

The potential energy of an electron in an orbit of hydrogen atom is -6.8 eV . The de-Broglie wavelength of the electron in this orbit is

( $r_o$ is Bohr radius)

If a radioactive substance decays $10 \%$ in every 16 hours, then the percentage of the radioactive substance that remains after 2 days is